Moon--: The oldest three-body problem

Martin C. Gutzwiller IBM Research Center, Yorktown Heights, New York 10598

The daily of the through the has many unusual features that a careful observer can discover without the help of instruments. The three different for of freedom have been known very accurately for 3000 , and the geometric explanation of the Greek was basically correct. Whereas Kepler’s laws are sufficient for describing the motion of the around the Sun, even the most obvious facts about the lunar motion cannot be understood without the gravitational attraction of both the Earth and the Sun. discussed this problem at great length, and with mixed success; it was the only testing ground for his Universal Gravitation. This background for today’s many-body theory is discussed in some detail because all the guiding principles for our understanding can be traced to the earliest developments of . They are the oldest results of scientific inquiry, and they were the first ones to be confirmed by the great physicist- of the 18th century. By a variety of methods, Laplace was able to claim complete agreement of celestial with the astronomical observations. Lagrange initiated a new trend wherein the mathematical problems of mechanics could all be solved by the same uniform process; canonical transformations eventually won the field. They were used for the first on a large scale by to find the ultimate solution of the lunar problem by perturbing the solution of the two-body Earth-Moon problem. Hill then treated the lunar as a displacement from a periodic that is an exact solution of a restricted three-body problem. Newton’s difficultly in explaining the motion of the lunar perigee was finally resolved, and the Moon’s orbit was computed by a new method that became the universal standard until after WW II. Poincare´ opened the with his analysis of in phase , his insistence on investigating periodic even in ergodic , and his critique of theory, particularly in the case of the Moon’s motion. Space , astrophysics, and the landing of the on the Moon led to a new flowering of . now has to confront many new data beyond the simple three-body problem in order to improve its accuracy below the precision of 1 arcsecond; the computer dominates all the theoretical advances. This review is intended as a case study of the many stages that characterize the slow development of a problem in from simple observations through many forms of explanation to a high-precision fit with the data. [S0034-6861(98)00802-2]

CONTENTS A. The traditional model of the Moon 600 B. The osculating elements 600 C. The lunar periods and Kepler’s third law 601 I. Introduction 590 D. The —Greek versus Babylonian A. The Moon as the first object of pure science 590 601 B. Plan of this review 591 E. The variation 602 II. Coordinates in the Sky 591 F. Three more inequalities of Brahe 602 A. The of the solar 591 VI. Newton’s Work in Lunar Theory 602 B. and and A. Short biography 602 592 C. and 592 B. Philosophiae Naturalis Principia Mathematica 603 D. The vernal 593 C. The rotating Kepler 604 E. All kinds of corrections 593 D. The advance of the lunar apsides 604 F. The measurement of time 593 E. Proposition LXVI and its 22 corollaries 605 G. The Earth’s 594 F. The motion of the perigee and the node 606 H. The measurement of the solar 594 G. The Moon in Newton’s system of the I. Scaling in the 594 (Book III) 606 III. Science without Instruments 595 VII. Lunar Theory in the 606 A. The lunar cycle and prescientific observations 595 A. Newton on the 606 B. 595 B. The challenge to the law of universal gravitation 607 C. The precise timing of the full 595 C. The for the Moon-Earth- D. The 596 Sun system 608 E. The Saros cycle 596 D. The analytical approach to lunar theory by IV. of Greek Astronomy 597 608 A. The historical context 597 E. The evection and the variation 609 B. The impact on modern science 597 F. Accounting for the motion of the perigee 609 C. The eccentric motion of the Sun 598 G. The annual equation and the parallactic D. The epicycle model of the Moon 598 inequality 609 E. The model for the outer planets 598 H. The computation of lunar tables 610 F. The Earth’s orbit and Kepler’s second law 599 I. The grand synthesis of Laplace 610 G. The of 599 J. Laplace’s lunar theory 611 H. Expansions in powers of the eccentricity 599 VIII. The Systematic Development of Lunar Theory 612 V. The Many of the Moon 600 A. The triumph of celestial mechanics 612

Reviews of Modern Physics, Vol. 70, No. 2, April 1998 0034-6861/98/70(2)/589(51)/$25.20 © 1998 The American Physical Society 589 590 Martin C. Gutzwiller: The oldest three-body problem

B. The variation of the constants 612 principal ideas as well as the crucial tests for our under- C. The Lagrange 613 standing of the . For simplicity’s sake I shall dis- D. The Poisson brackets 614 tinguish three stages in the development of any particu- E. The perturbing function 614 F. Simple derivation of earlier results 615 lar scientific endeavor. In all three of them the Moon G. Again the perigee and the node 615 played the role of the indispensable guide without whom IX. The Canonical Formalism 616 we might not have found our way through the maze of A. The inspiration of and 616 possibilities. B. Action-angle variables 616 The first stage of any scientific achievement was C. Generating functions 617 reached 3000 years ago in when elemen- D. The canonical formalism in lunar theory 618 tary observations of the Moon on the were E. The critique of Poincare´ 618 F. The expansion of the lunar motion in the made and recorded. The relevant numbers were then parameter m 619 represented by simple arithmetic formulas that lack any . Expansion around a Periodic Orbit 619 in terms of geometric models, let alone physical A. (1838–1914) 619 principles. And yet, most physicists are not aware of the B. Rotating rectangular coordinates 620 important characteristic frequencies in the C. Hill’s variational orbit 620 that were discovered at that time. They can be com- D. The motion of the lunar perigee 621 pared with the of elementary , our E. The motion of the 622 F. Invariant tori around the periodic orbit 622 present- understanding of which hardly goes beyond G. ’s complete lunar 623 their numerical values. H. The lunar ephemeris of Brown and 623 The second stage was initiated by the early Greek phi- XI. Lunar Theory in the 20th Century 624 losophers, who thought of the universe as a large empty A. The recalcitrant discrepancies 624 space with the Earth floating at its center, the Sun, the B. The Moon’s secular 625 Moon, and the planets moving in their various orbits C. Planetary inequalities in the Moon’s motion 626 around the center in front of the background of fixed D. Symplectic geometry in 626 E. Lie transforms 627 . This grand view may have been the single most F. New analytical solutions for the main problem significant achievement of the mind. Without the of lunar theory 627 Moon, visible both during the and during the day, G. Extent and accuracy of the analytical solutions 628 it is hard to imagine how the Sun could have been con- H. The fruits of solving the main problem of lunar ceived as moving through the just like the Moon theory 628 and the planets. The Greek mathematicians and as- I. The modern ephemerides of the Moon 630 tronomers were eventually led to sophisticated geomet- J. in gravitational problems 630 ric models that gave exact descriptions without any hint K. The three-body problem 631 List of Symbols 632 of the underlying physics. References 633 The third stage was reached toward the end of the 17th century with the work of . His grand opus, The Mathematical Principles of Natural Philoso- phy, represents the first endeavor to explain observa- I. INTRODUCTION tions both on Earth and in the on the basis of a If there be nothing new, but that which is few physical ‘‘laws’’ in the form of mathematical rela- Hath been before, how are our brains beguiled, tions. The crucial test is the motion of the Moon to- Which, laboring for invention, bear amiss gether with several related phenomena such as the The second burden of a former child! and the of the . This first effort at (Shakespeare, Sonnet 59) unification can be called a success only because it was able to solve some difficult problems such as the inter- A. The Moon as the first object of pure science action of the three bodies Moon-Earth-Sun. Modern physics undoubtedly claims to have passed When we try to understand a special area in physics already through stages one and two, but have we ourselves, or when we teach the basics of some specialty reached stage three in such areas as elementary- to our students, there is no better way than to go physics or ? Can we match Newton’s feat of through the most important steps in their historical or- finding two mathematical relations between the four rel- der. While doing so, it would be a pity if we did not evant lunar and solar frequencies that were known in make comparisons with the historic progression in re- antiquity to five significant decimals? The gravitational lated fields and identify the common features that help three-body problem has provided the testing ground for us to establish a successful and convincing picture in any many new approaches in the three centuries since New- area. The Moon’s motion around the Earth offers us the ton. But we are left with the question: what are we look- prime example in this respect. ing for in our pursuit of physics? Although we think primarily of the planets orbiting The Moon as well as elementary particles and cosmol- the Sun as the fundamental issue for the origin of mod- ogy are problems whose solutions can be called quite ern science, it was really the Moon that provided the remote and useless in today’s world. That very quality of

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 591 detachment from everyday makes them prime ex- finements that are needed to exploit fully what Newton amples of pure science, objects of without ap- had only tentatively suggested. parent purpose, such as only human beings would find The great mathematicians of the 18th century suc- interesting. In following the development in the case of ceeded in clearing up Newton’s difficulties with calculat- the Moon over the past three centuries we get an idea of ing the motion of the lunar perigee. Section VII tries to what is in store for us in other fields. give an idea of their straightforward, but somewhat clumsy, methods. Laplace was able to carry out all the necessary calculations, but his grand unification of all celestial mechanics came at a high price; physics was B. Plan of this review again in danger of getting lost. The three principal coordinate systems in the sky are The next three sections are more technical in content. described in Sec. II; they are based on the local horizon, They try to provide a glimpse of the general methods on the , and on the . The relations be- that were proposed in order to deal with the difficult tween these coordinate systems are fundamental for un- three-body problem Moon-Earth-Sun. Lagrange’s idea derstanding the process of observing and interpreting of ‘‘varying the constants’’ is discussed in Sec. VIII; a the results of the observations. The various definitions of few successful examples of his method still retain some time in astronomy are recalled, as well as the measure- intuitive appeal. Section IX describes the origin of the ment of linear distances in the solar system, which plays canonical formalism and its first use on a grand scale by ´ a special role in celestial mechanics because the equa- Delaunay to find the lunar trajectory. Poincare showed tions of motion scale with the distance. the ultimate futility of any expansion in the critical pa- Section III is devoted to the prescientific and the ear- rameter that caused Newton so much grief. Toward the liest scientific achievements in the search for under- end of the 19th century Hill approached lunar theory by standing of the lunar motions. The more obvious fea- starting with a periodic orbit of a perturbed dynamical tures that are easily seen with the bare eye can then be system. The many advantages of this method are dis- explained. The basic periods can be obtained from - cussed in Sec. X, including the complete ephemeris of servations near the horizon, i.e., near the time of moon- Brown and Eckert that was basic for the pro- rise or moonset. The Babylonians in the last millennium gram and the implementation on a modern computer. A somewhat haphazard survey of other developments B.C. developed a purely numerical scheme for predicting the important events in the lunar cycle. Their great in the 20th century, as well as some older but timely achievement was the precision measurement of the vari- problems related to the Moon, are brought up in Sec. ous fundamental frequencies in the Moon’s motion. XI. Modern technology in connection with the space program is responsible for many improvements both in In the last few centuries B.C., the Greeks developed a picture of the universe that is still essentially valid in our observing and in understanding the dynamic system time. The solar system is imbedded in a large three- Moon-Earth-Sun. High- computers have moved dimensional , which is surrounded by the fixed the emphasis away from the general theory of the three- stars. The Sun, the Moon, and the five classical planets body problem toward a better look at the detailed fea- move along rather elaborate orbits of various , the tures in many of its special cases. We have come full- Moon being by far the closest to the Earth. The basic circle to watching elementary phenomena, but they physics such as the conservation of angular present themselves on the screen of a monitor rather is hidden in these purely geometric models. The discus- than as the Sun or the Moon on the local horizon. sion in Sec. IV will hardly do justice to this great ad- vance in our understanding of the universe around us. II. COORDINATES IN THE SKY Some of the Greek data were refined by Islamic as- tronomers, but even the awakening in the 15th and 16th A. The geometry of the solar system century, in particular the great treatise by Copernicus, did not improve the calculation of the Moon’s motion, Any account of the motions in the Moon-Earth-Sun as will be shown in Sec. V. Physics came into the picture system has to start by defining the basic coordinates. when Kepler got a chance to interpret Brahe’s data, and Our cursory discussion describes the technical aspect of had the marvelous idea of looking at the stars the Greek universe and still represents the fundamental with the newly invented . The explosive accu- approach to running a modern observatory. The geocen- mulation of observations without a useful theory led to tric viewpoint is unavoidable as long as we are depen- stagnation at the end of the 17th century, not unlike dent on that are fixed on the ground or are high- physics at the end of the 20th century. attached to a . The big breakthrough came with the publication of For more details the reader is encouraged to take a the Principia by Newton in 1687. The theory of the look at the Explanatory Supplement to the Astronomical Moon became the great test for Newton’s laws of mo- Ephemeris, a 500-page that is published by the tion and universal gravitation, as will be described in US Naval Observatory and the Royal Greenwich Obser- Sec. VI. His truly awesome (and generally quite unap- vatory. Among the many introductory texts on spherical preciated) results in this area are well worth explaining astronomy are the classics by Smart (1931) and Woolard in detail before getting into the inevitable technical re- and Clemence (1966). The reader may also find some

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 592 Martin C. Gutzwiller: The oldest three-body problem useful explanations in more elementary textbooks like for refraction is least. The ’s position is defined by those of Motz and Duveen (1977) and Roy (1978). the angle between its and the observer’s me- Three conceivable places for the observer can serve as ridian, called the ␹; it is measured from the the origin of a coordinate-system: (i) topocentric, from observer’s meridian toward . The hour angle of any the place of the observatory on the of the Earth, star increases from 0 at its to exactly 360°ϭ24 (ii) geocentric, from the center of the Earth, or (iii) he- at its next transit, after one sidereal day. liocentric, from the center of the solar system. Each co- The Sun goes around the celestial once in one ordinate system on the requires for its , moving in a direction opposite to the daily motion definition a or, equivalently, a direction perpen- of the fixed stars. It makes on the average only 365.25 dicular to the plane. A full is transits in one year, whereas every fixed star makes obtained by adding the distance from the observer. 366.25 transits. Therefore, the sidereal day is shorter by The whole machinery of these reference systems was 1/366.25, or a little less than 4 , than the mean the invention of the Greeks as part of their purely geo- solar day. metric view of the universe; it is a crucial preliminary The horizon system with the x,y,z axes pointing step toward our physical picture of the world. The con- , , and toward the , can be transformed struction is completed by defining a full-fledged Carte- into the equatorial system with the xЈ,yЈ,zЈ axes point- sian coordinate system, which is centered on one of the ing south on the equator, east on the horizon, and to- three possible locations for the observer. The ordinary ward the pole, by a rotation around the common y formulas for the transformations from one Cartesian axis through the colatitude ␾¯ ϭ90°Ϫ␾. The position of a system to another can be used, rather than the less fa- star in the horizon system is given by the coordinates miliar formulas from spherical . (x,y,z)ϭ(cos ␨ cos ␺,Ϫcos ␨ ␺,sin ␨), whereas in the equatorial system (xЈ,yЈ,zЈ)ϭ(cos ␦ cos ␹, B. Azimuth and altitude—declination and hour angle Ϫcos ␦ sin ␹,sin ␦). The transformation is given by ¯ 0 ϩ ¯ The local plumbline fixes the point Z overhead, the xЈ cos ␾ sin ␾ x (local) zenith, on the imaginary spherical surface around yЈ ϭ 010 y . (1) us. The horizon is composed of all the points with a ͩ zЈͪ ͩ z ͪ ͩ Ϫsin ␾¯ 0 cos ␾¯ ͪ zenith distance equal to 90°. The center P for the appar- ent daily rotation is the north ; its distance A star is said to rise when its altitude becomes positive from the zenith is the colatitude ␾¯ ϭ90°Ϫ␾ where ␾ is by passing the eastern half of the horizon, and to set as it the geographical latitude for the place of observation. A dips into the western half of the horizon. The azimuth through P and Z defines the north point N ␺0 of a star with declination ␦ at the time of its setting, of the horizon, as well as the other three cardinal points i.e., when its altitude ␨ϭ0, follows from on the horizon, east (E), south (S), west (W). sin͑␦͒ The location of an object X is defined by moving west sin͑␺ Ϫ90°͒ϭ . (2) 0 cos ␾͒ from the south point on the horizon through the azimuth ͑ ␺, and then straight up through the altitude ␨, counted At the latitude of ␾ϭ40°, and for ␦ positive toward the zenith. The altitude ␨ of an object ϭ23.5° which is the declination of the Sun at the sum- determines its visibility at the location of the observer. mer , we find that the Sun sets ␺0Ϫ90°ϭ31°22Ј More importantly, the refraction of the light rays in the on the horizon toward north from the point west. Earth’s is responsible for increasing the ob- The upper limit for the declination of the Moon has a served value of the altitude by as much as 30Ј when the cycle of about 18 years, during which it varies from object is near the horizon. The refraction was not under- 18°10Ј to 28°50Ј. The corresponding moonsets, there- stood by the Greeks; nor even by , who fore, which are easily observed with the , vary gives different values for the Sun and for the Moon, from 24° north from the point west all the way to 39° at although the values themselves are quite good. the latitude of New York. The second spherical coordinate system has the main direction pointing toward the north celestial pole P. Any half of a great circle from the to the C. Right ascension—longitude and latitude is called a meridian. The directions at right angle to the north pole form the , which The difference in hour between two fixed stars intersects the horizon in the two cardinal points east and remains constant. Any feature Q on the celestial sphere west. Each star X has its own meridian. The declination may serve as a reference; its hour angle is called the ␦ is the of X from the equator, as mea- ␶. The right ascension ␣ of any star X is sured along its meridian, positive to the north and nega- then defined by tive to the south of the equator. The meridian through ␣ϭ␶Ϫ␹. (3) the zenith Z and S is the observer’s meridian. When the meridian of X coincides with the observer’s The right ascension ␣ increases toward the East as mea- meridian, the star is said to transit or culminate. At that sured from Q, opposite the apparent motion of the ce- moment the altitude of X is greatest, and the correction lestial sphere.

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 593

The apparent path of the Sun through the sky with 1980). The ephemerides correct all the geocentric data respect to the fixed stars is a great circle, which is called such as the right ascensions and for the the ecliptic, with the pole K in the ; ‘‘mean equator’’ and the ‘‘mean equinox of date.’’ The it changes only very slowly with time. An object X in the adjective ‘‘mean’’ always applies to changes in some pa- sky can be found by starting from the reference point Q rameter after its periodic variations have been elimi- and going east along the ecliptic through the longitude ␭, nated. and then toward K through the latitude ␤. This reference point Q is generally chosen to be the vernal equinox, i.e., the place where the ecliptic inter- E. All kinds of corrections sects the celestial equator, and where the Sun in its ap- parent motion around the ecliptic crosses over into the For objects in the solar system, a correction is neces- northern side of the equator; it is also called the First sary to refer any observations to the center of the Earth, Point in Aries (the of the Ram). This point rather than to some location on its surface; the data have has a slow, fairly involved, retrograde motion with re- to be reduced from the topocentric to the geocentric. spect to the fixed stars, i.e., it moves in the direction This correction for the parallax is very large for the opposite to the motion of virtually all bodies in the solar Moon, almost as much as 1°, whereas for all other ob- system. Its longitude with respect to a given fixed star jects in the solar system it is of the order of 10Љ at most. decreases by some 50Љ per year. It changes the apparent position with respect to the fixed The transformation from the equatorial to the ecliptic stars depending on the distance from the zenith. To system is made with the common x axis pointing toward make things worse, Zach (1814) showed that the local Q, causes the local plumbline to differ significantly 10 0 from the averaged normal. The shift from topocentric to cos ␤ cos ␭ geocentric coordinates is made by formulas such as Eq. cos ␤ sin ␭ ϭ 0 cos ␧0 sin ␧0 (1) that include the radial distances. ͩ sin ␤ ͪ ͩ 0 Ϫsin ␧ cos ␧ ͪ The objects in the solar system have generally a finite 0 0 on the order of seconds of arc (and therefore do not cos ␦ cos ␣ twinkle!), except for the Moon and the Sun, whose ap- ϫ cos ␦ sin ␣ (4) parent sizes are very nearly equal and close to 30Ј on the ͩ sin ␦ ͪ average. The Moon’s boundary is clearly jagged on the order of seconds of arc because of its ; this where ␧ is the angle between the equator and the eclip- 0 outline of the lunar shape changes because of the tic. Moon’s residual rotation with respect to the Earth. Although many instruments have been constructed The bare eye is able to work down to minutes of arc, since antiquity to measure directly the ecliptic coordi- and requires correction only for refraction and the par- nates (see the detailed description of Brahe’s instru- allax of the Moon. Isolated observations with a moder- ments by Raeder et al.), they are not very exact. Most of ate telescope can distinguish features down to seconds the precision measurements of the lunar position are of arc that can be seen in good photography. Positions made by timing the Moon’s transit day after day by mu- on the celestial sphere to a second of arc, however, re- ral quadrants, meridian circles, and other transit instru- quire not necessarily large telescopes but very stable and ments, and then transforming to the ecliptic coordinates precise mountings so that many data can be taken over with the help of the above formulas. extended intervals of time.

D. The vernal equinox F. The measurement of time During antiquity the vernal equinox was in the con- stellation of Aries, and was associated with the The time interval between the transits of the Sun var- Sun’s being located at the beginning of the Ram. But ies considerably throughout the year. The mean solar since then, the vernal equinox has moved into the ‘‘pre- day is defined with the help of a fictitious body called the ceding’’ constellation of Pisces, the Fishes. The astro- mean sun (MS), which moves on the equator with uni- logical literature, however, still associates the constella- form speed. The difference of the right ascension for the tions in the Zodiac with the twelve periods of the solar mean sun minus the right ascension for the real Sun is cycle as they were in antiquity. Therefore, gentle reader, called the , ␣MSϪ␣S . This has to be beware, although you might think of yourself as a Lion, added to the Sun’s right ascension ␣S to make it increase the Sun was in the sign of the Crab when you were ! uniformly. The ecliptic is inclined toward the celestial equator by The hour angle for the mean sun at the Greenwich the obliquity of the ecliptic ␧0 (really the inclination of Observatory is called the Greenwich mean astronomical the Earth’s axis with respect to the ecliptic), about time. UT is Greenwich mean astronomi- 23°30Ј. The exact motion of the Earth’s axis has a num- cal time plus 12 hours, so that the transit of the mean ber of periodic terms, collectively called , that sun occurs at 12 hours, and mean midnight at 0 hours. are of the order of 9Љ (see , , and Bender, UT is the basis for all the local standard ; they

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 594 Martin C. Gutzwiller: The oldest three-body problem differ from UT simply by adding or subtracting a fixed many other efforts to find a better figure, this value was (not necessarily integer, as in the case of ) number essentially accepted even by Kepler and his contempo- of hours (Howse, 1980). raries. The difference between the apparent motions of the The relative distances for all the planets from the Sun real and the mean sun is shown on any good . were quite correctly known ever since antiquity. The ab- The pointer’s at the time of the mean local noon solute distances, however, were only obtained at the end is the ; it has the shape of an unsymmetric and of the 17th century by a concerted effort to measure slanted figure 8 along the north-south direction. various in the solar system, mostly by observ- ing the transits of and across the face of the Sun from different locations on the Earth. Neverthe- G. The Earth’s rotation less, even Newton and his immediate successors did not have good figures for the solar parallax, and they all The rate of the Earth’s rotation decreases because the used widely different values at different times. That did tides in the act like a brake; the mean solar day not impair Newton’s theory of universal gravitation, and increases by roughly 1 each century. From its application to the solar system, all because of scaling the vast literature on this subject let me mention the invariance! classic study by Newcomb (1878 and 1912) and its mod- ern versions by Marsden and (1966), Martin (1969), McCarthy and Pilkington (1979), and I. Scaling in the solar system and (1988). The accumulated lengthening of the day adds up to more than two hours over 20 centuries, Newton derived the complete form of Kepler’s third or equivalently, the Earth has fallen behind by about 30° law for the Sun and an isolated which attract each in the rotation around its own axis. The reports of other with a that varies as the inverse square of can be used to determine the history of the their distance. In modern notation, let n be the angular Earth’s rotation. A very detailed account of this argu- (ϭ2␲ divided by the of the planet’s ment is found in the books of Robert R. Newton (1976, orbit), a the semi-major axis of the planet’s orbit, G0 the 1979). (in the appropriate units), and M The real trouble with the Earth’s rotation is that it and m the masses of the Sun and of the planet; then 3 3 decreases sometimes in fits, and occasionally even n a ϭG0͑Mϩm͒. (5) up again. It was decided, therefore, in the 1950s to use the Earth’s orbit around the Sun as the base for Let us now write the equations of motion for an arbi- , and to speak about Ephemeris trarily complicated system of points that interact Time (ET). The difference between the two times, Uni- only through the gravitational between versal and Ephemeris, is . has them; each mass ␮ occurs only in the product G0 ␮.If already been superseded by Atomic Time (AT), which is any solution of these equations has been found, i.e., an based on atomic clocks, i.e., it is completely independent explicit expression for each coordinate as a function of of the motions in the solar system. time that satisfies the equations of motion, then another solution is obtained by multiplying all the distances with an arbitrary factor ␬, provided all the products G0 ␮ are 3 H. The measurement of the solar parallax changed into ␬ G0 ␮. If we stick with the same units of time, length, and mass, so that the gravitational constant For a long time, all measurements of the linear dis- G0 keeps the same numerical value, then we effectively tance between any two objects could be made in only assume that each mass has been increased by the factor one of two ways: either by directly applying a measuring ␬3. rod or by triangulation from a base that was short This scaling preserves the average of each enough to be measured directly with a measuring rod. In planet and the Sun; the actual size of a star depends on this manner of Cyrene in the early third the interaction of the gravitational with forces of century B.C. obtained a fair value for the circumference a completely different . No other forces were of the Earth. Since the Moon has a large parallax, i.e., known in Newton’s time; only rough assumptions could the apparent radius of the Earth as seen from the Moon be made, such as the incompressibility of a fluid or the is about 1°, the Greeks concluded correctly that the equation of of an ideal gas. A good value for the Moon’s distance from the Earth is about 60 Earth radii. solar parallax was secured after the in A short treatise by of , also from 1769, and thus the value of the product G0 M for the the early third century B.C., has survived, ‘‘On the Sizes Sun was finally known, but not the value of the mass and the Distances of the Sun and the Moon,’’ and a itself for the Sun, nor for any body in the solar system. translation can be found in Heath (1913). It proposes to The gravitational constant G0 was finally measured by determine the exact moment of half moon simply by Henry (1798) with a torsion balance in a fa- looking for the time when the lighted portion of the mous experiment that measured the gravitational force Moon is exactly a half circle. Aristarchus concludes that between two balls made of lead. With G0 known, the the ratio of the Sun’s distance to the Moon’s distance is average density of the Earth can be obtained; that was about 20 (the correct figure is close to 400); in spite of the intention of Cavendish, whose paper is entitled ‘‘Ex-

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 595 periments to determine the density of the Earth.’’ Re- our present-day Iraq, were the first to start truly markably, the correct result had been guessed by New- scientific investigations, some time after 1000 B.C. The ton as about five times the density of . remaining record is substantial but fragmentary: broken Because of scaling invariance, inaccurate values of G0 pieces of tablets that were found among the ruins of affect neither the astronomical observations nor the the ancient cities. They were brought to and theory of the motions in the solar system. The observa- America by the thousands during the 19th century, not tions yield precise values for the mass ratios, and that is necessarily by archaeologists, and are now kept in places all the theory requires. Many of the problems with dis- like the British Museum, Columbia, and Yale Univer- tances in the solar system were finally solved in the most sity. recent times, particularly in the case of the Moon. On These clay tablets are densely covered on both sides the one hand, space probes circling the Moon in a very with letters and numbers that were inscribed with a low orbit produce a good value for its relative mass di- chisel before the firing of the clay. The interpretation of rectly from Kepler’s third law. On the other hand, the this ‘‘cuneiform’’ writing took many decades of patient reflectors left on the surface of the Moon by the astro- effort and is one of the miracles of archaeology. Several nauts provide the to measure directly the hundred tablets are devoted to astronomical pursuits; distance of the Moon from the Earth through the reflec- the oldest of them are from as far back as the 7th cen- tion of laser pulses, giving rise to contemporary eph- tury B.C., the time when the city of Babylon rose to the emerides of unprecedented accuracy. Thus distances leadership of the Middle East; the most recent are from have finally replaced angles as the fundamental variables the 1st century B.C., three centuries after the Greeks un- in the description of the Moon’s motion. der the Great had conquered that whole part of the world and Babylon had fallen into ruins. III. SCIENCE WITHOUT INSTRUMENTS There are extensive diaries covering six to seven on one tablet reporting the state of the sky from A. The lunar cycle and prescientific observations day to day. The data on the Moon include first and last visibility, the stars it passed close by, and the time dif- Humanity must have noticed for a long time the ferences between rising and setting for both the Sun and Moon’s fundamental cycle of about 30 days. Its risings the Moon around the time of the . These last and settings follow a similar pattern in the course of one data are directly related to the apparent speed of the to that of the and in the course of Moon, and bring out one of the basic periods in the one year, but their spread on the horizon varies greatly. lunar motion. Since this kind of analysis is generally rec- It reached a minimum in 1997 of only Ϯ24° at New ognized as the very first scientific activity of humanity, it York’s latitude, much smaller than the Sun’s Ϯ31°; it is worth describing in more detail. will increase over the coming nine years and then reach The sources from this era were finally deciphered at a maximum of Ϯ39° in 2006. the end of the 19th century by a small group of German The Moon moves about 13° in one day with respect to Jesuit priests. But the texts are difficult to understand, the fixed stars. When the dark edge of the Moon moves and the number of experts in this area since then is very over a star, the apparent width of the light source can be small. Neugebauer (1975, 1983) and van der Waerden measured very simply by the time interval for its gradual (1974, 1978) will provide the reader with an introduction disappearance. (In this manner the apparent width of to the available literature. As examples of the patient certain light sources with large redshifts was first estab- work required in this field and related to the Moon, let lished; it was found to be ‘‘quasistellar,’’ thereby justify- me mention Aaboe (1968, 1969, 1971, 1980), Sachs ing their name as .) (1974), and Brack-Bernsen (1990, 1993). Other cycles were known in many early . When first shows up in the ’s early light at C. The precise timing of the full moons the beginning of August, the Nile is likely to start flood- ing two later. Venus changes back and forth be- Let us call r, t, and s the times when the Moon rises, tween evening star and morning star five times in eight transits, and sets; and let us also call rЈ, tЈ, and sЈ the years. makes a full swing around the Zodiac in corresponding times for the Sun. According to a time- about 12 years, more exactly 7 times in 83 years. The honored custom in lunar theory: if z designates any cyclic nature of these phenomena probably had a pro- quantity for the Moon, then zЈ designates the similar found effect on religion and philosophy; they were re- quantity for the Sun. corded in the early writings of many . But The differences sЈϪr and rЈϪs are positive before they cannot be called science as yet because no effort the full moon, whereas the differences rϪsЈ and sϪrЈ was made to pin down the exact timings and, in particu- are positive after the full Moon. They are less than one lar, the fluctuations around the basic repetitions. hour around the time of the full moon and can easily be measured with rather primitive water clocks. They were B. Babylonian astronomy consistently recorded in the diaries, stated in the Baby- lonian units of time-degreeϭ1/360 of a day ϭabout 4 The people living in Mesopotamia, the country in the minutes, and sometimes even further divided into six Middle East between the rivers Tigris and Euphrates, parts. It is not known whether the recorded times were

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 596 Martin C. Gutzwiller: The oldest three-body problem for first visibility and last appearance on the horizon, or Since 235 lunations make 19 years within two hours, the last and first contact with the horizon. The positions of 223 lunations constitute 18 years and 11 days. There are the Moon and the Sun on the horizon are hardly ever exactly 242 swings on the horizon, while the lunar speed directly opposite each other. has completed quite exactly 239 cycles. At the same time Since the differences sЈϪr and rЈϪs change sign in the Moon has completed exactly 241 trips around the the interval of 24 hours, the time of their vanishing can Zodiac. be calculated by assuming their linear decrease. More- This new period of 223 lunations in 18 years and 11 over, the total change of each time difference measures days is called the Saros period; it can be recognized very the speed of the Moon relative to the Sun at the time of clearly in the sequence of eclipses. The Babylonians the . kept a careful record especially of the lunar eclipses There is no evidence that the Babylonians had any since they can be observed from many places and are geometric picture of the events in the sky. But many clay easier to spot. Although the solar eclipses are as fre- tablets with long and elaborate tables of numbers were quent, they are harder to notice if they are partial, and found; they are undoubtedly the results of sophisticated total solar eclipses are very rare in any one location. numerical models for the lunar and solar cycles, espe- uses three consecutive lunar eclipses from a cially of the events near a full moon. Babylonian record of the 8th century B.C. in order to compute the parameters for the lunar motions. The D. The Metonic cycle Babylonians were not able to make predictions, but they issued warnings for the eclipses to occur at cer- Most experts believe that the Babylonians found their tain times of the year. procedures for reducing the data simply by staring long There are many computed lists of lunar and solar enough at long rows of numbers. These data can be gen- eclipses in historical times; the best known and for many erated artificially on the computer nowadays, and one purposes quite sufficient are those by von Oppolzer can try to imitate research as it was done by the Baby- (1887) and Meeus, Grosjean, and Vanderleen (1966), lonians. As a simple example, the reader could look at both of them with maps to show the strip of totality in Goldstine’s ‘‘New and Full Moons from 1001 B.C. to A.D. the solar eclipses. The physical significance of the near- 1651,’’ published in 1973. periodicities in the Saros cycle have been discussed only A 19-year period is found immediately that does ex- very recently by Perozzi, Roy, Stevens, and Valsecchi tremely well. It manages to predict the full moons con- (1991, 1993). sistently within two days; there are 235 lunar cycles dur- As if this coincidence of the four periods in the Moon- ing this time so that the average lunar cycle lasts 19 Earth-Sun system in such a short time span were not times 365.25 divided by 235ϭ29.53085 mean solar days enough, the Earth rotates exactly 6585.321 times around compared to the modern value of 29.53059 days. its own axis in 223 synodic months. If we wait for three The 19-year cycle became the base for the prevailing Saros cycles, the Earth will have rotated almost exactly in the Middle East; it is generally called the back to its previous orientation, and the eclipse will be Metonic cycle because it was instituted in Athens as the seen again from the same place at the same time of the basis for the Greek in 432 B.C. by the astrono- day. mer . The Jewish calendar has a cycle of 19 years A pair of solar eclipses three Saros cycles apart are of with 12 short years of 12 months and 7 long years of 13 special interest to the author, since he was able to watch months; holidays can therefore be defined with respect the second, whereas the first took place while he was still to the nearest . waiting to be born. Indeed, the meeting of the Dynami- The Christian calendar pays only lip service to the cal Astronomy Division of the American Astronomical moon with a highly arbitrary division of the year into 12 Society (where the author reported his work comparing months whose lengths vary from 28 to 31 days. Never- the lunar calculations of Eckert and Bellesheim with theless the 19-year cycle remained alive in Christianity, earlier work) was timed to take place at Stanford Uni- as can be seen in one of the famous ‘‘Books of Hours’’ versity at the end of February 1979 so that the partici- from France around 1400 (Zwadlo, 1994) in which the pants could repair to Oregon to watch the total eclipse decorative calendar carries symbols to calculate the lu- of the Sun on the morning of February 26. Its earlier nar cycle for any time in the future. The version had been visible in northern Manhattan and is based on the opposite decision, i.e., to define the year Westchester County in the morning of January 24, 1925. as 12 lunar cycles, so that 19 years are short of 7 full It was the first time that the most recent lunar theory of moons. Therefore the Islamic year-count, which started G. W. Hill and E. W. Brown was used for the prediction. in ϩ622, is ticking at a faster rate than the Jewish and I shall say more of this event in Sec. XI.A. Christian counts. The Babylonians came tantalizingly near a good theory for their observations. As an example, they had E. The Saros cycle divided the year into two unequal intervals, each roughly one half-year; the Sun was moving at constant A period of 223 lunations (new moons) emerges from speed in each of them and the total ground covered was watching both the spread of the moonsets on the hori- equal to 360°; the slow speed was given to the interval zon and the speed of the Moon near the full moons. covering spring and , whereas the high speed

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 597 was given to the fall- interval. A smooth sinu- made the first catalog of about one thou- soidal curve was approximated by a step function, but sand stars, giving their longitude and latitude and esti- the geometric origins, let alone the physics, were never mating their brightness on a scale from one to six which suspected. is still in use, although greatly refined. He described the orbits of the two ‘‘luminaries,’’ Sun and Moon, and the five classical planets, in terms of epicycles, and deter- IV. THE GOLDEN AGE OF GREEK ASTRONOMY mined the relevant parameters. He devised a method for predicting the occurrence of solar and lunar eclipses, A. The historical context and he discovered the precession of the equinoxes. To set the stage for this last (but not least) creation of the Greek miracle, one has to be reminded of the time B. The impact on modern science frame in which astronomy came to full flower. After the defeat of the Persians in 479 B.C. under the leadership of Greek astronomy managed somehow to grow from Athens and Sparta, the Athenian democracy experi- the prescientific stage and, with the help of some input enced an economic and cultural explosion that lasted for from the Babylonian accomplishments, ended up pro- about 50 years. In 431 B.C., a motley collection of Greek posing the first complete and scientifically viable picture city-states started to grind down the military power of of the solar system. By the time the Romans took con- Athens in the Peloponesian war. It ended in 404 B.C. trol of the Eastern Mediterranean because the Greeks with Athens accepting the terms dictated by Sparta. had been fighting one another for three centuries, as- The main achievements of this delta function in space tronomy had solid foundations as a science in the mod- and time can still be admired. Scientific astronomy got ern sense. The idea of first observing and measuring the its start in 432 B.C. when Meton introduced the 19-year phenomena in the sky and then predicting future events cycle into the Athenian calendar. He presumably mea- on the basis of a mathematical model became the goal of sured the exact date of this solstice; historians debate all the other sciences that are concerned with the outside whether he knew about the 19-year cycle from the Baby- world. The description of the solar system has become lonians or had found it independently. the basic example of a valid scientific picture. Our intu- Greek science in general got its momentum from phi- ition and our understanding are still solidly based on the losophy in the fourth century B.C. Some of the basic ideas from antiquity. ideas in astronomy seem to have come from and Astronomy in antiquity had nothing to do with com- , as well as from their students and followers. plicated structures of crystalline that moved the Meanwhile, Greece had been forcibly unified under solar system around the Earth. The only extant, com- Philip of Macedonia, and his son, Alexander the plete account of astronomy, the Great, conquered all of the present-day Middle East in- of Ptolemy, is a monograph in the modern sense: the cluding Central and Pakistan. Only then do we en- various coordinate systems in the sky are first described counter the great mathematicians, and Apollo- exactly as we did in the second chapter, along with the nius, master of the conic sections, as well as , relevant ; the theory of the solar motion in the third century B.C. and of the much more involved lunar motion is ex- Two elementary treatises, ‘‘On the moving sphere’’ plained on purely geometric grounds, and the prediction and ‘‘On risings and settings,’’ by survive of the lunar phases and of the two kinds of eclipses is from this time. They show that the spherical of discussed; then follows the study of the fixed stars and the sky, with its great circles, its daily rotation, and the the precession of the equinoxes; finally, the apparent path of the Sun along the ecliptic was generally ac- motions of the two inferior planets (Mercury, Venus) cepted; the Greeks had gone way past the Babylonians and of the three superior planets (Mars, Jupiter, ) in finding the geometric underpinnings of astronomy. are treated in the second half of the monograph on the Shortly thereafter appeared the paper of Aristarchus basis of the geocentric universe. An English translation of Samos, ‘‘the ancient Copernicus’’ in the words of his by Taliaferro was published in 1938 as part of Volume translator, Sir Thomas Heath (1913), ‘‘On the Sizes and 16 of Great Books of the ; book-length Distances of the Sun and the Moon’’ (see Sec. II.H). The analyses were undertaken by Pedersen (1974) and Neu- heliocentric cosmology of Aristarchus was first men- gebauer (1975). tioned by Archimedes in his ‘‘Sand Reckoner,’’ where The history of ancient Greek astronomy has been he tried to estimate the volume and content of the uni- studied in great detail, and there are many comprehen- verse (Heath, 1897; see also Dijksterhuis, 1987). As- sive accounts for the interested and educated layperson tronomy was bound to take off at this juncture! in the principal European languages. The astronomers And yet, we have to wait one more century for Hip- in the 16th and 17th centuries had studied their Greek, parchus in the middle of the second century B.C., before as well as their medieval Islamic and Jewish, ancestors we get a complete view of astronomy in the modern with great care, and some of these works were available sense. Unfortunately, only two of his minor works have in their original as well as in translation. Whereas survived, so that our knowledge of his great accomplish- there is at present a whole industry concerned with the ments comes from Ptolemy’s account, another three advances during the 16th and 17th centuries, from which centuries later, i.e., from the second century A.D. we have an abundance of sources quite easily accessible,

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 598 Martin C. Gutzwiller: The oldest three-body problem very few general titles covering Greek and medieval as- 2␲ in one sidereal month and the cover- tronomy have been added in this century. Here is an ing 2␲ in one anomalistic month, from one apogee to the incomplete, but representative, list: Berry (1898), next. The difference ␭¯Ϫl covers 2␲ in about nine years. (1906), Heath (1913), Dicks (1970), Pedersen The first term describes a uniform motion on a large and Pihl (1974), Neugebauer (1975). circle around the origin, the deferent, while the second term describes the motion around a small circle that rides on the first one, the epicycle. Notice that this mo- C. The eccentric motion of the Sun tion reduces to the eccentric motion of the preceding The Greek astronomers invented different geometric section when the two angles increase with time at the constructions to represent the main results of their ob- same rate. The second term on the right then reduces to servations. We shall look at four examples of the most a translation by the constant ␧a. important models: (i) the eccentric circle for the Sun The epicycle construction can also be applied to the around the Earth in this section, (ii) the epicycle for the motion of the Sun around the Earth. The two angles Moon around the Earth in the next section, (iii) the may not move at the same speed for two reasons: (i) if equant for the outer planets in Sec. IV.E, and (iv) the the solar orbit remains stable with respect to the fixed evection for the three bodies, Moon, Earth, and Sun in stars, but the coordinates are fixed to the vernal equinox Sec. V.D. The motion always takes place in the ecliptic, Q, the Sun then seems to move an additional 50Љ each i.e., in a fixed plane, and one special feature in the ob- year before it gets to the aphelion; (ii) the aphelion ac- served motion is accounted for. The purpose of this ex- tually moves forward very slowly with respect to the ercise is to demonstrate the increasing of fixed stars; this motion of 12Љ per year was discovered by these motions and, with the benefit of hindsight, to the Islamic astronomers in the early . The watch the struggle with a principle of modern mechan- aphelion then moves away from the equinox at 62Љ per ics, the conservation of . year, and it passed the already in A.D. First, we deal with the slow apparent motion of the 1250 (see Sec. II.D). Sun in spring and summer, in contrast to its fast motion in fall and winter. The Sun moves with uniform speed on E. The equant model for the outer planets a circle of radius aЈ whose center is at some distance ␧ЈaЈ from the Earth. The two parameters in this model Both the eccenter and the epicycle motions explain are ␧Ј and the direction of the aphelion (largest distance the motion in longitude, but they suffer from a basic flaw from Earth), which lies in the direction of the center. that was already noticed at the end of Sec. IV.C: the They were determined by Hipparchus on the basis of angular speed and the distance from the Earth vary by two time intervals, from the vernal equinox to the sum- the same amount. Ptolemy became aware of this diffi- mer solstice and from the summer solstice to the autum- culty when he tried the eccenter model for the outer nal equinox. Hipparchus found 1/24 and 65°14Ј, in fair planets, and his remedy turns out to be one of the most agreement with modern values. ingenious contributions to astronomy. If the eccenter model for the Sun’s motion is taken at As the outer planets are watched from the Earth, they face value, the eccentricity ␧Јϭ1/24 would indicate that move in the forward (Eastern) direction along the eclip- the Sun’s distance from the Earth varies by this amount. tic most of the time, but at regular intervals they reverse The total intensity of the Sun’s light would vary by their course and go in the Western direction for awhile. Ϯ2␧ЈϭϮ1/12 These conclusions from the eccenter The midpoint of this reversal is called the opposition, model for the Sun were not appreciated until Kepler when the outer planet culminates exactly at midnight, examined the physical consequences of the antique i.e., the planet, the Earth, and the Sun lie on one straight models. line, with the Earth in the middle; the planet is at its brightest because nearest to the Earth. The motion of the outer planet around the Sun can be inferred from D. The epicycle model of the Moon the consecutive oppositions, while the varying amplitude of the reversal motion measures directly the relative size This construction is best described if Cartesian coor- of the Earth’s orbit compared with the distance of the dinates (x,y) are used in the ecliptic, and they are com- outer planet. bined into one complex number u, Ptolemy noticed that the eccenter model for the outer ¯ ¯ planet, after its eccentricity ␧ had been adjusted to give uϭxϩiyϭaei␭͑1ϩ␧eϪil ͒ϭaei␭ϩ␧aei͑␭Ϫl ͒, (6) p the correct time intervals between the oppositions, where we have now two angles, the ␭¯ yields twice the observed variation of the reversal mo- and the mean anomaly l . Here the word anomaly refers tion. Without being aware of the physics involved, he to the angle from the apogee to the Moon as seen from was effectively trying to conserve the angular momen- the Earth, whereas the longitude is reckoned from the tum of the outer planet around the Sun. Here is his in- reference point Q; again the adjective mean designates genious solution of this puzzle. these angles after the elimination of the periodic terms In the heliocentric picture the outer planet moves on (see Sec. II.D). Each ‘‘mean’’ angle increases linearly an eccentric circle, whose center O is shifted away from with time at its own rate, the mean longitude covering the Sun in S by ␧paЈ, but the planet does not run around

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 599

O at uniform speed. Rather, an equant point SЈ is con- G. The elliptic orbit of Mars structed at the distance 2␧paЈ from S, in the same direc- tion as the center O. The planet is made to move at Kepler now faces the main part of his epic struggle, constant angular speed around this equant point SЈ, namely, to find the exact orbit of Mars around the Sun. rather than around the center O. The outer planet varies The accurate knowledge of the Earth’s orbit around the its speed on the eccentric circle by twice as much as the Sun and Brahe’s observations over 20 years show that variation of its distance from the Sun. the orbit of Mars is not an eccentric circle. The New Astronomy gives a detailed report of all the detours and lucky incidents that finally led to the idea of the ellipse with the Sun in one of the foci, and the times propor- tional to the area. F. The Earth’s orbit and Kepler’s second law The role of the mean anomaly is taken over by the area that is swept out by the radius from the Sun to More than a thousand years were to pass before Mars, counted from the perihelion. If this area is nor- Ptolemy’s clever constructions were seriously ques- malized to 2␲ for one complete orbit, it is represented tioned. The reader is encouraged to consult the collec- by the angle l Ј, whose value increases linearly with tion of articles by Owen Gingerich (1993) for a more time. The true anomally fЈ and the vЈ detailed discussion of the development that led from keep their original meaning as the angles seen from the Ptolemy to Copernicus and finally to Kepler. Here is a Sun and from the center of the orbit, all of them mea- very brief account. sured from the perihelion. When Tycho Brahe died in 1600 he left his successor a The mathematical relations between these angles and treasure trove of the most meticulous observations with with the distance from the Sun turn out to be elemen- the best instruments that he was able to build. Kepler tary, but still quite tricky. Leaving out the primes on the studied the orbit of Mars because it has the largest ec- following formulas, one gets for the distance centricity among the classical planets (excepting Mer- cury, which is difficult to observe) and could be expected a͑1Ϫ␧2͒ rϭ , (7) to yield the most telling clues for the renewal of as- 1ϩ␧ cos f tronomy. But before starting this challenging task, he had to know exactly the orbit of the Earth around the where a is the semi-major axis and ␧ the eccentricity. In Sun because, after all, that was the base for Brahe’s Cartesian coordinates with the x axis in the direction of data. Almost one-fourth of Kepler’s New Astronomy, the perihelion, the ellipse is given by the equations published in 1609, is devoted to this preliminary . xϭr cos fϭa͑cos vϪ␧͒, Max Caspar (1929), the editor of Kepler’s Collected Works, has published a beautiful translation of the New yϭr sin fϭaͱ1Ϫ␧2 sin v. (8) Astronomy into German. Aside from a detailed para- The connection with the mean anomaly l is given by phrase by Small (1963), English speakers had to wait Kepler’s equation, until 1992 for a translation by Donahue (1992). Kepler triangulates the Earth’s orbit with the help of a vϪ␧ sin vϭl ϭn͑tϪt0͒, (9) fixed base, for which he uses the position of Mars at where t0 is the time of perihelion passage, and the mean regular intervals of 687 days, the period of Mars in its motion n is the mean angular speed, 2␲ divided by the orbit around the Sun S. He is tremendously pleased with period T0 . his discovery that the equant model is also applicable to the Earth E. First he argues that the Earth’s speed near the perihelion P and the aphelion A is inversely propor- H. Expansions in powers of the eccentricity tional to the distance rЈ from the Sun. Since he is con- vinced that the Earth’s motion is determined by the Sun, The eccentric anomaly v is of no interest, but it can- he then generalizes this idea: if the Earth’s orbit, say the not be avoided when the radius r and the eccentric circle, is broken up into short intervals, the f are directly expressed in terms of the mean anomaly l , total time to get from A to E would be proportional to i.e., the time t and the eccentricity ␧. The relevant ex- the sum over all these short intervals where each is mul- pressions can only be given as expansions in l tiplied with its distance from S. whose coefficients are power expansions in ␧. These ex- Although Kepler takes the trouble to show that this pansions are not hard to get; but when the way of calculating the earth’s motion differs only insig- Friedrich tried to compute them to high nificantly from the equant construction, he finds this sum order, he found it necessary to invent the functions that very cumbersome to compute. He now searches for an now carry his name! Notice that the mean anomaly l is easier way of relating his construction by small intervals counted from perihelion, a convention that we shall to the real Sun in S. He hits upon his second law; the keep from now on. time for the Earth to get from A to E is proportional to Only the terms to order ␧2 will be listed: the area that is swept out by the vector from S to E. He shows that this third way of computing is consistent with r 1 1 ϭ1ϩ ␧2Ϫ␧ cos l Ϫ ␧2 cos 2l ϩ , (10) the two other models for the Earth. a 2 2 ¯

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5 fϭl ϩ2␧ sin l ϩ ␧2 sin 2l ϩ . (11) 4 ¯ These formulas will eventually reappear when the mo- tion of the Earth around the Sun is taken into account explicitly to get the corrections for the motion of the Moon around the Earth. Finally, we list two more expressions for the same ex- pansions in order to make the connection with the epi- cycle theory, 1 r 1 3 1 FIG. 1. The basic lunar model from antiquity (adopted ever ͑xϩiy͒ϭ ejfϭeil 1Ϫ ␧2Ϫ ␧eϪil ϩ ␧eil since) consists of an orbital plane for the Moon containing an a a ͫ 2 2 2 epicycle for its orbit; the crucial parameters a,␧,␥ and the 1 3 three angles l,g,h have their modern interpretation. ϩ ␧2eϪ2il ϩ ␧2e2il ϩ . (12) 8 8 ¯ͬ The simple epicycle idea has become a Fourier expan- linear (increasing for f and g, while decreasing for h) sion. Equivalently, plus various periodic terms that average to 0. 1 r 1 ͑xϩiy͒ϭ eifϭeil 1Ϫ ␧2Ϫ␧ cos l ϩ2i␧ sin l a a ͫ 2 1 i ϩ ␧2 cos 2l ϩ ␧2 sin 2l ϩ , (13) B. The osculating elements 2 4 ¯ͬ Assuming that the Moon’s position *x and its momen- where the real part indicates the correction in the dis- * tance r, while the imaginary part describes the correc- tum p with respect to the Earth are known at some tion in the true anomaly f. In lowest order, the latter is time t, its total energy (kinetic plus potential with re- obviously twice as large as the former. The factor 2, be- spect to the Earth) gives the semi-major axis a; its an- * tween Ϫ␧ cosl and 2i␧ sin l , is designed to preserve gular momentum L yields not only the inclination ␥ and the angular momentum and was correctly given in Ptole- orientation h of its orbital plane, but also its eccentricity my’s equant model. ␧; finally, its so-called -Lenz vector, * * F ϭ *p , L ϩG EM2*x /r, (14) V. THE MANY MOTIONS OF THE MOON ͓ ͔ 0 gives the location of the perigee, i.e., the angle g with A. The traditional model of the Moon respect to the node, and from there the true anomaly f. The masses of the Earth and of the Moon are called E A plane through the center of the Earth is determined and M, while G is the gravitational constant. at an inclination ␥ of about 5 degrees with respect to the 0 These elements a, ␧, ␥, h, g, f for the Moon give the ecliptic. The Moon moves around the Earth in that plane on an ellipse with fixed semi-major axis a and ec- parameters of the Kepler ellipse that fits the lunar tra- centricity ␧ of about 1/18. The Greek model was quite jectory most closely at the time t; they have complicated similar, except that the ellipse was replaced by an eccen- variations with the time t. The linearly increasing parts tric circle. in the angles get special names and symbols; they are The plane itself rotates once every 18 years in the used as the basis for all the future computations. The backward direction, i.e., against the prevailing motion in mean anomaly l is the linearly varying part of the true the solar system, while keeping its inclination constant. anomaly f; the mean argument of the latitude F is the The perigee of the Moon, its point of closest approach to linear part of the distance fϩg from the node; the mean the Earth, makes a complete turn in the forward direc- longitude ␭ is the linear part of the distance fϩgϩh tion in about nine years. from the reference Q. The following picture (see Fig. 1) emerges: first we fix Whereas a single variable, the mean longitude, is suf- the direction of the spring equinox or some fixed star ficient for describing the complete motion of a planet near it as the universal reference Q in the ecliptic: around the Sun, three angles are required for the Moon: counting always from west to east, we determine the the mean longitude ␭ for the main motion around the angle h from Q to the ascending node, i.e., the line of Earth, the mean argument of latitude F for the motion intersection for the Moon’s orbit with the ecliptic where out of the ecliptic, and the mean anomaly l for the ra- the Moon enters the upper side of the ecliptic; from dial motion. The osculating elements a, ␥, and ␧ have there we move by an angle g in the Moon’s orbital plane only periodic variations. But, according to Fig. 2, it is until we meet the perigee of the Moon; and finally we quite misleading to think of the lunar orbit as having a get to the Moon by moving through the true anomaly f. fixed eccentricity like ␧Х1/18, because the value of the All these three angles have a double time dependence: osculating eccentricity varies enormously and quite fast.

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Moon and aЈ for the Sun can be regarded as defined in terms of the mean motions n and nЈ by Eqs. (15), or the other way around.

D. The evection—Greek science versus

The Babylonians knew that the full moons could be as much as 10 hours early or 10 hours late; this is due to the eccentricity ␧ of the Moon’s orbit. But the Greeks wanted to know whether the Moon displays the same kind of speedups and delays in the half moons, either FIG. 2. The effective eccentricity of the lunar trajectory as a waxing or waning. The answer is found with the help of function of time; the abscissa gives the time in synodic months, a simple instrument that measures the angle between starting with the year 1980; the traditional picture of the the Moon and the Sun as seen from the Earth. Moon’s motion is obviously not adequate (see Gutzwiller, The half moons can be as much as 15 hours early or 1990, page 60). late. With the Moon moving at an average speed of slightly more than 30Ј per hour (its own apparent diam- eter!), it may be as much as 5° ahead or behind in the C. The lunar periods and Kepler’s third law new/full moons; but in the half moons, it may be as much as 7°30Ј ahead or behind its average motion. This To each rate of change for an angle with linear time new feature is known as evection. dependence corresponds a period that was well known Ptolemy found a mechanical analog for this peculiar to the Babylonians and Greeks: complication, called the crank model. It describes the T1ϭtropical month (equinox to equinox)ϭ27.32158 angular coupling between Sun and Moon correctly, but days, it has the absurd consequence of causing the distance of T2ϭanomalistic month (perigee to perigee)ϭ27.55455 the Moon from the Earth to vary by almost a factor of 2. days, In the thirteenth century Hulagu Khan, a grandson of T3ϭdraconitic month (node to node)ϭ27.21222 days, Genghis Khan, asked his vizier, the Persian all-round measured in mean solar days. The fifth decimal corre- genius Nasir ed-din al Tusi, to build a magnificent obser- sponds to 1 second of time and was correctly known to vatory in Meragha, Persia, and write up what was known the Greeks. The mean longitude ␭ increases by 2␲ in the in astronomy at that time. Ptolemy’s explanation of the period T1 , the mean anomaly l increases by 2␲ in the evection was revised in the process. In the fourteenth period T2 , and the argument of latitude F in the period century Levi ben Gerson of Avignon in southern France T3 . seems to have been the first astronomer to measure the By combining the tropical month with the period for apparent of the Moon (see Goldstein, 1972, the Sun’s returning to the spring equinox, 1997). Shortly thereafter Ibn al-Shatir of Damascus in Syria proposed a model for the Moon’s motion that co- T0ϭtropical yearϭ365.2422 days, incides with the theory of Copernicus two centuries we obtain the second most familiar period in this system, later. The crank model was replaced by two additional the average time between new moons, which turns out epicycles, yielding a more elaborate Fourier expansion to be TϭT T /(T ϪT )ϭ29.53059 days. The mean in our modern terminology (see Swerdlow and Neuge- 0 1 0 1 bauer, 1984). Dϭ␭Ϫ␭Ј increases by 2␲ in one synodic With the improvements of the Persian, Jewish, and month. Arab astronomers, as well as Copernicus, the changes in The Greeks related all events in the sky to the spring the Moon’s apparent diameter are still too large with equinox, but an inertial system of reference is preferable Ϯ10%. As in Kepler’s second law, the Fourier expan- when doing physics. The return of the Moon and the sion (12) has to include epicycles both in the backward Sun to the same fixed star defines the sidereal month and in the forward direction, in the ratio 3:1. Thus one T1ϭ27.32166 days and the T0ϭ365.257 eventually finds for uϭxϩiy the expansion days. n 3 1 1 Let be the rate of increase of the Moon’s mean aeiD 1Ϫ ␧eϪil ϩ ␧eϩil ϩ ␦eϩ2iDϪil longitude ␭, and nЈ the rate of increase of the Sun’s ͩ 2 2 2 mean longitude ␭Ј. Then Kepler’s third laws are, in the 3 complete form that was first given by Newton, Ϫ ␦eϪ2iDϩil 2 ͪ 2 3 2 3 n a ϭG0͑EϩM͒, nЈ aЈ ϭG0͑SϩEϩM͒. (15) iD ϭae „1Ϫ␧ cos l Ϫ␦ cos͑2DϪl ͒ Only the products G0M, G0E, and G0S appear in the following discussion. The semi-major axes a for the ϩ2i␧ sin l ϩ2i␦ sin͑2DϪl ͒…. (16)

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The second line shows the maximum deviation from the vertical around the takes about 18 years. Brahe the uniform angular motion to vary between Ϯ2(␧Ϫ␦) recognized that this motion is not quite uniform, but acts in the full/new moons and Ϯ2(␧ϩ␦) in the half moons. like a deferent with a small epicycle of radius 9Ј that Therefore we finally get ␧ϭ.055 and ␦ϭ.011. The dis- turns twice every synodic month. It is as if the Moon’s tance of the Moon varies at most by Ϯ6.6%, and the orbital plane straightens up a bit when facing the Sun, apparent diameter of the Moon varies between 28Ј and which is reasonable on physical grounds. 32Ј. The net effect on the lunar latitude is similar to the evection: the main term, ␥ sin (F) (see Sec. V.B), has to E. The variation be corrected by a term proportional to sin (FϪ2D) (see Sec. V.D) with an amplitude of Ϫ9Ј. It looks like the At the end of the 16th century, Tycho Brahe mea- axis of a gyroscope oscillating while going around its sured continuously for over twenty years that fixed cone. It is accompanied by a periodic shift of the happened in the sky. He was lucky because he witnessed line of nodes that amounts to 96Ј, a phenomenon that is in 1572 the last big in our (the super- usually called a . nova of 1604 seen by Kepler was much smaller), and we By the middle of the 17th century, the astronomy of have been waiting for any supernova in our galaxy ever the solar system had reached a point where any further since. He also saw a large in 1577 and was able to along the same lines could only confuse the show that it was outside the sphere of the Moon. He did new picture of the universe that Copernicus, Brahe, and all that with his bare eyes and those of his assistants, Kepler had created. With the help of the telescope there after he had built the largest and best instruments ever, was no lack of serious effort; observational methods im- and reached a precision of 1 minute of arc. He published proved rapidly and led to an accumulation of new data a detailed description of his magnificent observatory on that needed more than just an increasing number of em- the Danish Hveen (see the English translation of pirical parameters. A first systematic survey of the astro- Raeder, Stroemgren, and Stroemgren, 1946). nomical observations during the 17th century was made Brahe’s lunar theory is contained in Part 1 of his Pre- by Pingre´, but was published only in 1901. Meanwhile paratory School for the New Astronomy, which was ed- Newcomb (1878) had collected all the relevant lunar ited by Kepler and published posthumously in 1602 data to 1900. Gingerich and Welther (1983) have com- (Volume 2 of the Complete Works). Dreyer’s biography pared astronomical tables from the 17th century with (1890) contains a short discussion of Brahe’s scientific modern computations. The situation in lunar theory work, whereas Gade’s (1947) is concerned with his tur- then had some similarities with our present conditions in bulent life; Thoren provides a special chapter on the nuclear and high-energy physics, where we seem to be theory of the Moon in The Lord of Uraniborg (Thoren, drowning in a flood of first-class data without a simple 1990). and efficient theory that can be generally understood Four new ‘‘inequalities,’’ i.e., periodic deviations from while giving good quantitative results. Of course, the the uniform motion of the Moon around the Earth, were man to change all that for the Moon’s motion was Isaac discovered by Brahe. Kepler tried to give all these mo- Newton. tions a physical interpretation, on the strength of his boundless imagination and without trying to figure out VI. NEWTON’S WORK IN LUNAR THEORY quantitatively how large they are. The most interesting of Brahe’s lunar discoveries is A. Short biography the variation, a not particularly informative name that has caused some confusion. It is the third largest correc- By the middle of the 17th century the passion for sci- tion to the longitude of the Moon: the anomaly causes ence had grown to such an extent that it became an deviations from the mean longitude up to 6°15Ј, the official function of the state. In 1666, under the leader- evection adds another 1°15Ј, while the variation ac- ship of the Louis XIV and his prime minister Col- counts for a further 40Ј. It depends on twice the mean bert, the French government organized its Royal Acad- elongation Dϭ␭Ϫ␭Ј, i.e., twice the mean angular dis- emy, where Christiaan became the best paid tance of the Moon from the Sun. The variation plays a member at the age of 37. In his efforts to build more crucial role in Hill’s theory of the Moon. reliable clocks, he discovered and then published in 1673 the law of : the is pro- F. Three more inequalities of Tycho Brahe portional to the mass and the square of the velocity, and inverse to the radius. He also visited several Brahe also found the ‘‘annual, inequality’’ with an am- times and participated in experiments to establish the plitude of 11Ј, which slows down the Moon in its motion laws of motion for bodies impacting on one another (see around the Earth when the Earth is near perihelion and , 1947). speeds it up near aphelion. Meanwhile, an obscure fellow at the University of Finally, Brahe found that the Moon’s orbital plane Cambridge had figured out the same laws of mechanics can be best understood by its vertical direction’s describ- on the basis of some exceedingly clever arguments such ing a small cone around the vertical to the ecliptic. The as bouncing a body inside a square box to get the cen- opening angle of this cone is 5°11Ј, and the motion of trifugal force. Isaac Newton, born on Christmas Day of

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1642 (the year Galileo died), had done entirely novel 1726, ‘‘the argument to prove that the moon is retained in work in mathematics, mechanics, and optics while a stu- her orbit by the force of is more fully stated.’’ It dent and then a fellow at Cambridge, but hardly any- sounds almost like ‘‘back to the drawing board.’’ body knew about it. In 1669, he was lucky to become the In the years after the completion of the Principia in Lucasian professor of mathematics upon the resignation 1687, Newton did some work in mathematics and optics; of the more ambitious Isaac . Newton acquired he also went back to alchemy, but he hardly touched the fame, and membership in the recently founded Royal topics in the Principia any more. He tried to get better Society of , with his work in optics and the con- observations of the Moon from the , struction of his reflecting telescope in 1671. But the pub- John , who was more interested in establish- licity and the scientific arguments were too much for ing his great star catalog, and the two parted ways after him, and he almost completely retired from contact with quarreling. any colleagues for over a decade, while he devoted his Newton left the pursuit of science altogether after his time mostly to theology and alchemy. breakdown in 1694. He became Warden and then Mas- His scientific talents were finally mobilized again ter of the Mint, a job which he pursued with great en- when the new secretary of the Royal Society, the as- ergy and in which he essentially controlled the circula- tronomer , went to see Newton in Au- tion of money in the . He became gust 1684 to get the answer to an important problem: If wealthy, but his scientific life was limited to presiding Huygens’ formula for the centrifugal force is combined over the Royal Society with an hand until his death with Kepler’s third law, the force that attracts the plan- in 1728. His pivotal role in the development of math- ets to the Sun or the to Jupiter is found ematics, astronomy, and physics is all concentrated in to vary inversely with the square of the distance. Halley relatively few years of his long life. had come to this conclusion in conversations with the Many biographies of Newton have been written, espe- physicist and the -architect cially in the last decades, for example, those of, Manuel at a meeting of the Royal Society in (1968), Westfall (1980), Christianson (1984), Gjertsen January 1684. But none of them had been able to show (1986), and (1997). There are always some new that Kepler’s first and second laws could be derived di- documents to be discussed and circumstances to be rectly from the assumption of this inverse-square-of-the- noted that were not appreciated in earlier reports. One distance force. Newton claimed that he had derived Ke- of the first biographies was written by Sir David Brew- pler’s laws in this way some time ago, but he could not ster of optics fame; it has the great virtue of pursuing find the relevant papers. Newton’s achievements and their further developments In November 1684 Halley received a manuscript from all the way to 1855 when it was published. Although Newton, ‘‘On the Motion of Bodies in an Orbit,’’ that many documents concerning Newton’s life and work contains all we know now about the motion on conic have been discussed at great length ever since, he re- sections, including even a discussion of the effect of a mains a lonely and mysterious figure with achievements resisting medium (see , 1983; Hall and Hall, 1962; to his credit that have no equal in the . Herivel, 1965; Mathematical Papers, 1967–1981; Cohen, 1978). Two years later, the printing of The Mathematical B. Philosophiae Naturalis Principia Mathematica Principles of Natural Philosophy (usually referred to as the Principia from the Latin title) got started under Hal- Newton’s monumental work is hard to understand ley’s watchful eye, and on July 5, 1687 the task was com- even in a modern translation. His great predecessors, pleted (Newton, 1687). This monumental work has 510 Galileo, Kepler, and Huygens, are generally easier to pages of tightly argued mathematical physics, all of them digest, perhaps because their achievements are simpler, conceived and written in two and a half years. The two- but also because they tried to explain themselves to page preface gives Halley some credit for his ‘‘encour- lesser mortals. Whereas the reader of a modern mono- agement and entreaties’’ to publish and offers the fol- graph is overwhelmed with intricate technical details, lowing remarks on Newton’s work concerning the Newton presents a sequence of statements about the Moon: geometric relationships in diagrams of deceptive sim- ‘‘But after I had begun to consider the inequalities of plicity. the lunar motions,..., I deferred that publication till I After two short introductory chapters entitled ‘‘Defi- had made a search into those , and could put forth nitions’’ and ‘‘Axioms or Laws of Motion,’’ Newton the whole together. What relates to the lunar motions (be- moves immediately to the core of the . Proposi- ing imperfect), I have put all together in the corollaries of tion I is the statement of Kepler’s second law for the Proposition LXVI,... .Some things, found out after the general case of a centripetal force, i.e., an attractive rest, I chose to insert in places less suitable, rather than force that depends only on the distance from the center. change the number of propositions and the citations.’’ The sixth corollary of proposition IV gives credit to Hal- Among the few subjects that Newton mentions in the ley, Hooke, and Wren, and various problems are solved preface to the second edition in 1713, he says that ‘‘the in the same section. Then comes the important proposi- lunar theory and the precession of the equinoxes were tion XI, in which the central force in a Kepler ellipse is more fully deduced from their principles.’’ And in the shown to vary as the inverse square of the distance. The equally short preface to the third and last edition of same thing is shown for hyperbolas and parabolas, and

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 604 Martin C. Gutzwiller: The oldest three-body problem proposition XV then proves Kepler’s third law, on page ideas on this subject to see how far he was able to get, 56 of a volume with 510 pages. although he abandoned his search while still far from his The remainder of Book I covers many problems that own goals. are connected with orbits whose shape is a conic section, e.g., finding the Kepler ellipse from a certain set of ini- C. The rotating Kepler ellipse tial conditions. The first statements are made that are equivalent to the conservation of energy. Section 9 will Lunar theory makes its first surreptitious appearance be discussed in detail below because it deals with trajec- in propositions 43 to 45, where Newton discusses the tories that result from rotating a stationary orbit. Section possibility of a Kepler ellipse rotating at some uniform 11 treats two-body problems without external forces and rate as it occurs for the Moon. He assumes a purely derives Kepler’s third law in its complete form [Eq. (15)] centripetal force, so that Kepler’s second law is still which is required for double stars. This section ends with valid. The apsides (places of largest and smallest separa- proposition LXVI, which is central for lunar theory. Sec- tion) rotate because the force is no longer assumed to be tion 12 covers the potential theory for bodies of spheri- inversely proportional to the square of the distance r. cal shape, and section 13 does it for bodies of arbitrary, The resulting trajectory looks like the petals of a flower. in particular ellipsoidal, shapes. The comparison with as- Let us set up the problem in the way in which it would tronomical observations is left to Book III (the last) en- appear in a modern textbook. A body of known mass titled ‘‘The System of the World,’’ where many of the m0 is moving around a fixed center, which is located at earlier results are clarified. the origin of a polar coordinate system (r,␾). The force There exists a small volume entitled A New and most is a known function F(r)ϭϪdV/dr in terms of the - V r Accurate Theory of the Moon’s Motion ‘‘whereby all her tential ( ). Newton introduces the idea of a conserved angular momentum L without defining any such quan- Irregularities may be solved, and her Place truly calcu- tity. The equation for the radial motion becomes lated to Two Minutes. Written by that Incomparable Mr. Isaac Newton, and published in d2r L2 m0 2 ϭF͑r͒ϩ 3 . (17) Latin by Mr. David in his Excellent As- dt m0r tronomy. London, Printed and sold by A. Baldwin in Newton’s proposition 44 says essentially the same in Warwick-. 1702.’’ It is more a description than an words. explanation such as Newton attempted in the Principia. The eccentricity ␧ is assumed to be small, so that the It contains, however, a very eloquent statement of New- orbit is very close to a circle, and the distance between ton’s attitude toward lunar theory. The preface ‘‘To the the two bodies varies only over a narrow range (r1 ,r2). reader’’ starts out: ‘‘The Irregularity of the Moon’s Mo- Newton now comes up with an ingenious trick that re- tion hath been all along the just Complaint of Astrono- veals his deep understanding of physics, although the mers; and indeed I have always look’d upon it as a great reader has to figure out in her own terms what exactly Misfortune that a Planet so near us as the Moon is,... goes on in Newton’s mind: Since the 1/r2 force works should have her Orbit so unaccountably various, that it is out so well, whatever the centrifugal force that goes as in a manner vain to depend on any calculation..., 1/r3, why not make an expansion for the arbitrary force though never so accurately made.’’ Notice the marvelous F(r) where only these two terms occur? definition of what we would call nowadays! See Newton treats as his second example the case in which Cohen (1975), as well as Waff (1976) and (1977). F(r)ϭϪconst r␯/r3, where the exponent ␯Ͼ0, and ␯ϭ1 A number of major as well as minor scientists have for the gravitational force. The change ⌿ in the angle ␾ taken the trouble to present Newton’s arguments for from one closest approach to the next becomes 2␲/ͱ␯, students in the sciences and for the educated layperson. which is the main result of Newton’s section 9 after an Outstanding among these authors are the astronomers 11-page argument. Sir George in 1834 and Sir John Herschel (son of William, the discoverer of ) in 1849, as well as D. The advance of the lunar apsides Henry Lord Brougham (formerly Lord Chancellor) and E. J. Routh, who wrote a very useful Analytical View of Without explaining what he is doing, Newton pro- Sir Isaac Newton’s Principia in 1855. Two recent books poses as the third example of the advancing apsides the make a valiant effort to introduce the reader to the un- case of a small perturbative force that is repulsive and familiar methods of the Principia: Brackenridge (1995) varies linearly with the distance, so that and Densmore (1995) cover only the proofs of Kepler’s G M m 0 0 0 2 laws whereas Chandrasekhar (1995) discusses both F͑r͒ϭϪ 2 ϩm0␻ r, (18) Books I and III; see the essay review by Westfall in Isis r (1996). where the circular frequency ␻ is our parameter to keep Lunar theory was the first instance in all the sciences the correct dimensions of a force. Its value remains open where sheer intuition was no longer sufficient to keep up at this point. with the rapidly increasing accuracy of the observations. The calculation follows the same pattern as in the pre- Nevertheless, we shall track down a few of Newton’s ceding section and yields

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FIG. 4. Modern version of Fig. 3 containing the ⌫ of the Earth-Moon that is shown to circle the Sun almost exactly on a Kepler ellipse. The vectors are x៝ from * the Earth in E to the Moon in M, X from the Sun in S to ⌫, * x៝ Ј from S to E, and ␰ from S to M.

corresponding Fig. 4 will be used as the modern equiva- lent, where the nomenclature agrees with the later chap- ters. The main proposition as applied to the lunar problem claims the following: The motion of the Moon around the Earth under the gravitational attraction of both the Earth FIG. 3. Newton’s Proposition LXVI from the first book of the and the Sun comes closer to the ideal Kepler motion, if Principia explains the essence of his lunar theory (see the quo- the Earth in turn is allowed to move under the gravita- tation in Sec. VI.A); its 22 corollaries cover 15 pages, and con- tional attraction of both the Sun and the Moon rather tain a single figure, which is repeated on every second page in than being forced to remain at a fixed position. the third edition. The Sun’s attraction to the Moon is repre- In Fig. 3 we have the Earth with mass E in T (tellus), sented by the line SL and is decomposed into the sum SM the Sun with mass S in S (sol), and the Moon with mass ϩML for the detailed discussion. M in P (planeta). In Fig. 4 the Moon’s and Earth’s loca- tions are designated by M and E, while ⌫ is the center of mass for the double -Moon. The vectors 3␻2 connecting various points in Fig. 4 are x៝ r ⌿Х2␲ 1ϩ 2 , (19) of length ͩ 2n ͪ pointing from E to M,x៝ Ј of length rЈ pointing from S to provided the second term is small. In the case of the E, ␰ of length ␳ pointing from S to M, and X៝ of length Moon, the angular speed is given by nϭ2␲/T1 , where R pointing from S to ⌫. T1 is the sidereal month. The of Newton’s argument lies in representing Without any explanation, Newton now proposes for the ‘‘accelerative force’’ of the Sun on the Moon by the the ratio ␻2/n2 the value 100/35745 and concludes that segment SL in Fig. 3. This segment is then decomposed the perigee advances by 1°31Ј28Љ. Only the third edition into the sum of the two segments SM and ML, where of the Principia, almost 40 years later, ends this whole ML is parallel to the line TP which connects the Earth section with the remark: ‘‘The apse of the Moon with the Moon. Referring to Fig. 4, we get the first com- is about twice as .’’ Nevertheless, after the data had ponent of the Moon’s perturbation by the Sun, been known with high accuracy for over 2000 years, 1 1 x៝Ј ͑x៝Ј,x៝͒ Newton was able to reduce the difference between the G0Sx៝Ј 3Ϫ 3 Х3G0S 4 , (20) anomalistic and the sidereal month to the ratio ͩ␳ rЈ ͪ rЈ rЈ 2 (T1 /T0) . The missing factor 2 in the motion of the which is clearly of the same order as the second compo- Moon’s perigee was finally explained 20 years after his nent, death. x៝ x៝ ϪG S ХϪG S . (21) 0 ␳3 0 rЈ3 E. Proposition LXVI and its 22 corollaries Newton now studies the effect of these perturbative accelerations on the around the Modern language and contemporary symbols like vec- Earth. According to Corollary 2, the Moon’s angular tors will be used in this section to explain the basic ideas momentum with respect to the Earth is larger in the in proposition LXVI. Nevertheless, it is a sacred duty to syzygies (new and full moon) than in the quadratures emphasize what we owe to Newton’s crucial insights, (half moons). Moreover, according to Corollary 3, the without which there would be no progress in under- Moon’s speed is larger in the syzygies than in the standing the motion of three masses that interact quadratures. Corollary 4 points out, however, that the through gravitational forces. All his reasoning over 15 lunar orbit is more highly curved in the quadratures than pages is based on a single diagram (Fig. 3), which is in the syzygies because the solar perturbation decreases repeated seven times, on every second page, to spare the the Earth’s attraction in the syzygies, while its radial reader the tedium of turning pages while thinking. The component helps in the quadratures. Corollary 5 ex-

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 606 Martin C. Gutzwiller: The oldest three-body problem plains that the Moon is further from the Earth in the noxes. If one adds up the number of pages in the Prin- quadratures than in the syzygies. But we still have to cipia that are devoted to lunar problems, their total account for the motion of the apsides. probably exceeds the space that is devoted to all the two-body problems, including the discussion of the plan- etary motions. F. The motion of the perigee and the node Book III, ‘‘The System of the World,’’ talks about the From Corollary 7 onward, Newton gets to the main Moon more directly rather than citing it only as an ex- job of estimating the strength of the solar perturbation. ample of some general proposition. The rotation of the Since rЈХaЈ, i.e., the semi-major axis of the Earth’s or- Earth is discussed, and its flattening at the poles is sug- bit around the Sun, one can simplify Eqs. (20) and (21) gested. The only evidence for this phenomenon at the by approximating, time was the length of the second , which was observed to be shorter near the Earth’s equator. That G0S G0S leads to the precession of the equinox by 50Љ/year, of Х ϭnЈ2, (22) rЈ3 aЈ3 which the Moon accounts for 40Љ/year and the Sun only 10Љ/year. The tides are also described in detail with with the help of Kepler’s third law [Eq. (14)]. The first some qualitative explanations. component [Eq. (20)] of the solar perturbation is always Then comes the rather audacious claim concerning repulsive, and its average over all directions reduces to the motions of the Moon that ‘‘all the inequalities of ϩ n 2r (3/2) Ј , while the second component [Eq. (21)] be- those motions follow from the principles which we have Ϫn 2r comes simply Ј . If these two contributions are laid down.’’ The discussion of proposition LXVI is taken M added and multiplied with the Moon’s mass , the av- up again with the relevant numerical figures this time. eraged force of the Sun on the Moon becomes The variation of Tycho Brahe is explained under the ϩ M n 2r ( /2) Ј . assumption of a . The perturbation of the In order to apply the result from Sec. VI.D, we have 2 2 Sun produces a closed oval orbit that is centered on the to set m0ϭM and ␻ ϭnЈ /2 in Eqs. (18) and (19). Since 2 Earth with the long axis in the direction of the quadra- (nЈ/n) ϭ1/178.7 is exactly twice the ratio 100/35745 tures. This ingenious idea would have to wait almost 200 which was used by Newton, we find the explanation for years before being taken up again by G. W. Hill (1877). his cryptic statement. His result can also be written in The motion of the nodes gets many pages of geomet- the form. ric discussion, including the libration of the inclination 2␲ 3 nЈ2 3nЈ2 and of the nodes. Various annual effects are described n ϭ Хn 1Ϫ , T ХT 1ϩ . (23) 2 T ͩ 4 n2 ͪ 2 1ͩ 4n2 ͪ and explained qualitatively, and even the figure of the 2 Moon is brought up. But neither the motion of the peri- Newton expands the scope of his arguments with the gee (with the missing factor 2) nor the evection are men- help of some striking images in the later corollaries. The tioned further. In the final analysis, many qualitative ex- mass of the Moon is distributed in a rotating ring around planations concerning the three-body problem are the Earth. This ‘‘lunar ring’’ is inclined by ␥ with respect advanced, but only two numbers, the motion of the to the ecliptic and experiences the pull of the Sun in the Moon’s perigee and of her node, are obtained. Nothing syzygies. Nowadays we would conceive of the lunar orbit of substance was added to this record for another 50 as a fast gyroscope whose axis of rotation is not quite years after the first appearance of the Principia. Never- perpendicular to the ecliptic. The change in angular mo- theless, Newton had succeeded in starting a grand unifi- mentum is due to the solar and yields the regres- cation to the point where hardly anybody could doubt sion of the lunar node, that his somewhat spotty results were only the first signs 2␲ 3 nЈ2 3nЈ2 of a whole new approach to the riddles of nature. n ϭ ϭn 1ϩ , T ϭT 1Ϫ . (24) 3 T ͩ 4 n2 ͪ 3 1ͩ 4n2 ͪ 3 The 22 corollaries in Proposition LXVI do not give VII. LUNAR THEORY IN THE AGE OF ENLIGHTENMENT the impression of being organized very systematically. A. Newton on the continent They look more like an accumulation of intuitive in- sights, all based on the gravitational interaction of three Although a new age in celestial mechanics as well as bodies and sometimes argued with a great deal of imagi- in many other branches of the sciences started with nation. The motion of the lunar nodes is seen as closely Newton, it almost looks as if nobody dared to expand related to the problem of the tides, as well as the pre- upon his great achievements while he was still alive. The cession of the equinoxes. United Kingdom, in particular, needed more than a cen- tury to liberate itself from his awesome and fearful pres- G. The Moon in Newton’s system of the world (Book III) ence. On the continent, however, there was a full flow- ering of his ideas, starting in the late 1730s, fifty years Newton tested his grand theory of universal gravita- after the Principia was published. By the end of the cen- tion almost exclusively by studying the motion of the tury the marriage of physics with mathematics had been Moon, including its effect on the motion of the Earth as consummated and was producing many healthy off- manifested in the tides and the precession of the equi- spring.

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 607

The French Academy of Sciences decided to test but without explaining how he had computed them. Newton’s prediction that the Earth has a flattened shape Now he was competing for another prize from the which is shorter along its axis of rotation by a fraction of French Academy (which he won as he did 10 others), a percent. A first expedition under the leadership of this one to explain some of the irregularities in the mo- Charles-Marie de left France in the spring tions of Saturn. Jupiter and Saturn are in a 2/5 reso- of 1735 to measure an arc of 3 degrees near Quito (Ec- nance, and ’s treatment showed that he had a lot to uador), from whence it returned successfully in 1744 af- say about the Moon as well. Clairaut pointed out that, as ter incredible adventures and hardships (Condamine, a member of the Academy, he like d’Alembert is not 1751). A second, more high-powered group headed by allowed to compete for the prize. Moreau de left in the spring of 1737 for Tor- All of them wrestled with the problem of the motion nea in Lapland, in the north of Sweden, and returned in of the lunar perigee. They were unanimous in claiming the summer of 1738 after a job done too well [see Mau- that Newton’s law of universal gravitation with the pertuis, 1756a and 1756b]. Their measure of one degree inverse-square-of-the-distance dependence does not ac- in the far north yielded some 700 meters more than near count for the observed value, which is larger by a factor Paris, too much by a factor of 2 (see Svanberg, 1835). But Newton was vindicated against some earlier mea- of 2, as Newton had already found. In addition, Clairaut surements by the Cassinis, father and son, who had com- (1747) now proclaimed as a great discovery that the dis- pared the length of one degree in the north and the tance dependence of the universal gravitation had to be 4 south of France. Voltaire (1738) had published a very modified for short distances by adding a term in 1/r (see popular and competent ‘‘Elements of Newton’s Philoso- the Ph.D. thesis of Craig Waff, 1976). He was immedi- phy,’’ and now had a good time greeting the heros re- ately taken to task by Georges-Louis Leclerc, Comte de turning from Lapland: ‘‘You have flattened the Earth (1747), his famous colleague from the section of and the Cassinis’’ (Terral, 1992). He also encouraged his , who was unwilling to believe that an im- friend, Gabrielle-Emilie de Breteuil, Marquise du Chat- portant principle of physics could end up leading to a elet, to translate the Principia into French: but her work fundamental force with a complicated mathematical appeared only in 1756, seven years after her death in form. Was that the last time a representative of the life childbirth. Meanwhile, two French scientist-priests, Tho- sciences entered into a lively debate with a theoretical mas Le Seur and Francois Jacquier (1739–1742), had physicist concerning the general principles of the physi- written an extensive commentary on the Principia in cal sciences? which many of the obscure passages are more fully ex- Clairaut (1752) went back to work a little harder; he plained; they included the three memoirs on the tides by pushed his approximations to higher order in the crucial Daniel , Colin McLaurin, and Leonard Euler parameter mϭnЈ/n, and found the required correction. that had received a prize from the French Academy in He deposited the relevant paper at the French Academy 1740. in January 1749 while announcing his results without ex- planation, and submitted his work to the Russian Acad- B. The challenge to the law of universal gravitation emy in St. Petersburg for a prize in lunar theory (Clair- aut, 1752). Euler was appointed a referee and joined to Even the theory of the Moon’s motion around the his report a voluminous treatise of his own, which was Earth had its moment of drama in this atmosphere of eventually published as a separate book, usually re- scientific excitement. It involved the three most talented ferred to as Euler’s first lunar theory (1753). Meanwhile, and productive mathematical physicists of the time. In d’Alembert (1749) published a treatise about Researches 1736, while a member of the St. Petersburg Academy, on the precession of the equinoxes and the nutation of the Leonard Euler had published the first textbook on Me- Earth’s axis. Together with he edited the fa- chanics, in which a plethora of problems were solved for mous Encyclope´die, which appeared from 1751 to 1766 the first time with the help of calculus. in 17 large of text plus 11 volumes of etchings. had entered the French Academy of Science at the age He also went back to the lunar problem and gave an of 16, had participated in the expedition to Lapland with algebraic, rather than numerical, calculation for the cor- Maupertuis, and had helped the Marquise du Chatelet in rect value of the motion of the perigee. her translation of Newton. Jean Le Rond d’Alembert Reading all this work nowadays is considerably easier (1743) was the author of the first treatise on Dynamics, than dealing with Newton, but the authors were still in which Newton’s laws were established on the basis of finding their way through the new language of analysis general principles relating to the nature of space and rather than elementary geometry. They did not always time. All three of them decided, simultaneously but in- find the shortest connections, so much so that Clairaut dependently, to put lunar theory on a firmer base. ended up with Hebrew symbols because the Latin and They all submitted their different versions during the Greek alphabets were too short. Luckily, their work has summer of 1747 to the Secretary of the French Acad- been analyzed more recently; first, in the Historical Es- emy, but found out only during the winter what the oth- say on the Problem of Three Bodies published in 1817 by ers had to say. Euler (1746) had published New Astro- Alfred Gautier; second, by Felix Tisserand in his classic nomical Tables for the Motions of the Sun and the Moon four-volume Treatise on Celestial Mechanics from 1889 the year before, telling the reader only how to use them, to 1896, whose third volume is entirely devoted to the

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 608 Martin C. Gutzwiller: The oldest three-body problem

ϱ theory of the Moon’s motion; third, in An Introductory G͑EϩM͒ GS ͑Ϫ͒jEjϪ1ϩMjϪ1 rj Treatise on the Lunar Theory by Ernest W. Brown of ϩ P ͑cos ␻͒, r R ͚ EϩM jϪ1 Rj j 1896. jϭ2 ͑ ͒ (31) * where cos ␻ϭ(*x ,X)/rR. The right-hand side of Eq. (30) becomes, leaving out again the gradX , C. The equations of motion for the Moon-Earth-Sun G͑SϩEϩM͒ EM system 1ϩ R ͫ ͑EϩM͒2 The complete equations for the three-body system ϱ ͑Ϫ͒jEjϪ1ϩMjϪ1 rj Moon-Earth-Sun will now be written down in the most ϫ͚ jϪ1 j Pj͑cos ␻͒ . (32) straightforward manner. Since the inertial mass of each jϭ2 ͑EϩM͒ R ͬ of these three bodies always cancels out the gravita- The trouble in lunar theory arises already in the quad- tional mass, it is simpler to speak directly about the ac- rupole term jϭ2. celerations rather than the forces. We use the nomencla- The quadrupole term in Eq. (32) is smaller than the ture of Fig. 4, and the masses are again designated by M monopole term by a factor (1/400)2 from the relative (Moon), E (Earth), and S (Sun). The accelerations are distances r/R, and another factor 1/80 from the relative listed as follows: masses M/E; it will be completely ignored. The solution to Eq. (30) is, therefore, the simple Kepler motion for GE GS the center of mass ⌫ around the Sun. The motion of the Ϫ x៝Ϫ ␰៝ of the Moon in M, (25) r3 ␳3 vector X៝ will henceforth define the plane of the ecliptic, where it moves according to Eqs. (7)–(13). The distance GM GS r in (7) will be called R, and the remaining symbols will ϩ x៝Ϫ x៝Ј of the Earth in E, (26) r3 rЈ3 be decorated with a prime, i.e., aЈ instead of a, ␧Ј in- stead of ␧, and so on. Kepler’s third law in the form of GE GM Eq. (15) follows immediately. ϩ x៝Јϩ ␰៝ of the Sun in S. (27) The approximate size of the quadrupole term in Eq. r 3 ␳3 Ј (31) can be estimated with the help of the Kepler’s third law (15) exactly as in Eq. (22). Its quotient by the mono- The main coordinates for the three-body system are 2 2 the vector x៝ from the Earth to the Moon and the vector pole term in Eq. (31) is ϳm ϭ(nЈ/n) , as Newton knew very well. The equation of motion (29) spells out X៝ from the Sun to the center of mass ⌫. Therefore we ‘‘The Main Problem of Lunar Theory,’’ that is to find find that the motion of the Moon relative to the Earth when the M E center of mass for Earth and Moon is assumed to move x៝ ЈϭX៝ Ϫ x៝ , ␰៝ϭX៝ ϩ x៝ . (28) on a fixed Kepler ellipse around the Sun, and assuming EϩM EϩM that the masses of Moon, Earth, and Sun are concentrated in their centers of mass. It takes a little manipulation to end up with the equa- tions of motion, D. The analytical approach to lunar theory by Clairaut d2x៝ EϩM GEM GSM GSE Clairaut and many celestial mechanicians after him, 2 ϭ gradx ϩ ϩ . (29) dt EM ͩ r ␳ rЈ ͪ including Laplace, chooses the ␾ rather than the time t as the independent variable. This prefer- for the Moon with respect to the Earth, and ence is natural when there are no good clocks available. Similarly, the variable 1/r rather than the radial coordi- d2X៝ MϩEϩS GSM GSE nate r represents the lunar parallax (after multiplication ϭ gradX ϩ , (30) dt2 S͑EϩM͒ ͩ ␳ rЈ ͪ with the equatorial radius of the Earth). Clairaut then manipulates the equation for the radial motion into the for the center of mass ⌫ with respect to the Sun. The form ϭ ៝ ϭ ៝ distances ␳ ͉␰͉ and rЈ ͉xЈ͉ are to be replaced by Eqs. 2 2 (28). These equations are exact and will form the basis d s F 2 ϩsϭ⍀, with sϭ Ϫ1, (33) for all our further work. d␾ G0͑EϩM͒r ៝ The vector X is about 400 times longer than x៝ , so that where ⍀ is a somewhat messy expression for the solar it is natural to expand the denominators in Eqs. (29) and perturbation. The parameter F in the definition of s is a (30). The becomes an expansion constant of motion that comes from the integration of in polynomials Pj , where each term can be the angular motion. interpreted as arising from a multipole. The choice of The motion of the perigee is taken into account from coordinates ensures that the terms cancel out, the very start, by inserting leaving the quadrupole term as the lowest-order pertur- bation to the direct interaction. Thus the right-hand side k rϭ . (34) of Eq. (29) becomes, leaving out the gradx , 1ϩ␧ cos ␮␾

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 609

The distance k and the rate ␮Ͻ1 will somehow emerge F. Accounting for the motion of the perigee from solving Eq. (33). ⍀ is now expanded as a trigono- metric series of ␾, since the time is everywhere ex- The complete first-order expression (36) for the in- pressed as a function of ␾ with the help of a formal verse radius k/r as a function of ␾ is now inserted into solution for the equation of the angular motion. The the radial equation (33) to get the corrections of second constant F turns out to be the average angular momen- order. In trying to cancel out all the terms proportional tum of the Moon, with Fϭnk2. to cos ␮␾, one now obtains The perturbation ⍀ acts like a feedback mechanism 3 225 for the harmonic oscillator on the left-hand side of Eq. ␮2ϭ1Ϫ m2Ϫ m3. (38) 2 16 (33) and leads to a of finite amplitude for all frequencies other than 1 that appear on the right-hand The second-order correction to ␮ is, therefore, side of Eq. (33). Any possible excitation at the fre- 225m3/32, compared to the first-order result 3m3/4 of quency 1, however, such as would be caused by the pure Newton. With 225/32Х7 and mХ3/40, the second term Kepler motion, will cause a shift in frequency from 1 to constitutes 7/10 of the first. ␮ rather than an amplitude that goes to ϱ. After all this work, Clairaut and d’Alembert were The terms of order 0 in the perturbation expansion of able to account for 85% of the motion of the lunar peri- s cancel one another, provided gee, compared to Newton’s 50%. We have to remind ourselves that the Greeks knew the correct value em- m2 3 2 3 2 2 n k ϭG0͑EϩM͒ 1Ϫ , ␮ ϭ1Ϫ m . (35) pirically to better than four decimals. Nevertheless, this ͩ 2 ͪ 2 work of the two French academicians convinced every- The first condition indicates that the rotating ellipse is body that Newton’s universal gravitation should be suf- shrunk compared to the unperturbed Kepler ellipse, ficient to explain all the motions of the Moon around the which is to be expected because of the additional repul- Earth. It also suggested that a perfect fit with the obser- sion due to the Sun. The second condition confirms vations would be hard to accomplish and could be far Newton’s result (23) that ␮Ϫ1ХϪ3m2/4. down the road.

E. The evection and the variation G. The annual equation and the parallactic inequality

A first-order approximation yields the correction to The algebraic expansion (36) provides a tool with the rotating Kepler ellipse (34), which the individual terms in the lunar motion can be derived separately. Brahe’s annual equation (see Sec. k V.F) was found to have a coefficient Ϫ13Ј, whereas the ϭ1ϩ␧ cos ␮␾ϩ␦ cos͑2Ϫ2mϪ␮͒␾ r observed value was only Ϫ11Ј8Љ. The negative sign in- dicates that the Moon falls behind in spring and catches ϩ␤ cos 2͑1Ϫm͒␾ϩ␣ cos͑2Ϫ2mϩ␮͒␾, (36) up in fall. where A correction of similar origin arises from the octupole term in the expansion (31) for the perturbation of the 15 5 ␦Х ␧m, ␤Хm2, ␣ХϪ ␧m2. (36Ј) Sun. This correction can be singled out because its coef- 8 8 ficient has a factor a/aЈ. The period is the synodic The lunar longitude ␾ as a function of time becomes month; such a motion was first noticed by (see next section). The calculation yields 73Љ for the am- ␾ϭntϩ2␧ sin ␮ntϩ2␦ sin͑2nϪ2nЈϪ␮n͒t plitude of this ‘‘parallactic inequality’’ in lowest order, 3m2 whereas the complete value is 125Љ. Again, we are short ϩ ␤ϩ sin 2͑nϪnЈ͒t by almost a factor of 2. ͩ 8 ͪ If the theory for this inequality could be improved, 2␣ the ratio a/aЈ could be obtained from the lunar orbit ϩ sin͑2nϪ2nЈϩ␮n͒t, (37) and, therefore, the solar parallax from the well-known 3 ratio Earth-radius/a. In this manner Tobias Mayer to the lowest order with respect to m. One recognizes found 8.6Љ, which differs insignificantly from the modern the anomaly, which has the amplitude 2␧, the evection value of 8.8Љ. A better value for the parallactic inequal- comes with a factor 2␦ϭ15␧m/4, and the variation has ity, however, is not easy to tease out of the observations. the amplitude 11m2/8. The aЈ, the fundamental measure of High-precision data are customarily expressed in sec- length in the universe, is better found by other observa- onds of arc, but we shall stick with the round figures in tions, such as the transits of Venus over the disc of the minutes of arc because they are more easily remem- Sun. bered. Therefore 2␧Х375Ј and mϭ1/13.3679Х3/40 with Such transits occur only every 120 years, and then the help of Eq. (36Ј) lead to (␤ϩ3m2/8)ϭ26Ј30Љ and they happen in pairs eight years apart, such as 1761 and 2␦ϭ52Ј36Љ, instead of 40Ј and 75Ј for the observed 1769 (see Woolf, 1959). The French Academy organized variation and evection. Thus the first-order corrections a large enterprise in 1761 which failed, however, due to yield only two-thirds of the observed values. bad luck and the great war between France and England

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 610 Martin C. Gutzwiller: The oldest three-body problem that spread over the whole . (This important event Mayer had required 14 linear combinations of angles was known as the French-Indian wars in America and in the expansion of the Moon’s longitude and identified led to the ouster of the French from Canada.) The sec- 8 more of them that were too small for the precision of ond opportunity in 1769 was more carefully planned and his . Charles published Lunar Tables in profited from some lucky breaks, such as Captain Longitude and Latitude According to the Newtonian ’s discovering Tahiti just in time, and the great Eu- Laws of Gravity in 1787, using all 20 combinations. (In ler himself observing both contacts from a station near 1763, upon the request of the Astronomer Royal, St. Petersburg. But even after discarding the more Nathaniel , he had gone with Jeremiah Dixon to the doubtful observations, the final results still varied be- American colonies in order to survey the boundaries be- tween 8.5Љ and 8.9Љ (Newcomb, 1891). tween , Maryland, Delaware, and Virginia; a fictional account of this adventure by Thomas Pynchon appeared in 1997 under the title Mason & Dixon.) There followed the Austrian Bu¨ rg in 1806, who listed 28 com- H. The computation of lunar tables binations in his table, although there were really 40 The simple results of the preceding sections demon- terms in longitude because both the parallactic inequal- strate that the theory had to be greatly refined before ity and the variation depended on the mean elongation complete agreement with the observations could be Dϭ␭Ϫ␭Ј. The last in this roster is the French academi- achieved. The lunar motions were represented as trigo- cian in 1812, who used 36 combinations of nometric series that involved the combination of four angles. angles. The coefficients in these expansions depended The comparison of individual tables grew more com- on a very small number of parameters, but all through plicated, and the search for the explanation of any dis- the 18th century the theory was not good enough to crepancies became more scientific. Methods of least carry out the required computations. squares were used for judging the quality of the last two Good predictions for the lunar position in the sky tables and for finding the best values for the mean mo- were necessary as a help in navigating across the oceans, tions and the so-called epochs, i.e., the value of the rel- since mechanical clocks did not run reliably for several evant angles at some arbitrary time such as midnight weeks or even months without interruption. The exis- before January 1, 1801. The root of the mean-square tence of the trigonometric series became an accepted deviation went down to 6Љ of arc, although exceptionally result of the general theory. But the values of the coef- an individual deviation might reach almost 60Љ. The co- ficients were obtained from fitting the observations, efficients in the trigonometric series for the longitude of rather than working out any algebraic formulas in terms the Moon were given to the tenth of a second of arc. The of the few parameters. observations were still ahead of the theory. Thousands of lunar positions were provided by the British Astronomers Royal, starting with Flamsteed, fol- lowed by the all-round genius Halley, then Bradley, the discoverer of the of light and the nutation of I. The grand synthesis of Laplace the Earth’s axis, and finally Maskelyne, who tested all kinds of new clocks against celestial observations. The Pierre Simon Laplace (1749–1827) is a well-known Continental astronomers also made many observations figure in the French scientific pantheon, not necessarily in the sky, but they did not accumulate the long runs of for his many scientific achievements, but rather for his measurements with the same instruments under similar philosophical approach to them and for his influence on conditions. A complete review of all the available data present-day institutions in France. Nevertheless, with his concerning the motion of the Moon was given by New- monumental four-volume work Traite´ de Me´canique Ce´- comb (1878, 1912). leste, divided into ten books and published from 1799 to Although Euler (1746) had already published some 1805 (the fifth volume, covering history and some gen- tables, and Clairaut (1754) as well as d’Alembert (1756) eral physics, followed 20 years later), Laplace validated followed a few years later, it was Tobias Mayer, profes- Newton’s claim that all of astronomy in the solar system sor of astronomy at the University of Goettingen, who can be reduced to the three laws of motion and universal set the new standards. The claim that his table of 1752 gravitation with the inverse square of the distance. fitted the observations within at most 1Ј, was reluctantly The whole work is characterized by its precise as well confirmed by his colleagues. After his death, his theory as concise language and its systematic buildup, starting was published in 1767, and his improved table appeared from general principles and ending with the fine details in 1770 with a preface by Maskelyne. Meanwhile his of comparison with the observations. An abbreviated widow was awarded 3000 pounds by the British Parlia- version with the title Mechanism of the Heavens was ment in 1763 to recognize her husband’s ability to ‘‘Dis- published in 1831 by Mary Sommerville, from a Scottish cover the Longitude at ,’’ while Euler got 300 pounds middle-class family. It contains a discussion of lunar for helping Tobias Mayer. Very entertaining accounts of theory far beyond what is offered in this review. She the competition between the Moon and the mechanical became well known and well established, but she could clock have been published recently by Andrewes (1993) not be elected to the Royal Society of London. Instead and Sobel (1995). the Fellows decided to have her marble portrait made by

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 611

Francis Chantrey and to have it stand in Burlington of the constants’’ of Lagrange in book VI, the first part House, their headquarters.1 of the third volume, which is devoted to the perturba- English-speaking readers have been blessed with a tions in the planetary orbits. But he preferred the more unique translation of the first four volumes of Laplace’s primitive approach of Clairaut and d’Alembert for the treatise. A self-made businessman from Boston, Moon. Nathaniel (1773–1838), not only translated The starting point for Laplace’s lunar theory is a but also added numerous comments to explain with in- modified Kepler motion in three dimensions. In addition finite patience exactly what Laplace was doing. The re- to the motion of the perigee ␮, there is now a motion of sulting four tomes (Bowditch, 1829, 1832, 1834, and the node ␯ already in the lowest approximation, 1839) contain about two and a half times as many pages 2 ͱ1ϩ␴ ϩ␧0 cos␮͑␾Ϫ␻͒ uϭ , ␴ϭ␥ sin␯͑␾Ϫ⌰͒. as the original and constitute a prime document of sci- k 0 ence in the early . Its author also wrote a (40) voluminous New American Practical Navigator that went through of editions, and he received many The eccentricity ␧0 and the inclination ␥0 differ some- honors for his efforts from academic institutions the what from the earlier definitions for ␧ and ␥. Together world over. with the angles ␻ and ⌰ they are the initial conditions The developments in Me´canique Ce´leste do not follow for the integration of the equations of motion. The lunar motion now looks like a dynamic problem some general method, nor do they pretend to provide a of two oscillators with feedback mechanisms. The fre- complete survey of the whole field, but they cover many quency has to shift for both the radial motion and the of the outstanding problems: the shapes and the rotation motion in latitude because otherwise there would be an of the bodies in the solar system, including the tides in infinite resonance with the motion in longitude and lati- the and in the atmosphere, as well as the rings of tude. The lowest approximation for ␮ is again given by Saturn, the motion of the individual planets and their Eq. (35), while the lowest approximation for ␯ is ob- with all the mutual perturbations and reso- tained from Eq. (40) and yields Newton’s result (24), nances, and finally a discussion of the , but ignor- ␯ Ϫ ϩ m2 ing the recent discovery of the . Laplace natu- 1Х 3 /4. rally prefers to present much of the successful work that The further development of the lunar motion at a he had done in the preceding 30 years. He tries to push right angle to the ecliptic follows the same pattern as the every topic to a perfect agreement between observation earlier calculations in Sec. VII.E. In complete analogy to and theory, and he succeeded in convincing the world the evection, the first-order corrected formula for the that such a goal could be attained. He backed up this latitude becomes claim with sophisticated arguments from his theory of 3m ␴ϭ␥ sin ␯͑␾Ϫ⌰͒Ϫ ␥ sin͓␯͑␾Ϫ⌰͒ probability. 0 8 0

J. Laplace’s lunar theory Ϫ2͑1Ϫm͒␾͒], (41) where we have kept only the lowest power of the m Book VII in the third volume treats the ‘‘Theory of ϭnЈ/n. In spite of its similar origin, the evection term in the Moon.’’ After 12 pages of general introduction, Eq. (36) is five times larger with a coefficient 15/8 in ␣ there follow 123 pages of hard analysis. Bowditch had to rather than the libration with 3/8 in Eq. (41). expand them to 331 pages in order to accommodate all Since ␥0 corresponds to 5°11ϭ311Ј, and 3m/8 his explanatory remarks. The is still the same as in Х(3/8)(3/40)ϭ9/320, this change in latitude amounts to the work of Clairaut and d’Alembert, but the motion in about 9Ј, which is the observed value. Equation (41) can latitude is taken into account from the start, and many also be interpreted as a motion of the nodes by finding more terms are included in the trigonometric series. The the longitude ␾ when the latitude ␴ vanishes. Thus ⌰ is basic equations are a fairly straightforward generaliza- found to vary as (3m/8)sin 2D, i.e., with an amplitude tion of Eq. (33), and demonstrate once more the con- of 1°37, in good agreement with the observations of trast with the various modern approaches. Brahe (cf. Sec. V.F). Again we find that the motion per- Starting from polar coordinates (r,␪,␾) for the Moon pendicular to the ecliptic comes out quite well in the with respect to the ecliptic, the equations of motion are lowest significant approximation, in contrast to the mo- written in terms of tion in the ecliptic. 1 A great deal of care is necessary if the method of uϭ , ␴ϭtan␪, ␾, (39) Laplace is to be carried to fourth or even fifth order. r cos␪ The details in Me´canique Ce´leste are not always easy to where the true longitude ␾ serves as the independent follow because the numerical value for the most impor- variable. The differential equations for u and ␴ look tant (and best known) parameter is used, namely, the almost exactly like Eq. (33). Laplace used the ‘‘variation ratio of the mean motions mϭnЈ/nХ3/40, or any func- tion of it. Also, there are subtle arguments of what we call ‘‘renormalization’’ because the starting parameters 1I happen to own the copy of Mechanism of the Heavens that in the theory are not always identical with the observed Sommerville dedicated to the sculptor Chantrey. ones.

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The work of Laplace had profound philosophical im- Section IX deals with (i) and (iii), because much of plications for the sciences because he was able to Poincare´’s work in celestial mechanics is directly based achieve what we would call today a ‘‘grand unification’’ on in the Hamilton-Jacobi version. for everything that was observed in the solar system. By contrast, Hill’s work throws a totally new light on the Since the main problem in lunar theory requires very motion of the Moon and will be discussed in Sec. X. few external parameters plus some initial conditions, it These developments, however, began with Lagrange, affords an extremely stringent test for both the underly- who seems to be the first mathematical physicist trying ing physics and the mathematical methods. The agree- to find general methods while solving special problems. ment with the observations of Bradley and Maskelyne Together with his much younger colleague Poisson, he averages around one arcsecond. Nothing of this kind worked out a general formalism that allowed a clearer had ever been achieved before! insight into the lunar motion. Laplace also investigated the effect of the planets on the Moon’s motion and found that it is mostly indirect, i.e., through the motion of the Sun around the center of B. The variation of the constants mass of the solar system. Halley had discovered that the Moon’s motion was accelerating slowly, and Laplace Euler’s first theory (1753) for the motion of the Moon found the cause in the variation of the Earth’s eccentric- was only mentioned in passing in Sec. VII.B. As in so ity. Tobias Mayer had found an inequality in longitude many other instances, Euler was far ahead of his time, that depended only on the position of the node, and but he followed up on his new approaches only to the Laplace now attributed it to the flattened figure of the extent required to solve the problem at hand. Some of Earth, whose value he determines thereby as 1/305. his most interesting results are found in the Appendix. The great endeavor of Laplace has left a deep mark Rather than solving the equations of motion in any of on our view of nature, but his methods for solving the the various coordinate systems, Euler’s simple idea was equations of celestial mechanics are no longer used. The to express the rate of change with time, for any of the many technical inventions that led to the success of this osculating elements in Sec. V.B. The time derivatives of project have either been abandoned as impractical or the parameters a, ␧, ␥, l 0 , g, h for the Moon are given become part of the common background in this field. directly in terms of the perturbation by the Sun. Thanks The guiding spirit of this great enterprise has influenced to his virtuoso skills in geometry and analysis he was many of our beliefs, but the huge bulk that supported able to carry out this program. the whole structure is mostly forgotten. Giuseppe Lodovico Lagrangia (1736–1813) spent his first thirty years in his native Torino, where he started most of his scientific work. He founded a new journal VIII. THE SYSTEMATIC DEVELOPMENT OF LUNAR together with some like-minded friends to publish his THEORY prodigious output. In 1766 he was called to Berlin as the successor of Euler, who was moving back to St. Peters- A. The triumph of celestial mechanics burg. Thanks to the eminence of Euler and Lagrange, Frederick the Great of Prussia succeeded in putting his The 19th century brought many new problems in ce- Royal Academy on the map. Some 147 years later, the lestial mechanics because of three major discoveries: (i) same academy offered its senior position to the 33-year- the major planet Uranus in 1781, (ii) four large asteroids old , a Swiss citizen (like Euler) who had from 1801 to 1807, and (iii) the last major planet, Nep- spent some of his youth in Italy, to complete the anal- tune, on the basis of the observed perturbations on Ura- ogy. nus and the calculations of Urbain Leverrier and John Although he got an early start on his seminal ideas, it Couch in 1846. took Lagrange more than 50 years of hard work to cast The second half of the 19th century brought some them into the simple shape that we learn about in clas- clarification into the many approaches to celestial me- sical and quantum mechanics. This long process began in chanics, particularly in three respects: 1774 with his research on the of the nodes and inclinations of planetary orbits. The adjective (i) one special approach to mechanics, now associ- secular refers to a slow change in the speed of the mean ated with the names of Hamilton and Jacobi, motion. Applying this research to lunar theory, he intro- seemed to become dominant; duced the angular momentum vector (in modern no- (ii) the lunar problem served as inspiration for Hill to menclature) of the Moon with respect to the Earth, and invent a completely new foundation; calculated its time rate of change when the lunar orbit is (iii) the many schemes for constructing approximate assumed to be circular in lowest approximation. solutions were finally examined from a purely The fruit of this approach came two years later when mathematical perspective by Poincare´. Lagrange (1776) derived his famous theorem on the sta- At the same time, the comparison of the theory with bility of the solar system. The semi-major axis aЈ for ever better observations was improved, and the compu- some particular planet like the Earth is given by the tations for the ephemerides were made more accurate total energy of its motion around the Sun, ϪG0SE/2aЈ. and practical. The only reason for the change of aЈ is the interference

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 613 of the other planets. Again we insert in lowest approxi- completed when he died. In 1808 he made a major mation the simple Kepler motions for each planet. breakthrough by finding a general method for solving The secular variation of the semi-major axes had been problems in mechanics. calculated earlier by Euler, who got rather large values. Lagrange does not speak of the momentum, but al- Laplace then carried Euler’s computation one step fur- ways of the velocity; even today mathematicians usually ther and showed that there was a subtle compensation of do not make that distinction, although it is already quite the two lowest orders. With Lagrange’s simple expres- clearly stated on the very first page of Newton’s Prin- sion for daЈ/dt it became almost obvious that there was cipia. In gravitational problems the distinction is some- no secular, i.e., constant as opposed to periodic, term. In what artificial, however, because the mass of any celes- Jacobi’s words, the proof of Lagrange was accomplished tial body always drops out of its equation of motion with one stroke of his pen (Jacobi, 1866). since the inertial mass equals the gravitational mass. In 1808 Poisson showed the absence of secular terms Lagrange’s reasoning can be best explained with the in second order provided there is no resonance between help of the Moon’s motion around the Earth. If there is any two planets, i.e., there is no linear relation with no perturbation, the complete solution of Kepler’s prob- simple integers between the mean motions of any two lem would be a set of functions, planets. Poincare´ (1899) pointed out in Chapter XXVI that Poisson excluded only terms like Bt from the x͑a,␧,␥,l 0 ,g,h;t͒, y͑a,...,h;t͒, z͑a,...,h;t͒, change of the semi-major axis, but admitted terms like x˙͑a,␧,␥,l ,g,h;t͒, y˙͑a,...,h;t͒, z˙͑a,...,h,;t͒. At sin(␣tϩ␤). Stability to second order only implies the 0 (42) return to the original values, but allows arbitrarily large excursions in between. It was finally shown that secular The coordinates and velocities depend on time in two terms could no longer be avoided in third order (see ways: through the explicit occurrence of t in the Kepler Tisserand’s discussion in Chapter XXV of Volume I). motion as represented in Eq. (42), and through the From 1781 to 1784 Lagrange generalized his method change of the parameters a, ␧, ␥, l 0 , g, h because of the by including what we now call the Runge-Lenz vector perturbation. [Eq. (14)]. The change of these quantities due to pertur- Since the coordinates (x,y,z) are explicitly known as bations leads to a change in a, ␧, ␥ and the angles h, g, functions of the parameters a,...,h and t, the perturb- l 0 . In this roundabout way, Lagrange calculated the ing potential W(x,y,z,t) is now viewed also as a func- time rate of change for the Kepler parameters a, ␥, etc. tion of these parameters. By a sequence of simple ma- Lagrange made a major effort to check whether his nipulations, Lagrange manages to write the equations of scheme gave the observed values for the planetary mo- motion in the following form: tions, to the point of critically evaluating various observ- Wץ ing instruments. Obviously, the separation between ob- da d␧ dh ͕a,a͖ ϩ͕a,␧͖ ϩ•••ϩ͕a,h͖ ϭϪ , aץ servation and theory was not yet as wide as it is today. dt dt dt During the same years Lagrange wrote his magnum Wץ opus, , the founding document for da d␧ dh ͕␧,a͖ ϩ͕␧,␧͖ ϩ•••ϩ͕␧,h͖ ϭϪ , (43) ␧ץ modern . The central idea of dt dt dt Lagrange’s approach was expressed in the ‘‘avertisse- ment’’ (i.e., preface), where he says ‘‘There are no fig- and four more equations for ␥, l 0 , g, h, where the ures in this treatise. The methods that I propose require Lagrange ͕␧,a͖ is defined by neither constructions nor mechanical reasoning, but only ˙ ˙ ˙ z,z͒͑ץ y,y͒͑ץ x,x͒͑ץ -algebraic operations that are bound to a regular and uni ͕␧,a͖ϭm0 ϩ ϩ , (44) ␧,a͒ͪ͑ץ ␧,a͒͑ץ ␧,a͒͑ץ ͩ form procedure. People who like analysis will see with pleasure that mechanics has become a part of it, and they with will be grateful to me for having expanded its range.’’ ˙xץ xץ ˙xץ xץ ͒ ˙x,x͑ץ ϭ Ϫ . ␧ץ aץ aץ ␧ץ ␧,a͒͑ץ

C. The Lagrange brackets The 6ϫ6 of the Lagrange brackets is regular, because it is basically the square of the Jacobian matrix In 1787 Lagrange joined the Royal Academy of Sci- for the functions (42). Calculating the individual brack- ences in Paris where he was treated with the utmost ets, however, requires all of Lagrange’s computing skills. respect to the point of being offered the doubtful privi- Most importantly, he shows that their partial derivative t vanishes. Each bracket is a combination of the sixץ/ץ lege of an apartment in the Louvre. In the same year the Analytical Mechanics was published, and Lagrange spent parameters a, ␧, ␥, l 0 , g, h; but t does not occur explic- most of his remaining 25 years in writing mathematical itly in them, even though it is present in the functions monographs, typically on the solution of algebraic equa- (42). tions and on the theory of functions. But he also worked The 6ϫ6 matrix of the Lagrange brackets turns out to hard on a second and expanded edition of the Analytical be so simple that the linear equations (43) can be in- Mechanics, of which the second volume was not quite verted to yield

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 614 Martin C. Gutzwiller: The oldest three-body problem

A,B͒͑ץ A,B͒͑ץ A,B͒͑ץ W 1ץ da 2 ϭϪ , (45) ͓A,B͔ϭ ϩ ϩ . (52) ͪ ͒ ˙z,z͑ץ ͒ ˙y,y͑ץ ͒ ˙x,x͑ץ ͩ l mץ dt na 0 0 2 W The 6ϫ6 matrix of the Poisson brackets is the inverse ofץ W 1Ϫ␧ץ dl 0 2 ϭ ϩ , (46) ␧ the 6ϫ6 matrix of the Lagrange brackets. Indeed, theץ a na2␧ץ dt na Poisson brackets for the Kepler problem are exactly the .W coefficients in the equations (45)–(50) of Lagrangeץ W 1Ϫ␧2ץ d␧ ͱ1Ϫ␧2 ϭ Ϫ , (47) l Lagrange did not live long enough to see Poissonץ g na2␧ץ dt na2␧ 0 brackets steal the show from his own brainchild. He had W tried to grasp the most basic elements of mechanics, andץ ␥ W cosץ dg ͱ1Ϫ␧2 ϭϪ 2 ϩ , (48) he almost succeeded, with the help of his method of ␥ץ ␧ 2 Ϫ 2ץ dt na ␧ na ͱ1 ␧ sin ␥ virtual displacements and of his general equations of W motion in terms of the kinetic and potential . Inץ ␥ W cosץ d␥ 1 ϭ Ϫ , the process, classical mechanics became an ever more 2 2 2 2 g abstract branch of science. In particular, lunar theoryץ ␥ h na ͱ1Ϫ␧ sinץ ␥ dt na ͱ1Ϫ␧ sin (49) after Lagrange turned more and more into a competi- tion among different computational schemes. Wץ dh 1 ϭϪ . (50) ␥ץ ␥ dt na2ͱ1Ϫ␧2 sin The reader should notice that the parameters a, ␧, ␥ change with time only through the partial derivatives of E. The perturbing function the perturbation W with respect to the angles l 0 , g, h. These derivatives are necessarily periodic with time, so In order to take advantage of our new equations of that a, ␧, ␥ are subject only to periodic changes in first motion, the perturbing potential W(x,y,z,t) has to be order. Lagrange’s theorem on the invariance of the rewritten. The Cartesian coordinates (x,y,z,) have to semi-major axes (the stability of the solar system) is a be expressed as functions of the Kepler parameters special case of this conclusion. (a,...,h) and time t as in Eq. (42). For many bodies in the solar system the eccentricity ␧ This messy calculation is carried out in exemplary and the inclination ␥ are small. In these cases the singu- fashion by Delaunay (1860, 1867; see Sec. IX.B). The larities on the right-hand sides of Eqs. (46) through (50) parameters ␧Х1/18, ␥¯ϭsin ␥/2Х1/22, and ␧ЈХ1/60 are can be avoided if the parameter pairs (␧,g) and (␥,h) considered as first-order quantities, whereas a/aЈ are replaced by (␧ cos g,␧ sin g) and (␥ cos h,␥ sin h). Х1/400 is treated as second order. All terms in W up to Poincare´ will use this idea when he discusses the conver- eighth order are listed. Moreover, terms whose argu- gence of power-series expansions in these parameters. ments contain l Ј but not l are carried to ninth order, The main task now becomes to write the perturbing po- while terms containing neither l nor l Ј are listed to tential W(x,y,z,t) as a function of parameters such as tenth order. These terms have a slow periodic variation, our (a,␧,␥,l 0 ,g,h). so that their integration introduces small denominators. The result of this computation is given explicitly on pages 33–54 of the first volume. It forms the basis for the remaining 1750 pages, which will be discussed in the D. The Poisson brackets next chapter. Here, we shall copy some of the very lowest-order terms, The 27-year-old Poisson was inspired by the paper 2 that the 72-year-old Lagrange read before the French G0Sa 1 3 3 3 1 WϭϪ ϩ ␧2Ϫ ␥2ϩ ␧Ј2Ϫ ␧ cos l Academy of Sciences. Within two months Poisson had aЈ3 ͩ4 8 8 8 2 found a significant shortcut by starting with the opposite 3 15 of Eq. (42): the constants of motion, say our usual col- ϩ cos 2͑hϩgϩl Ϫ⌽͒ϩ ␧2 cos 2͑hϩgϪ⌽͒ lection for the Kepler problem, are given explicitly in 4 8 terms of the Cartesian coordinates (x,y,z) and their ve- 3 locities (x˙ ,y˙ ,z˙ ). ϩ ␥2 cos 2͑hϪ⌽͒ , (53) It follows then with relative ease that 8 ͪ W where we have set the Sun’s mean longitude l ЈϩgЈץ Wץ Wץ da ϭϪ͓a,a͔ Ϫ͓a,␧͔ Ϫ•••Ϫ͓a,h͔ , ϩhЈϭ⌽. hץ ␧ץ aץ dt The general term contains a cosine of the argument in W the formץ Wץ Wץ d␧ ϭϪ͓␧,a͔ Ϫ͓␧,␧͔ Ϫ•••Ϫ͓␧,h͔ , (51) .͒ h j1͑hϩgϩl ϪhЈϪgЈϪl Ј͒ϩj2l ϩj3l Јϩj4͑gϩlץ ␧ץ aץ dt and similar equations for the remaining four parameters. (54) The ͓A,B͔ between any two functions Its coefficient is a polynomial, where the individual term A(x,y,z,x˙ ,y˙ ,z˙ ) and B(x,y,z,x˙ ,y˙ ,z˙ ) is defined as is a rational number that gets multiplied with

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 615

2 G0Sa eters in the Earth’s gravitational field, as first established ͑a/aЈ͒k1␧k2␧Јk3 sink4͑␥/2͒. (55) aЈ3 by Clairaut (1743). The answers were finally figured out by Laplace, who found the correction ␦␤ϭ The coefficient j1 and all the exponents k are у0. The Ϫ8Љ.382 sin(l ϩgϩh) in latitude and ␦␭ϭ7Љ.624 sin h following relations hold: j1 and k1 are simultaneously in longitude. The first had been observed by Tobias even or odd; j4 and k4 are both even; k2 , k3 , k4 are Mayer without interpretation, and the second was dis- larger, respectively, than ͉j2͉, ͉j3͉, ͉j4͉ by an even num- covered subsequently by Bu¨ rg and Burckhardt. This ber у0. problem plays a central role in our times for the trajec- Any physical understanding of the results is possible tories of artificial satellites around the Earth (see end of W only if the individual terms in the expansion of can be Sec. XI.E). tracked down to the original expression for the solar perturbation. Looking at the 22 pages of Delaunay’s ex- pansion, it is clear that only the very lowest terms, such G. Again the perigee and the node as Eq. (53), can be identified and interpreted. Unfortu- nately, it lies in the very nature of modern theoretical The improved values of Clairaut and d’Alembert for physics that such obvious reductions are no longer fea- the motions of the perigee and of the node can be ob- sible. Our gain in precision does not necessarily come tained rather easily from Lagrange’s equations (47)– with a better understanding. (50) in the following ingenious manner. Only the terms proportional to ␧2 and ␥2 in the perturbation (53) are F. Simple derivation of earlier results taken into account. If the longitude of the perigee is called ␻ϭgϩh, one finds that Some of the principal perturbations in the lunar tra- 2 jectory were calculated in the preceding section just as d␧ 3m ϭϪ 5␧ sin 2͑␭ЈϪ␻͒, they were explained for the first time by the French dt 4 mathematicians in the 18th century. These same results d␻ 3m2 can now be obtained much faster with the help of ϭ „1ϩ5 cos 2͑␭ЈϪ␻͒…, (58) Lagrange’s method. The expansion (53) has to be in- dt 4 – serted into the equations of motion (45) (50), which d␥ 3m2 then have to be integrated. Since the n is ϭϪ ␥ sin 2͑␭ЈϪh͒, expressed through Kepler’s third law [Eq. (15)], i.e., n dt 4 3 2 ϭͱG0(EϩM)/a , the mean anomaly at has to be dh 3m redefined, ϭϪ 1Ϫcos 2͑␭ЈϪh͒ , (59) dt 4 „ … where ␭Јϭl ЈϩgЈϩhЈ is the mean longitude of the Sun. l ϭntϩl 0ϭ͵ ndtϩl 1 , (56) We shall assume that ␭ЈϭnЈtϩ␭0Ј where nЈ is known. in order to avoid terms of the type t sin l ; l 1 serves as Some small coupling between these two sets of equa- the new Kepler parameter. tions has been neglected. The two pairs of equations From the first line in Eq. (53) we get immediately were found by Puiseux in 1864 to be completely inte- 2 2 2 grable, with solutions that use only elementary func- dl 1 7 nЈ dg 3 nЈ dh 3 nЈ ϭϪ , ϭ , ϭϪ , (57) tions. dt 4 n dt 2 n dt 4 n Rather than displaying the full solutions, it is sufficient in agreement with Newton’s results (23) and (24). The to say that h, the longitude of the node, is found to increase on the average at the rate nЈ(1Ϫͱ1ϩ3m/2), correction to the longitude l ϩgϩh, i.e., ␦(l 1ϩgϩh), is found to decrease at a rate nЈ2/n. It is directly related whereas the longitude of the perigee ␻ϭgϩh increases to a renormalization of the average distance of the at the rate nЈ(1Ϫͱ(1ϩ3m)(1Ϫ9m/2)) where m Moon from the Earth, and to correction (35) of Kepler’s ϭnЈ/n. When these expressions are expanded in powers third law. of m, one finds The differential equations (45)–(50) are solved in low- 2 ˙ 3m 9m est approximation by integrating the right-hand sides hϭϪnЈ Ϫ ϩ¯ , under the assumption that a, ␧, ␥ have constant values, ͩ 4 32 ͪ while l , g, h increase at a fixed rate with time. The first 3m 225m2 term in the second line of Eq. (53) yields Tycho Brahe’s g˙ ϩh˙ ϭϩnЈ ϩ ϩ . (60) ͩ 4 32 ¯ͪ variation. The second term in the second line of Eq. (53) yields Ptolemy’s evection, and the third line in (83) gives Remarkably, these are the correct values for the first the lowest-order correction (41) to the motion in lati- two terms of the expansions in powers of the relevant tude. parameter m [cf. Eq. (33)]. Lagrange’s equations of motion (45)–(50) can also be In his discussion of Newton’s lunar theory, Tisserand used to find the corrections in the Moon’s motion for the (Vol. III, p. 44) mentions the collection of Newtonian nonspherical nature of the Earth. The perturbing poten- manuscripts that the Count of Portsmouth left to the tial depends on the declination of the Moon with respect University of Cambridge. A committee, including to the Earth’s equator and on the geophysical param- Stokes and Adams, examined the papers and found im-

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 616 Martin C. Gutzwiller: The oldest three-body problem portant results in only three areas: lunar theory, atmo- The main purpose of present-day attention to spheric refraction, and the form of a solid of least resis- Hamilton-Jacobi theory is the connection between clas- tance. Newton had investigated the motion of the sical and quantum mechanics. This possibility was not perigee for an orbit of small eccentricity, and had for- foreseen in the first half of the 19th century. The theory mulated two lemmata as if for a fourth edition of the found relatively few applications and was probably re- Principia. They are equivalent to the second equation sponsible for some illusions that still persist. Indeed Ja- (58), except that the coefficient 5 becomes 11/2 for rea- cobi and all his successors (with the important exception sons that are not well understood. But even so, there is a of Poincare´) seem to imply that most dynamic systems significant improvement for the motion of the perigee are integrable, i.e., they have as many integrals of mo- over Newton’s earlier value. (See the comments of tion as degrees of freedom, if you are only smart enough Chandrasekhar, 1995.) to find them. Although there is some merit in this idea for the double-planet Earth-Moon moving around the massive IX. THE CANONICAL FORMALISM Sun, the construction of the required constants of mo- tion cannot be accomplished by some miraculous appli- A. The inspiration of Hamilton and Jacobi cation of Hamilton-Jacobi theory. Although canonical (1805–1865) is a romantic transformations go back to Hamilton and Jacobi, their figure in the best tradition of the early 19th century, practical use in the service of cannot supremely gifted and at times deeply unhappy. Yet his be traced back to them. enormous talents were widely appreciated, and he re- ceived many honors, such as being at the of the B. Action-angle variables first 14 foreign associates in the U.S. National Academy of Sciences at its founding in 1863. His unique achieve- The first large-scale application of canonical transfor- ment in physics was to recognize the deep analogy be- mations was worked out by Charles-Euge`ne Delaunay tween optics and mechanics (see Hamilton’s Mathemati- (1816–1872), a prominent engineer, mathematician, as- cal Papers, 1931 and 1940). tronomer, professor, and academician. In 1846 he pro- The analog of ’s principle in optics is the varia- posed a ‘‘New Method for the Determination of the tional principle of Euler and Maupertuis in mechanics Moon’s Motion’’ on which he worked for the following (Euler, 1744). The relevant characteristic function is the 20 years all by himself while teaching and writing vari- integral of the momentum at constant energy EϭTϩV ous textbooks and monographs. The detailed record of along a trajectory from the initial point (x0 ,y0 ,z0)to these heroic labors is contained in two monumental the endpoint (x,y,z). But in 1834, Hamilton considered tomes, each over 900 pages, published in 1860 and 1867. the variation for the more general integral of LϭTϪV There is no reason to believe that Delaunay knew over the trajectory from the initial spacetime anything about the work of Hamilton and Jacobi, and (x0 ,y0 ,z0 ,t0) to the final (x,y,z,t). He also wrote down yet there is no better example of the use of canonical the first-order partial differential equation for these transformations in perturbation theory. Delaunay wrote characteristic functions. In 1837 he had an extended cor- with great precision and clarity, with the necessary detail respondence with Lubbock on the motion of the Moon, to check on his computations. And indeed, that has been although his interests remained mostly in optics. done with the tools of both the 19th and the 20th cen- (1804–1851) was Hamil- tury. ton’s equal in mathematical talent, but he seems to have In a major change, Delaunay describes the pure been better organized and more effective. In mechanics, Kepler motion of the Earth-Moon system in terms of an he started in 1837 where Hamilton left off and created a angular momentum ␮L instead of the usual semi-major whole body of theory that has become the foundation of axis a. The energy becomes the modern approach to classical mechanics. It is well G EM ͑G EM͒2 EM explained in his famous Vorlesungen u¨ber Dynamik, 0 0 Ϫ ϭϪ␮ 2 2 , where ␮ϭ . which he held 1842–43 in Ko¨ nigsberg, now Kaliningrad. 2a 2␮ L EϩM They were published by Clebsch only in 1866 together (61) with some further well-written, but so far unpublished, With a few obvious modifications, one recognizes the notes on the theory of perturbation. energy levels of the . Jacobi’s main emphasis was on obtaining the charac- Two minor changes involve the angular momenta, teristic function from its first-order partial differential ␮G and ␮H instead of the eccentricity ␧ and the incli- equation, the classical analog of Schro¨ dinger’s equation. nation ␥, This first-order partial differential equation is solved ex- 2 plicitly whenever there is a sufficient number of con- LϭͱG0͑EϩM͒a, GϭLͱ1Ϫ␧ , HϭG cos ␥. stants of the motion, e.g., in the Kepler problem, and the (62) solution yields the orbit directly in terms of the relevant The division of the three angular momenta by the re- parameters. The procedure is explained in all the stan- duced mass ␮ simplifies some of the formulas, but we dard textbooks on classical mechanics with various de- shall continue to speak about the angular momenta grees of abstraction. L,G,H. They are paired with the mean anomaly l , the

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 617 distance g from the ascendant node to the perigee, and C. Generating functions the longitude h of the ascendant node from the refer- ence direction Q in the ecliptic. In modern language, we The Swedish mathematician is generally have the ‘‘actions’’ L,G,H and the angles l , g,h; they credited with having developed the use of generating are commonly called the Delaunay variables. functions in performing canonical transformations The equations of motion now take the standard form around 1916, although there were precursors. In his for a perturbed system, Ph.D. thesis of 1868, Tisserand discusses Delaunay’s re- sults in this manner (see Tisserand Vol. III, Chap. 11); ´ ⍀ץ ⍀ dhץ ⍀ dgץ dl ϭ , ϭ , ϭ , (63) Poincare also uses generating functions in his Nouvelles .H Me´thodes as if they were common propertyץ G dtץ L dtץ dt There is a set of old actions and angles, K, L, G, H, ⍀ץ ⍀ dHץ ⍀ dGץ dL ϭϪ , ϭϪ , ϭϪ , and k, l , g, h, and a set of new actions and angles, KЈ, h LЈ, GЈ, HЈ, and kЈ, l Ј, gЈ, hЈ. The generating functionץ g dtץ l dtץ dt where ⍀ is now the kinetic plus divided F depends on the new actions and the old angles; the by ␮, transformation is given by the formulas Fץ Fץ Fץ Fץ 2 „G0͑EϩM͒… ⍀ϭϪ ϩW͑x,y,z,t͒. (64) Kϭ , Lϭ , Gϭ , Hϭ , hץ gץ lץ kץ 2L2 Fץ Fץ Fץ Fץ Notice that the derivatives in Eq. (63) no longer require kЈϭ , l Јϭ , gЈϭ , hЈϭ . HЈץ GЈץ LЈץ KЈץ the special precaution of Eq. (56) in order to prevent the appearance of terms where the time t multiplies a trigo- (66) nometric function of the angles. Naturally, the generating function has to be chosen to The expansion (53) of W in powers of ␧, ␥, ␧Ј, and advance the solution of the problem at hand. a/aЈ can be used again; indeed, it was worked out by In contrast to Delaunay we shall now assume that the Delaunay in exactly this form (see Sec. VIII.E). But the resulting does not differ by Kepler parameters a, ␧, ␥ are now expressed in terms of much from the identity. This allows us to transform at L,G,H, the same time several terms of the type A cos ␪ where A L2 G2 is a function of the old actions such as Eq. (55), and ␪ is some linear combination with integer coefficients of the aϭ , ␧ϭͱ1Ϫ 2 , G0͑EϩM͒ L old angles such as Eq. (54). Thus we start from the ␥ 1 H Hamiltonian in the form sin ϭ Ϫ . (65) 2 ͱ2 2G ⍀ϭB͑K,L,G,H͒ϩA1 cos ␪1ϩA2 cos ␪2ϩ¯ . (67) Obviously, Delaunay has made a compromise between two viewpoints: on the one hand, the Kepler parameters where A1 ,A2 , . . . are small compared to B. Then we are very helpful in understanding a particular term in ⍀ construct a generating function as a sum of several and estimating its ; on the other hand, the pieces, computations are made easier by the simplicity of Eqs. FϭKЈkϩLЈl ϩGЈgϩHЈhϩF1ϩF2ϩ¯ . (68) (63) compared to Lagrange’s equations (45)–(50). For the sake of symmetry in the nomenclature, a where F1 is designed to take care of A1 cos ␪1 , and F2 of fourth degree of freedom is introduced to take care of A2 cos ␪2 , etc. the Sun’s motion. Its action variable is called K, and its Since the time does not occur explicitly in the old angle is kϭl Ј. A fourth pair of equations will appear in Hamiltonian ⍀, the new Hamiltonian ⍀Ј is obtained Eq. (63), and the total potential ⍀ will include an addi- from the old one simply by replacing in ⍀ the old actions tive term nЈK to make sure that the angle l Ј increases and angles by their expression in terms of the new ones. at the constant rate nЈ. This fourth degree of freedom Equations (66) are applied to Eq. (68) and inserted into does not interact with the other three, but the energy of Eq. (67). The choice of the functions F1 , F2 , and so on the Earth-Moon system depends on time through l Ј. is dictated by the requirement that the new Hamiltonian This situation is described nowadays as due to 3 plus 1/2 in terms of the new actions and angles have no more degree of freedom. terms such as A1 cos ␪1 , A2 cos ␪2 , etc. Delaunay uses the expansion of the perturbation W in The condition for the vanishing of the term A1 cos ␪1 Sec. VIII.E. Each term in this trigonometric expansion is in ⍀ then yields transformed away individually, one after the other, by a A1͑LЈ,GЈ,HЈ͒sin ␪1 procedure that yields all the higher-order corrections. F ϭϪ , (69) 1 ͑i ␻ ϩi ␻ ϩi ␻ ϩi ␻ ͒ The factors (55) determine which of the new terms to 10 0 11 1 12 2 13 3 retain and how far to carry the procedure. Although the and similar expressions for F2 , and so on. The integers whole process is very systematic, it is still beset by many i10 ,i11 ,i12 ,i13 are the same as in ␪1ϭi10kϩi11l ϩi12g details that require tremendous attention as well as al- ϩi13h. The ␻’s are the frequencies of the undisturbed most infinite patience. system and are given by the standard formulas

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 618 Martin C. Gutzwiller: The oldest three-body problem

B for the latitude includes the angles of 423 terms coveringץ Bץ Bץ Bץ ␻ ϭ ϭnЈ, ␻ ϭ , ␻ ϭ , ␻ ϭ . H an additional 52 pages. The coefficient for each term is aץ G 3ץ L 2ץ K 1ץ 0 (70) polynomial in mϭnЈ/n, ␧, sin(␥/2), ␧Ј, a/aЈ, all with rational numbers. 49 corrections in these polynomials This is where the small denominators raise their ugly were necessary for the longitude and 45 for the latitude. heads. With Von Zeipel’s method, one can obtain the An earlier suggestion by Andoyer (1901) was confirmed, generating function for the lowest-order corrections sim- namely that most of the terms of order 8 and 9 are er- ply by looking at the Hamiltonian ⍀. In order to pro- roneous, altogether a somewhat disappointing compari- ceed further, one must expand the transformation for- son. mulas from the old action angles to the new ones, to Delaunay’s work is a benchmark for what one human higher powers in A1 , A2 , etc. The results of these ex- individual is able to accomplish without the help of com- pansions have to be inserted into Eq. (67), and this last puting machines. His results can be looked up in a good step will produce additional terms that are independent library, since they were published in 1860 and 1867. The of the angles, as well as terms with new combinations of more extensive work that came out of the computers in angles ␪. But these new terms in the Hamiltonian ⍀Ј are the 1970s and 1980s, however, is not easily available be- expected to be smaller than those that were eliminated cause both the programming and the technical prowess by the generating function (68). Finally the lunar coor- of the machines have changed so rapidly. Whoever dinates have to be expressed in terms of the new action- wants to find out what has already been done, beyond a angle variables, a big job. qualitative account, is almost forced to do the whole job all over again with the help of whatever means are avail- D. The canonical formalism in lunar theory able at the time. Delaunay provided important insights into the con- Delaunay first carries out a set of 57 transformations vergence of series expansions in celestial mechanics. and eliminates all terms in the original Hamiltonian of This convergence is poor in the ratio mϭnЈ/n, which is order lower than four. The detailed record of this major known to very high accuracy. Although Delaunay tried task makes up the first volume; the main effort goes into to go to ninth order in m, some of his results are still finding the new terms of higher order in the Hamil- insufficient, e.g., his expression for the motion of the tonian that are produced by the transformations of the perigee is still in error by 10Ϫ4. On the other hand, his lower-order terms. Since the remaining 440 transforma- theory is analytic in all the variables and yet comparable tions no longer interfere with one another, they are in accuracy to the best of his time, i.e., Hansen’s, who equivalent to a single transformation with a generating had used numerical values for all the variables from the function to lowest order. The numerical accuracy of the very start. Moon’s longitude is not quite satisfactory at this point, however, and some of the earlier operations have to be improved. Therefore, 8 new canonical transformations E. The critique of Poincare´ are added, for a grand total of 505. Since the motions of the angles are given by the de- At the end of the 19th century, classical mechanics rivatives (70), the term B(K,L,G,H) in the final Hamil- took a decisive turn away from the happy optimism of tonian is the most important result. Deprit, Henrard, its earlier practitioners. Physicists are finally waking up and Rom of the Boeing Scientific Laboratories in Seattle at the end of the 20th century to an unpleasant reality reported in 1970 that Delaunay’s final expression for B that the mathematicians, followed by and as- has to be corrected by subtracting the single term tronomers, have known for a long time: even simple dy- 2 2 2 2 (5/8)nЈ a m sin (␥/2)␧Ј , where the lowest term in per- namic systems, with only two degrees of freedom and 2 2 turbation (53) is (1/4)nЈ a . The effect of this correction conserved energy, such as the double pendulum, have on the rate of change with time for the angles l 0 , g, h in very complicated motions as a rule. The elementary ex- the original Kepler problem can be obtained directly amples of the textbooks like Kepler’s motion for an iso- from Eqs. (46), (48), and (50). For the motion of the lated planet are not typical at all of most of the realistic node, the correction is systems in nature. dh 5 nЈ2 This capital discovery can be safely attributed to ⌬ ϭ m␧Ј2, (71) Henri Poincare´ (1854–1912), whose first series of scien- dt 16 n tific papers is concerned with the qualitative behavior of whereas the correction vanishes for the motion of the the solutions of ordinary differential equations. This perigee, d(gϩh)/dt. This correction amounts to about early work led him into celestial mechanics, in particu- 10Ϫ5 of Newton’s result Ϫ3nЈ2/4n. lar, into a study of periodic solutions of the full three- The same authors published in 1971 a more detailed body problem and its various special cases. This activity comparison of Delaunay’s results with their own compu- received a strong stimulus when King Oscar II of Swe- tations, which were carried to higher order and will be den announced a prize for the best scientific paper to discussed in Sec. X. Delaunay’s final expression for the prove (or disprove) the stability of the solar system. Moon’s longitude is a trigonometric series in the angles Poincare´ was eventually declared the winner although (54) of 460 terms covering 53 pages, while his expression he was unable to give a definitive answer to the main

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 619 question, and his long paper was published in 1890 in ␳ϭͱ2͑LϪG͒, ␴ϭͱ2͑GϪH͒ (72) Acta Mathematica (see Barrow-, 1996). Then he followed up with what is considered his mas- to form the new pairs (␳,gϩh) and (␴,h). Obviously, ␳ terpiece, a three-volume work entitled The New Meth- is of the order ␧ͱL, and ␴ is of the order ␥ͱL. With the ods of Celestial Mechanics, which appeared in 1892, further change in variables to (␰1 ,␩1) and (␰2 ,␩2), 1893, and 1899. Its English translation has just been pub- where lished, a century later. This is not the place to give even ͑␰1 ,␩1͒ϭ␳„cos͑gϩh͒, sin͑gϩh͒…, a brief account of this monumental work, except to say that the Moon plays a special role as the first example ͑␰2 ,␩2͒ϭ␴͑cos h, sin h͒, (73) for which a periodic orbit was chosen as the starting we have again two pairs of conjugate variables. point for a new approach in mechanics, an idea that was These Poincare´ variables can be used directly in the first proposed in 1877 by Hill to explain the Moon’s mo- perturbation W to arrange the many terms in decreasing tion (see next section). Working through the more than order of importance according as the powers of m, ␳ 1200 pages of mathematical argument is not for the 2 2 2 2 ϭͱ␰ ϩ␩ , ␴ϭͱ␰ ϩ␩ , ␧Ј, and ␣. Each term has a faint-hearted and can be frustrating because Poincare´ 1 1 2 2 ‘‘characteristic,’’ insists on using a fairly abstract language, in spite of the evident inspiration from physics, astronomy, and celes- m␮1␳␮2␴␮3␧Ј␮4␣␮5, (74) tial mechanics. whose exponents ␮ give an approximate idea of its rela- Three volumes of Lectures on Celestial Mechanics tive size. Generating functions are used exactly as de- were published in 1905, 1907, and 1910. They were nei- scribed in Sec. IX.B. ther intended as a rehash of the New Methods of Celes- Equations (60), however, show that the denominator tial Mechanics nor as a competition to the classic Trea- in Eq. (69) may bring about a division by various powers tise of Celestial Mechanics by Felix Tisserand, in four of m, up to the third in the special case when the motion volumes (1889, 1891, 1894, 1896), of which the third is of the node h˙ /n and the motion of the perigee (g˙ entirely devoted to the Moon. The standard methods are ˙ ´ subjected to a mathematical scrutiny in order to estab- ϩh)/n enter with the same multiple. Poincare calls de- lish their legitimacy, in particular with respect to the nominators of this type ‘‘analytically very small,’’ in con- convergence of the approximation schemes. The first trast to denominators where they enter in the ratio 2 to volume treats the motion of the planets and is domi- 1, which he calls ‘‘numerically very small’’ because of nated by the methods of Hamilton and Jacobi. The first the empirical values of the lunar periods in Sec. V.C. part of the second volume discusses the purely technical Therefore certain unavoidable canonical transforma- problem of expanding the Hamiltonian for the perturba- tions will lower the exponent ␮1 in the characteristic. tion calculation, while the second part examines the Poincare´ goes through a careful and rather lengthy mathematical justification for the lunar theory of Hill examination of all the many cases that might arise and and Brown. The rather hefty third volume treats the how they change the characteristic in the individual many kind of tides, in oceans and rivers, in the Earth’s terms of the solution. The interference of the small pa- , and in the stars, both in theory and in observation. rameters besides m complicate the discussion, but ulti- mately the presence of three degrees of freedom leads to disaster: If Delaunay’s expansion is pushed far enough, terms are found where the exponent of m is negative! F. The expansion of the lunar motion in the parameter m This conclusion defeats the whole purpose of the theory, even though it happens only when the expansion In 1908 Poincare´ published a 40-page paper in the is driven quite far, much further than in Delaunay’s Bulletin Astronomique with the title ‘‘On the small divi- work. Paradoxically, his incomplete solution might still sors in the theory of the Moon.’’ Its length is due to the provide an excellent numerical accuracy. But this state many different cases that have to be taken up in the of affairs is worse than in an ordinary asymptotic expan- argument, but it is rather straightforward and its nota- sion, like that for the Bessel functions where the coeffi- tion stays close to the special conditions of the Moon’s cients increase so fast, e.g., like factorials, that the posi- motion. Rather than discussing the convergence of the tive powers of the expansion parameter are defeated. In solution that comes out of perturbation theory, the main the lunar theory negative powers in the most crucial pa- issue is more drastic. The question to be answered will rameter cannot be avoided at all! be phrased carefully in order to get a clear reply. The lunar problem in the form (63), with the time replaced by the angle k as in Sec. IX.B, has four pairs of X. EXPANSION AROUND A PERIODIC ORBIT action-angle variables plus the small parameters m ϭnЈ/n, ␧, ␥, ␧Ј, and ␣ϭaЈ/a. The two pairs (K,k) and A. George William Hill (1838–1914) (L,l ) are left as they are, but the pairs (G,g) and (H,h) are replaced by two others in order to reflect the Hill’s career reflects life in the United States at the fact that both the eccentricity ␧ and the inclination ␥ are end of the 19th century, with one glaring exception. He small. In agreement with Eq. (62) and the end of Sec. never adjusted to the amenities and strains of regular VIII.C, we shall use academic and scientific life, even after his work was

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 620 Martin C. Gutzwiller: The oldest three-body problem widely appreciated and he received many honors. When burg under Euler’s direction with the help of fellow aca- his Collected Works were published in 1905, Poincare´ demicians, J.-A. Euler (his son), Krafft, and . It wrote the 12-page introduction (in French), where he introduces two new ideas that turned out to be critical: says ‘‘This reserve, I was going to say this savagery, has The Moon is described by Cartesian coordinates with been a happy circumstance for science, because it has al- respect to the ecliptic, and this reference system turns lowed him to complete his ingenious and patient re- around the Earth with the Moon’s mean motion n in searches.’’ longitude. This trick eliminates the linear term in time Hill’s father moved to the countryside a few years af- from the longitude, just as there is none in latitude and ter George’s birth in New York City, to start farming in sine parallax. Euler also is the first to order the terms in West Nyack, 30 up the Hudson river. Since his the expansion of the lunar motion according to their mathematical ability had been noticed, he was sent to characteristics like Eqs. (55) or (74). Rutgers College in New Jersey, where he was lucky to But Hill makes a crucial modification by letting the find a first-class teacher who made him study the classi- coordinates turn with respect to the ecliptic at the rate cal works from the 18th and early 19th century. In 1861, of the mean motion of the Sun nЈ rather than of the he joined the staff of the scientists working in Moon. The acceleration of the Moon in this rotating sys- Cambridge, Massachussets, on the American Ephemeris tem is now changed by 2nЈ(Ϫdy/dt,ϩdx/dt,0), the and Nautical . negative of the force, and by nЈ2(Ϫx,Ϫy,0), After became Superintendent of the the negative of the centrifugal force. Nautical Almanac in 1877, Hill started working on the The series (31) is again the basis for the expansion of theory of Jupiter and Saturn. His results, with the title the solar perturbation, but the Legendre polynomials New Theory of Jupiter and Saturn form volume III of are handled differently. Instead of treating rj as a factor j the Collected Papers, and occupy a hefty tome of more in front of Pj , Hill puts it inside, so that r Pj becomes a than 500 pages. It was a cornerstone in Newcomb’s great homogeneous polynomial of degree j in the lunar coor- project of revising all the data for the orbits in the solar dinates. (In most solid-state applications of atomic system. (Physicists are aware of this enterprise only be- theory, the Legendre polynomials of low order are al- cause it definitely established the missing 42Љ in the cen- ways written in this explicit manner.) tennial precession of Mercury’s perihelion, which were Hill’s purpose is to take into account the terms that the best data for the confirmation of Einstein’s theory of depend only on the ratio mϭnЈ/n. Therefore he sets ; see 1921 and 1958.) After a ten- RϭaЈ, and all the corrections in powers of ␧Ј are left year stay in Washington, D.C., Hill retired in 1892 to his out in lowest approximation. Moreover, he stops with beloved farm in West Nyack. the solar quadrupole, because the higher multipole Hill’s great contribution to all of mechanics (including terms have additional powers in ␣ϭaЈ/a. With nЈ2 3 its quantum version), and in particular to its celestial ϭG0S/aЈ , the perturbation now reads simply branch, is contained in the 1877 paper On the Part of the 3 1 Motion of the Lunar Perigee which is a Function of the VϭϪnЈ2 x2Ϫ ͑x2ϩy2ϩz2͒ . (75) Mean Motions of the Sun and the Moon. It was followed ͩ2 2 ͪ 1878 by a more detailed version Researches in Lunar The work of Newton, then of Clairaut and Theory. In the Collected Works, these papers appear as d’Alembert, and eventually of many others, including numbers 29 and 32, among some 80 others that cover a particularly Delaunay, had shown that the expansion in wide range of topics. powers of m was the main culprit for making lunar Lunar theory at that time came in several different theory so difficult and unsatisfactory. Poincare´’s results versions besides Delaunay’s. The theory of Lubbock and (at the end of the last section) were still in the future. de Ponte´coulant appeared in the 1830’s, and was a Hill was the first to treat the difficulties with the motion clever mixture of elements from earlier efforts that of the perigee completely, and separately from all the could be systematically expanded in powers of the other complications in the motion of the Moon. Kepler parameters. Hansen (1838, with additions in 1862–1864), on the other hand, started from equations C. Hill’s variational orbit close to Lagrange’s variation of the constants (45)–(50). But he did not develop a systematic perturbation theory, With the quadrupole potential (75) added to the stan- although he did carry his computations to very high pre- dard gravitational attraction between Earth and Moon, cision. His final expression for the lunar coordinates be- there is now a well-defined mechanical problem of mo- came the accepted standard for the remainder of the tion in three dimensions that has to be solved com- 19th century, until it was replaced in 1923 by Ernest W. pletely, at least for sufficiently small values of m. Before Brown’s extension of Hill’s work in all the national eph- one writes down its equations explicitly, the length scale emerides. is normalized to the semi-major axis a according to Kepler’s third law (15). The time variable ␶ is normal- B. Rotating rectangular coordinates ized to the length of the synodic month, ␶ϭ͑nϪnЈ͒͑tϪt ͒. (76) Hill’s inspiration must be connected with Euler’s Sec- 0 ond Theory, which was published (1772) in St. Peters- With the complex notation

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 621

uϭxϩiy, vϭxϪiy, r2ϭx2ϩy2ϩz2ϭuvϩz2, d d ␨ϭexp͑i␶͒, Dϭ ϭ␨ , (77) id␶ d␨ the equations of motion now are u 3mЈ2 D2uϩ2mЈDuϪ ϩ ͑uϩv͒ϭ0, (78) r3 2 v 3mЈ2 D2vϪ2mЈDvϪ ϩ ͑uϩv͒ϭ0, (79) r3 2 z D2zϪ ϪmЈ2zϭ0, (80) r3 where we have introduced the new parameter mЈ ϭnЈ/(nϪnЈ)ϭ1/(number of synodic months in one year). Since our dynamic system does not have any time de- pendence, there is an integral of motion C which Hill calls the Jacobian integral, 1 1 3 ͑ϪDuDvϩDz2͒Ϫ Ϫ mЈ2͑uϩv͒2ϭC. (81) 2 r 8 Normally, one would first of all define the energy of a trajectory by fixing the value of C, but Hill decides to find a solution that is defined by its period right from the start, not by its energy. His ingenious idea is that the real trajectory of the Moon must be close to a periodic orbit with the correct FIG. 5. Hill’s diagram of the variational orbit for different lengths of the synodic month: aϭ12.369 lunations per year, period. Among all the known correction terms in the like our Moon, bϭ4, cϭ3, dϭ1.78265 lunations leading to a accepted description of the Moon’s motion, only Tycho cusp. Poincare´ showed that this series continues smoothly, Brahe’s variation is retained because it has the period of leading to an ever increasing loop around the half moon. the synodic month. The resulting ‘‘variational orbit’’ lies in the ecliptic and has a slightly oval shape which is cen- tered on the Earth, with the long axis in the direction of At the end of this long paper, Hill obtains the varia- the half moons, as Newton had foreseen. tional orbit for increasing values of mЈ. The number of The required periodicity with the synodic month is synodic months per yearϭ1/mЈ comes down from 12.37 enforced by postulating a solution in the form through the integers until it reaches the critical value 1.78265 where the oval shape acquires a cusp in the half 1 ϱ i␶ j Ϫj moons (see Fig. 5). Hill thought the satellite would not u0͑␶͒ϭ ͑x0ϩiy0͒ϭe a0ϩ͚ ͑aj␨ ϩaϪj␨ ͒ , a ͩ jϭ1 ͪ reach the half moon position any longer for supercritical (82) values of mЈ. But Poincare´ showed that the sequence of variational orbits continues with the satellites now mak- a power series with positive and negative integer expo- ing a short regressive motion around the half moon in nents. If the orbit is symmetric with respect to the x axis, the rotating frame, before taking up again the principal then u (Ϫ␶)ϭu* , and all the coefficients in the series 0 0 motion in the forward direction. are real. The symmetry with respect to the y axis is A recursive computation for the variational orbit was equivalent to the point symmetry at the origin, so that set up by Schmidt (1995) with relative ease, provided the u (␶ϩ␲)ϭϪu (␶), and all the odd-numbered coeffi- 0 0 program can handle rational arithmetic. The expansion cients vanish. of u is made in powers of mЈ where each term is a finite Hill’s 1878 paper contains a detailed and complete ac- 0 polynomial in ␨ and ␨Ϫ1 whose order does not exceed count of his work in finding the variational orbit in the the power of mЈ. Each next-higher order follows from ecliptic, zϭ0. The problem is nonlinear because of the the lower-order terms in a standard recursion applied to terms with 1/r3 in Eqs. (78) and (79). The coefficients in Eq. (78). Eq. (82) are expanded in powers of mЈ to exponents that guarantee 15-figure accuracy! Poincare´ (1907) made Hill’s procedure more transparent in his Lectures on Ce- D. The motion of the lunar perigee lestial Mechanics. With the help of the Jacobian integral, Hill was able to manage his computations in rational The real trajectory of the Moon is now constructed as arithmetic, and in such a way that he gained a factor mЈ4 a small displacement from the variational orbit. To at each step. make the distinction, coordinates of the variational orbit

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 622 Martin C. Gutzwiller: The oldest three-body problem will carry the suffix 0, while the displacement in first fashion, starting from Eq. (84). The discussion of ‘‘Hill’s order carries the suffix 1. The perigee and the node will equation’’ in this case is simpler, and the origin of ‘‘Hill’s be treated simultaneously because they lead to almost determinant’’ is easier to understand; but Adams does the same equations. Our presentation will differ some- not get the formula (87) for ⌬. what from the classical work of Hill. It is worth emphasizing the discoveries that are hid- The linearized version of Eqs. (78)–(80) is written as den in this method. First, a periodic orbit is found in 2 some appropriate reference frame that has to be chosen ͑DϩmЈ͒ u1ϩM͑␨͒u1ϩN͑␨͒v1ϭ0, on the basis of physical intuition. Then the dynamic 2 ͑DϪmЈ͒ v1ϩM͑␨͒v1ϩN*͑␨͒u1ϭ0, (83) neighborhood of this orbit is examined by linearizing the 2 equations of motion. The result of this approach is al- D z1Ϫ2M͑␨͒z1ϭ0. (84) ways an equation that reads exactly like Hill’s equation The variational orbit (82) enters these equations (86). through The Moon’s motion at right angle to the ecliptic is 2 2 2 z mЈ 1 3mЈ 3u0 described by a first-order displacement 1 from the M͑␨͒ϭ ϩ 3 , N͑␨͒ϭ ϩ 5 , (85) variational orbit, 2 2r0 2 2r0 ig0␶ Ϫig0␶ iz1͑␶͒ϭe G͑␶͒Ϫe G*͑␶͒, (88) where the complex conjugate N* has v0 replacing u0 . Hill’s work is concerned with the two equations (83); where G is also complex valued, satisfying the periodic- we shall not discuss their solution in detail, but explain ity conditions G(␶ϩ␲)ϭG(␶). We assume the expan- briefly in the next section how to solve the simpler equa- sion tion (84) in a more direct manner. The differential equa- ϩϱ tions (83) and (84) are linear with coefficients that are 2j G͑␶͒ϭ ␬j␨ , (89) periodic functions of the independent variable ␶; their ͚Ϫϱ general discussion goes back to Floquet (1883). The where the coefficients are real. The function ⌰ in Hill’s trivial solution (Du0 ,Dv0,0) is eliminated by consider- ing only the displacement w in the ecliptic that is locally equation is now the function M(␨) in Eq. (85) from at a right angle to the variational orbit. It satisfies ‘‘Hill’s which one gets the coefficients ␪j . equation,’’ The coefficients in Eq. (89) can be viewed as a vector ϩϱ ␬, and Hill’s equation is easily reduced to a system of 2 2j linear equations, D wϭ⌰w, with ⌰ϭ ␪j␨ and ␪Ϫjϭ␪j . (86) ͚Ϫϱ ␪͑g0͒␬ϭ0. (90) The periodic function ⌰ is a rather complicated expres- The infinite matrix ␪(g0) has the same structure as the sion in terms of the variational orbit (82). This oscillator matrix in the preceding section that led to the determi- is driven parametrically with the frequency 1, but it re- nant ⌬(c0). The motion of the lunar node is obtained by sponds with the different frequency c that gives the mo- 0 requiring that the determinant of ␪(g0) vanish. In terms tion of the perigee. of the angles g and h, we get the more familiar expres- By a daring maneuver, Hill transforms his equation to sions for the mean motion of the perigee and of the an infinite set of homogeneous linear equations with a node, determinant ⌬(c0) that can be reduced to the simple form c0 g0 g˙ ϩh˙ ϭ 1Ϫ n, h˙ ϭ 1Ϫ n. (91) ͩ 1ϩmЈͪ ͩ 1ϩmЈͪ 1Ϫcos c0␲ ⌬͑c0͒ϭ⌬͑0͒Ϫ . (87) 1Ϫcos ͱ␪0 F. Invariant tori around the periodic orbit

The solution of ⌬(c0)ϭ0 is thereby reduced to an ex- pansion of ⌬(0) in powers of mЈ4. The motion of the Condition (90) for the coefficients in the displacement z1 leaves an arbitrary constant to be determined for the perigee c0 is obtained to very high accuracy. Poincare´ remarks in his preface to Hill’s Collected Papers: ‘‘In this motion (88) at right angle to the ecliptic. This parameter work, one is allowed to perceive the germ of most of the plays the same role as an and is clearly progress that Science has made ever since.’’ related to the ordinary inclination ␥. The term jϭ0in Eq. (89) by itself describes the essential feature of the motion at right angle to the ecliptic, so that ␬0 can be E. The motion of the lunar node taken as a measure of the effective inclination ␥;it serves as the initial condition in Eq. (84). The first application of Hill’s method obviously con- In the same manner, the condition ⌬(c0)ϭ0 with Eq. cerns the motion of the lunar node. The luckless Adams, (87) leaves us with the choice of one real number that who was a close second to Leverrier in the discovery of determines the scale of the displacement u1 in the eclip- the planet , published a short notice in 1877 af- tic. The number is equivalent to the effective eccentric- ter reading Hill’s great paper. He had obtained the mo- ity ␧ of the lunar orbit, and acts like an initial condition tion of the node some years before in exactly the same for the equations of motion (83).

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 623

The multiplicity of solutions for the lunar problem can The coefficients Apqrs are real polynomial series in be given a geometric interpretation. The ‘‘flow’’ of the the variables ␧, ␥, ␧Ј, ␣ϭa/aЈ, lunar trajectories is embedded in a phase space of six dimensions, three for the Cartesian coordinates and abcd a b c d Apqrsϭ ͚ Cpqrs ␧ ␥ ␧Ј ␣ . (94) three for the components of the momentum. This six- abcd dimensional space is naturally reduced to the five- The coefficients C depend on the frequency ratio mЈ as dimensional surface of constant energy by the Jacobian well as on the mass ratios M/E and (EϩM)/S and sat- integral (81). isfy the inequalities The most general trajectory for the Moon is given in aу p , bу q , cу r . (95) lowest approximation by the variational orbit (82) in the ͉ ͉ ͉ ͉ ͉ ͉ ecliptic, with the displacements (86) and (88). While the The expression ␮ϭ␧a␥b␧Јc␣d is called by Brown the variational orbit is covered in one synodic month corre- characteristic of a particular term in Eq. (93). The lowest sponding to the time variable ␶, the anomaly l ϭc0␶ characteristic that is compatible with a given set of ex- runs with the anomalistic month, and the argument of ponents (p,q,r) is its principal characteristic. The successive terms in Eq. (93) can be regarded as latitude Fϭg0␶ runs with the draconitic month. The re- sult is a three-dimensional torus surrounding the varia- higher-order displacements from the variational orbit. tional orbit, all embedded in a five-dimensional space. Each one can be determined by insertion into Eqs. (78)– (80), which have to be augmented with the additional The speeds c0 and g0 at which any trajectory itself around the variational orbit are independent of the terms of the solar perturbations that were left out of Eq. (75). The characteristics in Eq. (93) provide a natural eccentricity ␧ and inclination ␥. They do not vanish as ordering in which the variational orbit represents the the torus winds itself ever more closely around the varia- order 0, while the displacements u and z are the first tional orbit. The higher-order corrections change noth- 1 1 order; they determine the higher orders. ing in this first-order picture. Hill’s theory gives a com- For each higher order an equation such as (83) and plete description of the flow in phase space near the real (84) is found, in which the new term in Eq. (93) appears lunar trajectory. It is less than what the ambitious earlier on the left-hand side, whereas the relevant terms of theories were trying to find, since it covers only a rela- lower order appear on the right-hand side. Therefore tively small part of the phase space for the original prob- the mechanical analog is now an externally driven para- lem (29). metric oscillator; its frequency is determined by the combination of zeta’s. Instead of Eq. (90) one gets an G. Brown’s complete lunar ephemeris inhomogeneous linear equation such as

Ernest William Brown (1866–1938) came from his na- ␪͑pc0ϩqg0ϩrmЈ͒␩ϭQ, (96) tive England in 1891 to teach mathematics at Haverford where g0 has been replaced by a linear combination with College, where he wrote his much appreciated Introduc- integer coefficients, pc0ϩqg0ϩrmЈ. The vector Q on tory Treatise on the Lunar Theory (Brown, 1896), and the right-hand side arises from the right-hand side in the the long series of papers (Brown, 1896–1910) to expand modified equations (83) and (84). Hill’s work into a complete description of the Moon’s Solving the equations for the lunar problem can now motion. In 1907 he went to , mainly be- be organized quite systematically according to the char- cause of promised support for the computing and pub- acteristics, and it is always clear before beginning the lishing of his lunar tables, and he eventually became the process of computing the right-hand sides in Eq. (96) first J. W. Professor of Mathematics. The tables which characteristics of lower order are able to contrib- finally appeared in 1919 and became the base for the ute to the given characteristic ␮, and how. The actual calculations of all the national ephemerides after 1923; solution at each step requires no more than solving an see the biography by Hoffleit (1992). inhomogeneous linear equation with always the same The complex notation (77) is now completed by the matrix. Even for high-accuracy work, the number of vec- definitions, tor components at any one step is no more than 20 be- i␶ ic0␶ϩl 0 ig0␶ϩF0 cause the functions (85) converge like power series in ␨ϭe , ␨1ϭe , ␨2ϭe , mЈ␨. imЈ␶ϩl Ј ␨3ϭe 0, (92) The matrix ␪ in Eq. (96) is regular unless, because of Eq. (90), qϭ1 and pϭrϭ0 in the argument of ␪. Such a where the new complex variables are associated with the thing can happen when two terms of comparatively high effective eccentricity ␧, the effective inclination ␥, and characteristic meet on the right-hand side of the equa- the eccentricity ␧Ј of the Earth-Moon’s orbit around the tions of motion (84). The system will respond with a Sun. The complete expansion of the lunar coordinates shift in frequency ␦g , and the required corrections of now becomes 0 the frequency can be worked out. Ϫ1 ␨ u p q r s ϭ Apqrs␨1␨2␨3␨ , (93) H. The lunar ephemeris of Brown and Eckert ͩ iz ͪ pqrs͚ Ϫ1 where even powers of ␨2 go with ␨ u, while the odd When Brown started to work on the new lunar theory powers of ␨2 go with iz. based on Hill’s ideas, he stated his aim very clearly. The

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 624 Martin C. Gutzwiller: The oldest three-body problem accuracy was to be such ‘‘that the coefficients of all peri- ucts. There are 985 terms from the solar perturbation odic terms in longitude, latitude, and parallax shall be and 642 additional terms for the planetary perturba- included which are greater than 0Љ.01, and that they shall tions, where all of the latter are smaller than 1 arcsec- be correct to this amount. The number of terms required ond. This ‘‘Improved Lunar Ephemeris’’ became the ba- is undoubtedly very great. The calculation of coefficients sis for the . up to sixth order inclusive with respect to the lunar eccen- Eventually Eckert decided to redo all of Brown’s tricity and inclination will be necessary; those of the sev- work with the algebra done on one of IBM’s all-purpose enth order may be replaced by their elliptic values.’’ electronic computers. After his retirement in 1967, he Hansen’s theory, which was in universal use then, was continued his project with the help of just one program- entirely numerical, while Delaunay’s, which was being mer. All terms in Eqs. (93) and (94) including the sixth order were going to be calculated with an accuracy of completed to include the effects of the planets, was en- Ϫ12 tirely algebraic. Brown (1896–1910) made an important 10 . After Eckert’s death in 1971, I accepted the job of compromise: the ratio mЈϭnЈ/(nϪnЈ) of the mean mo- seeing the work to its conclusion. A detailed comparison tions would be replaced by its numerical value from the with related work (Gutzwiller, 1979) showed that the very start; its value was extremely well known and very expected accuracy had been achieved, but the rigid goal unlikely to change due to new observations; any expan- of all sixth-order terms was unrealistic because of the sion in powers of mЈ seemed to require the most incredible proliferation of minute terms without practi- troublesome mathematics, and the results were gener- cal significance. ally no guarantee of improvement in the general accu- Dieter Schmidt (1979, 1980a) of the University of Cin- racy. On the other hand, leaving ␧, ␥, ␧Ј, and ␣ as alge- cinnati proceeded independently to redo Brown’s calcu- braic quantities was part of the whole development; lations with the help of his own program for the alge- their values were more subject to revision, and keeping braic manipulation of large trigonometric series. With them algebraic would facilitate the comparison between the large increase in the accuracy of the lunar data, it seemed natural to include all the terms larger than different theories. Ϫ12 The resulting Tables of the Motion of the Moon (1919) 10 , independent of their order. Since Schmidt’s re- are the last of their kind; the first tables of similar but sults could be directly compared with the results of Eck- much simpler design are found in Ptolemy’s Almagest. ert’s project, the two works were reported together Their use requires only looking up with interpolation, (Gutzwiller and Schmidt, 1986). The conclusions from and addition of numbers in order to get the coordinates this state-of-the-art computation will be discussed in the of the Moon in the sky for any instant of time. A lot of next section. goes into making the task of the computer, a human being at that time, straightforward and reliable. XI. LUNAR THEORY IN THE 20TH CENTURY In 1932 proposed the use of mechanized bookkeeping machines for the purposes of celestial me- A. The recalcitrant discrepancies chanics. This project was realized by Wallace J. Eckert, professor of astronomy at , who es- The last chapter in the third volume of Tisserand’s tablished in 1933 the T.J. Watson Astronomical Com- Treatise on Celestial Mechanics (1984) deals with the puting Bureau with the help of IBM’s chairman. During ‘‘Present State of Lunar Theory’’; it does not cover the second World War, Eckert was the director of the Brown’s Complete Ephemeris and the finishing touches U.S. Nautical Almanac Office at the Naval Observatory by Eckert, nor the critical evaluation of Poincare´. Nev- in Washington, D.C., where he took special pride in de- ertheless, Tisserand’s conclusions are more farsighted signing the first Air Almanac that was produced without than he might have expected: he acknowledges some human intervention and presumably free of printing er- fundamental difficulties and hopes for a major discovery rors. After the war, he became the first Ph.D. to be hired without knowing where it could come from. by IBM and was asked to run its first research labora- The general agreement between Hansen’s tables of tory, to be located at the campus of Columbia Univer- 1857 with the work of Delaunay made it obvious that sity. the three-body problem of Moon-Earth-Sun had finally Meanwhile it had become clear that, while the been solved to the required precision. The most recent claimed accuracy had indeed been achieved in Brown’s improvements of lunar theory by Hill and Adams fully original papers, the tables did not quite live up to expec- supported the work of Hansen and Delaunay on the tations. In the process of making the results usable for main problem, i.e., on the motions of the pure three- the practical calculation of ephemerides, a number of body system. The work of Newcomb on this important short-cuts had been adopted that reduced their preci- question can be followed in Archibald’s bibliography sion. In 1954, Eckert and his collaborators published a (1924). Moreover, Hansen reported complete agreement careful list of all coefficients in the expansion (93) that with the observations from 1750–1850 to within errors Brown had obtained and converted them directly to of 1Љ to 2Љ at most. While this claim seemed correct, it similar expansions for the lunar longitude, latitude, and came as a big surprise that the agreement deteriorated parallax. In using this list, however, the computer was almost immediately to the point where some of the er- expected to multiply each coefficient with its appropri- rors increased from 5Љ by 1870, to 10Љ by 1880, and 18Љ ate trigonometric function, and then add up all the prod- by 1889.

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 625

The sources of these discrepancies are not easy to pin observations. The various sections of this Section are down. A major source of uncertainty arises from the intended to give the reader a brief glimpse of some of acceleration of the Moon’s motion, which was discov- the efforts in this area. ered by Halley and then explained by Laplace. But a Meanwhile the three-body problem in its full general- careful recalculation by Adams produced a value only ity is still with us and occupies many volumes of papers half of what was required by the observations. A judi- in learned journals. Modern computers are the main re- cious change in the parameters and in the initial condi- source for exploring all the possibilities of the general tions is not sufficient; the perturbations by the planets theory; there are many applications in the solar system are not nearly large enough to bring the errors down. and even some outside. But the Moon-Earth-Sun system Finally an empirical inequality with a period of 273 years is no longer among the active if one looks at the was recommended, without any attempt to explain its scientific literature. size or even its period. In 1923 the Tables of Brown became the generally B. The Moon’s secular acceleration accepted basis for the computation of lunar positions, but their accuracy suffered a serious blow during the In 1693 Halley compared the dates of some well- of January 24, 1925, mentioned in Sec. documented lunar eclipses: the longitude of the Moon could not be written as a linear function of time plus any III.E. The path of totality crossed the upper half of number of sinusoidal corrections. A quadratic term in Manhattan, with the southern boundary of the shadow time was necessary to achieve a decent fit. Empirically, at 96th Street. Consolidated , New York City’s the lunar motion was found to suffer a small accelera- electric utility company, posted 149 pairs of observers on tion. the roofs along Riverside Drive on every block, in order The longitude of the Moon can be represented sche- to determine the exact line of the Moon’s shadow (see matically by the formula TIES, 1925). Only one observer expressed doubt about the Sun’s being covered by the Moon on his location. ␭ϭ␭ ϩntϩ␴͑t/c ͒2ϩ A sin͑␣tϩ␤͒, (97) The line could be pinned down within 100 meters. 0 Jul ͚ carried a headline on the next where cJul designates one Julian century of 36525 mean day: Where was the Moon? It turns out that the Moon solar days. The sum is extended over all the known pe- was four seconds late, not much by the standards of a riodic perturbations, from the Sun, the other planets, the casual observer, but much more than the accuracy that shape of the Earth, and so on. The value of ␴ was de- Brown had tried to achieve. An error of 4 time-seconds termined during the 18th century by various astrono- translates into an error of more than 2 arcseconds in mers to be about 10 arcseconds. longitude, roughly speaking. The precision of the shad- The glory of discovering the main cause belongs to owline, however, translates to an accuracy of better than Laplace (1787), who attributed it to the change in the 0.1 arcsecond in latitude. Actually the profile of the eccentricity ␧Ј of the Earth’s orbit. The formal calcula- Moon varies by a few kilometers (Bailey’s beads) with tion is based on the second-to-last term of the first line 2 its exact orientation, and that may have been part of the in Eq. (53), i.e., the term proportional to ␧Ј in the per- problem with the longitude. turbation W. The Astronomical Almanac for 1995 gives With the arrival of computing machinery in the 1930s only the very rough formula ␧Јϭ0.01671 043 the processing of data and the preparation of ephemeri- Ϫ0.00000 00012d, where d, is the interval in days from des could be accomplished without the incredible drudg- 1995 January 0, 0 hours. The coefficient of the quadratic ery of earlier times. Celestial mechanics received an term in Eq. (97) is found to be 10.66 arcseconds. enormous impulse in the late 1950s from the sudden Adams (1853, 1860) pursued this calculation to in- clude higher corrections in the powers of m and found push for a large program of . Lunar that all the higher powers in m decreased the value of theory in particular profited from President Kennedy’s the coefficient ␴ to 6.11Љ, too small by a factor of 2. It decision to have humans visit the Moon by the end of also became clear that the rotation of the Earth is bound the 1960s. Indeed, there followed a prolific outpouring to slow down because angular momentum gets trans- of interesting work, which will be reviewed somewhat ferred to the Moon due to the braking action of the summarily in this Section. tides. Part of the lunar acceleration is an ‘‘optical illu- At the end of the 1990s it has to be admitted, how- sion’’ because it is really due to the slowing down of the ever, that Tisserand’s diagnosis is still valid. The main Earth’s rotation. problem of lunar theory has been completely solved for When the Moon acquires additional angular momen- all practical purposes, but there have been no major dis- tum, its potential energy with respect to the Earth in- coveries in getting a more direct analytical approach, creases, whereas its decreases. The Moon and we are still wrestling with uncertainties in the com- goes into a higher orbit where its speed with respect to parison with the observations of almost 1 arcsecond. the fixed stars decreases while its apparent speed with There is a long list of difficulties outside the strict con- respect to the Earth increases. The slowing down of the fines of the three-body problem that have to be taken Earth’s rotation is well documented in prehistoric (geo- into account in order to improve the agreement with the logic) times. The trouble in historic times is the irregular

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 626 Martin C. Gutzwiller: The oldest three-body problem nature of this process, which finally forced the transition ‘‘about ten days’ work suffice for the elaboration’’ of from the mean solar day to the mean solar year as the Jupiter’s action on the Moon. But the 16 large pages of standard scale of time. arithmetic in Hill’s Collected Mathematical Works do not The ancient observations of eclipses, both lunar and look very encouraging! solar, have been discussed with ever increasing sophisti- cation for the purpose of determining the secular accel- eration of the Moon as well as the rotation of the Earth; D. Symplectic geometry in phase space some of the more recent work is due to Spencer-Jones (1939), R. R. Newton (1970, 1979), Morrison (1978), and Whereas the Lagrange-Poisson method provides a Stephenson (1978). Modern measurements are reported modicum of physical insight at the price of some con- and critically assessed in monographs like that of Munk ceivable confusion, the canonical transformations on top and MacDonald (1960), collections of special articles of Delaunay’s work are straightforward. The real chal- like those of Kopal (1961) and Brosche and Su¨ nder- lenge comes with extending the Hill-Brown results for mann (1978), and Conference Proceedings like those ed- the main problem of lunar theory to cover the perturba- ited by Marsden and Cameron (1966), Calame (1982), tions due to the Earth’s shape and due to the other plan- and Babcock and Wilkins (1988). The data are quite ets. confusing to the layperson (see Van Flandern, 1970). The difficulty arises from Brown’s decision to insert the numerical value of mϭnЈ/n from the very begin- C. Planetary inequalities in the Moon’s motion ning, rather than to expand in powers of m as Delaunay and Hill did. The Moon’s semi-major axis a determines The presence of the other planets in the solar system her mean motion n in longitude according to Kepler’s acts as an external disturbance. Their effect on the mo- third law, and that parameter enters the solution of the tion of the Moon is weak, so that they are generally main problem through m. The equations of motion, ei- investigated only after the main problem has been ther in the Lagrange-Poisson form [Eqs. (45)–(50)] or in solved. Three ways to go about this task will be dis- the canonical form [Eq. (63)], require the derivatives of cussed briefly in this section: the variation of the con- the perturbation with respect to a. How can we find the stants of Lagrange and Poisson of Sec. VIII, the canoni- derivative with respect to a parameter when only its nu- cal formalism of Hamilton and Jacobi of Sec. IX, and merical value is present in the solution? the method of Hill and Brown that was explained in Sec. Since the action L is directly related to the semi- X. major axis a by Eq. (62), the derivative with respect to L It is unlikely that the Moon exerts any kind of pertur- can be used. Brown (1903, 1908) invokes the symplectic bation on the planets of the solar system with the excep- nature of the trajectories in phase space to obtain the tion of the Earth; nor is the direct perturbation of the required derivatives of his solution with respect to the planets very effective. But Newton’s proposition LXVI actions L,G,H. Unfortunately, this most imaginative (see Sec. VI.E) reminds us that not only the planets, but application of the symplectic geometry of classical me- the Sun itself has to respond to gravitational forces. chanics in phase space is not generally known. Meyer Therefore we have to take into account the of and Schmidt (1982) have given a much clearer and the Sun to the motion of the planet in its Kepler ellipse. shorter version of Brown’s original explanations. As a particularly instructive example, Jupiter with Brown’s method can be understood from looking at 1/1000 is about 1000 solar radii away, so that the special case in which we know the derivatives of the the center of mass Sun-Jupiter lies just outside the Sun. trajectory with respect to G,H,l ,g,h, but have used a This leads to a noticeable indirect effect on the Moon. numerical value for L. We write down explicitly the The first method is similar to the discussion in Sec. Poisson bracket, ͒ ˙x,x͑ץ ͒ ˙x,x͑ץ xץ ˙xץ ˙xץ xץ VIII.F concerning the effect of the Earth’s shape on the Moon’s orbit. The main job is to write down the energy ͓x,x˙ ͔ϭ Ϫ ϩ ϩ ϭ1. H,h͒͑ץ G,g͒͑ץ lץ Lץ lץ Lץ ,of the gravitational interaction between the Moon and say, Jupiter using the expansions of the Moon’s longi- (98) tude, latitude, and parallax in terms of the angles After dividing this equation with the known function l )2, the first two terms become simply the timeץ/xץ) l ,g,h,l Ј from Delaunay’s work. The mass and the orbit l . The value ofץ/xץ L divided byץ/xץ of the planet are assumed to be known; its Kepler pa- derivative of L is then obtained by integrating over time; theץ/xץ rameters are kept constant in first approximation. The planetary terms in addition to the solar perturbation ⍀ constant of integration is determined by the required are inserted into Lagrange’s equations (45)–(50). symmetries and the other Poisson brackets. By the end of the 19th century it became clear that the Brown used the same idea to check the internal con- method of canonical transformations is by far the most sistency of his solution. The Poisson brackets are more efficient. Therefore the work of Delaunay on the solu- demanding than it appears at first, because their right- tion of the three-body problem is the prerequisite. The hand sides are constants. If the time derivatives are mul- planetary terms in the Hamiltonian become the object tiplied out, all the periodic terms have to cancel out. of some further canonical transformations, in von This check was also carried out on the computer by Zeipel’s or any other form. Hill (1885) claims that Schmidt (1980).

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 627

E. Lie transforms the 1960s and early 1970s provided the incentive for many celestial mechanicians to improve the available As electronic computing became more efficient in the calculations of the Moon’s motion. It is important to 1960s, and financial support for lunar theory became distinguish two types of solutions for the Moon’s trajec- available, it was natural to emulate Delaunay’s great tory. So far we have discussed almost exclusively the feat with different means. A first try by Barton (1966) on main problem as defined at the end of Sec. VII.C, and its the basis of a straightforward repetition, however, only solution was always represented as a Fourier series in got as far as calculating the perturbation function with the four angles l ,g,h,l Ј. They will be called ‘‘analyti- the addition of ninth- and tenth-order terms. The ca- cal’’ solutions to distinguish them from the ‘‘practical’’ nonical transformations themselves, even in von Zeipel’s solutions that are represented by a direct numerical in- form, did not lend themselves easily to a systematic pro- tegration of the equations of motion. The latter include cedure that could be programmed. The expressions in right away the effect of all the other perturbations on the second line of Eq. (66) have to be inverted. In many the Moon besides the influence of the Sun. practical cases this inversion depends on the possibility A first effort went into completing some work that of expanding F as well as the Hamiltonian of the system Airy (1889) had left unfinished and that had been criti- in a power series with respect to a small parameter ␮. cized by Radau (1889). The idea is simple: the best avail- The method of Lie transforms avoids the inversion by able solution is substituted into the relevant equations of aiming directly at the transformation that represents the motion, and the necessary corrections are determined by old angles y and old actions Y in terms of the new angles varying the coefficients in the expansion of the solution. x and new actions X. The transformation formulas are Eckert and Smith (1966) started from Brown’s solution power series in ␮ where the zero order is the identity. and achieved residuals in the 13th to 15th decimals by The Lie derivative LFf of a function f(y,Y) with respect solving 10 000 equations of variation. Although the best to a generating function F(y,Y) is the Poisson bracket computers of the time were run for several hundred ͓F,f͔, and plays an essential role. The formalism is hours, the report of the results still creates the impres- worked out in a fundamental paper by Deprit (1969). sion of a tremendous effort in manual labor. The given Hamiltonian ⍀(y,Y;␮,t) is transformed The next great enterprise was already mentioned at into a particularly simple form such as ⌶(x,X;␮,t). The the end of Sec. IX because it tries to obtain a solution new Hamiltonian ⌶ may be required to have no more that is an improvement on Delaunay’s classic work. It terms in the angles x, at least up to a certain power of ␮. was called Analytical Lunar Ephemeris (ALE) by its At each step in the recursion, the function F(y,Y) has creators at the Boeing Scientific Research Laboratories, to satisfy a first-order partial differential equation. This Deprit, Henrard, and Rom (1970, 1971c). Regrettably, new scheme becomes a straightforward algorithm that only certain parts of it have been reported in the scien- can be programmed very efficiently. Once the generat- tific journals, presumably because the total output of ing function F is known, the same algorithm can be used data is too large. The method of Lie transforms was used to express the old action angles (y,Y) in terms of the to get a completely algebraic solution in all the param- new ones (x,X). eters with all the coefficients as rational numbers. The Many different detailed procedures were worked out work of Delaunay can now be used with confidence in the late 1960s and early 1970s to realize and apply the since the corrections are easily accessible. Lie transforms in celestial mechanics, as well as to prove Among the fascinating results is a short note by De- their equivalence. Hori (1963) seems to have been the prit and Rom (1971) dealing with the long-period term first to try a new approach to lunar theory; see Stumpff in the Moon’s longitude. It arises from the combination (1974) for a more recent account. In particular, the re- 3␭Ϫl Ϫ2F, whose period can be found from the figures lations between the formalisms of Hori (1966, 1967) and in Sec. V.C to be 183 years. It leads to a denominator of Deprit (1969) are treated by Kamel (1969), Henrard that Poincare´ called ‘‘nume´riquement tre`s petit’’ in con- (1970), and Jefferys (1970), Mersman (1970, trast to the term ‘‘analytiquement tre`s petit’’ that led to 1971), Henrard and Roels (1973), Rapaport (1974), and the disaster at the end of Sec. IX. The longitude was Stumpff (1974). One application by Deprit and Rom found to contain the term (1970) that is closely related to the lunar problem con- cerns the main problem of satellite theory: a satellite 315 m␣␧␥2␧Ј sin͑3lЈϪlϪ2Fϩ3D͒, (99) circling the Earth is subject to the perturbation of the 128 Earth’s quadrupole moment. The most striking result of which Laplace thought ‘‘quasi impossible’’ to predict this method, however, is the complete recalculation of from the theory. Its amplitude turns out to be com- Delaunay’s theory by Deprit, Henrard, and Rom pletely negligible. (1971a), to be discussed in the next section. A project that is related to ALE was conceived by Henrard (1978, 1979) under the name of Semi- F. New analytical solutions for the main problem of lunar Analytical Lunar Ephemeris (SALE). It starts with a theory completely analytical solution of Hill’s problem, i.e., the lunar trajectory in three dimensions if the Sun’s pertur- The by the U.S. National bation is reduced to its average quadrupole field [Eq. Aeronautics and Space Administration (NASA) during (75)] in the neighborhood of the Earth. The solar eccen-

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 628 Martin C. Gutzwiller: The oldest three-body problem tricity ␧Ј and the ratio of the semi-major axes ␣ϭa/aЈ of the Earth’s equatorial radius over the distance of the are neglected in this first step. Their effect is then Moon, but this ratio becomes an angle if it is set equal to treated as a perturbation to the fully algebraic solution the sine of an angle.) of Hill’s problem. The numerical agreement with ALE is The cutoff for the listing in ELE was chosen at excellent. 0.000005Љ for the longitude and latitude, which allows A completely independent approach was first con- correct rounding to the fifth decimal. [The long-period ceived by Chapront-Touze´ (1974, 1980) at the Bureau inequality (99) of Laplace barely makes the grade with a des in Paris. The solutions are assumed to be coefficient 0.00000592Љ.] This threshold is to be com- the trigonometric series in the relevant angles, each with pared with the largest terms, 22639.55Љ sin l in longitude its own rate of change that has to be determined. The and 18461.40 sin F in latitude. They are generally agreed basic program has to manipulate these large series and as the best fit to the observations and effectively define then match their coefficients. In contrast to the projects the eccentricity ␧ and the inclination ␥ (see the discus- mentioned so far, this effort has been pursued system- sion in Sec. X.F.) The cutoff for the parallax is atically to yield a complete ephemeris for the solar sys- 0.00000 01Љ and has to be compared (in this cosine ex- tem under the abbreviation ELP 2000 because the vari- pansion) with the constant term 3422.452Љ, which is ous constants were adjusted to that epoch (see the again generally agreed to be the best observed value. further discussion in Sec. XI.J). The planetary perturba- The cutoffs were chosen somewhat differently in tions, the effect of the Earth’s and the Moon’s shape, SALE and ELP 2000, so that the total number of terms and even relativistic corrections are all taken into ac- in the expansions for the polar coordinates is not exactly count, but the long-period term [Eq. (99)] is making the same. For SALE, ELP 2000, and ELE there are trouble! 1177, 1024, and 1144 terms in longitude, 1026, 918, and Finally, there is the work of Eckert that was men- 1037 terms in latitude, as well as 669, 921, and 915 terms tioned at the end of Sec. X; it was based on Brown’s in parallax. The agreement between ELP 2000 and ELE development of Hill’s approach to the lunar problem. is practically perfect, with 1 coefficient in longitude dif- Since the Lunar Laser Ranging (LLR) allowed the lunar fering by 2 in the last (fifth) decimal, 1 coefficient in distance to be measured with a precision approaching a latitude and 3 in sine parallax differing by 1 in the last few centimeters out of 400 000 kilometers, or 10Ϫ10,it decimal. The agreement with SALE in the last decimal seemed reasonable to Gutzwiller and Schmidt (1986) has only a few more discrepancies. This almost complete that all terms in the expansion be calculated down to coincidence between three large data sets that are based that level and be correct to 12 significant decimals. Such on entirely different computations shows that the main a requirement could be met if the calculations were problem of lunar theory is solved correctly. made with so-called ‘‘double precision,’’ which guaran- The reader might be interested in the distribution of tees at least 14 decimals. The result would be called the coefficients c in the Fourier expansions according to ELE for Eckert’s Lunar Ephemeris. their size. Each bin is defined by its leading decimal As the work proceeded beyond Eckert’s original when c is written in seconds of arc. For the lunar longi- plans, however, the term (99) made its appearance with tude and latitude we have, starting with log10 cу4, a magnification by a factor of 2000 because of its small through 4Ͼlog10 cу3, all the way down to Ϫ4Ͼlog10 c denominator. It was necessary to use ‘‘extended preci- уϪ5; for the sine parallax the counts are shifted down- sion’’ and lower the cutoff to a level of 10Ϫ17, which ward by one bin. The number of coefficients in these would correspond to a distance of a few nanometers on bins are: for the longitude (1, 2, 10, 14, 32, 56, 192, 154, the Moon. That is physically quite absurd and betrays a 268, 376), for the latitude (1, 1, 5, 7, 31, 49, 94, 151, 220, mild form of chaos even in the Moon’s motion. The au- 357), and for the sine parallax (1, 1, 3, 4, 19, 28, 55, 96, (Gutzwiller, 1979) showed, in the case of Eckert’s 140, 238, 328) with one additional bin. original work, that the terms below a certain threshold Although these counts cover 10 powers of 10, no create a noise whose root-mean-square of the amplitude simple model for the proliferation of terms in the Fou- is almost 10 times the threshold. rier series seems to work well. If the series are truncated by retaining only the terms with a coefficient above some threshold, the neglected terms generate a noise whose root-mean-square is larger than the threshold G. Extent and accuracy of the analytical solutions roughly by a factor 10 (Gutzwiller, 1979). The data were taken from the tables of Gutzwiller and Schmidt (1986), Various modern methods for solving the main prob- probably the last and most accurate record ever avail- lem of lunar theory were described in the preceding sec- able in print. tion. Whereas SALE and ELE are at least partially ana- lytic, ELP aims directly at finding the Fourier expansion H. The fruits of solving the main problem of lunar theory of the lunar trajectory with purely numerical coeffi- cients, not unlike Airy’s method (see Eckert and Smith, Artificial satellites for the Earth and other planets, as 1966). All three calculations eventually yield the expan- well as a visit to the Moon by human beings, became a sions for the polar coordinates of the Moon: longitude, reality shortly after lunar theory had arrived at a suffi- latitude, and sine parallax. (The sine parallax is the ratio ciently accurate and trustworthy solution of the three-

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 629 body problem Moon-Earth-Sun. This sequence of events In 1693 Giovanni Domenico published the fol- was no accident, of course; the scientific achievements in lowing three laws: celestial mechanics and the technical progress in more (i) the Moon rotates at a constant of mundane fields had grown simultaneously to the point one full rotation per sidereal month; where they could be joined in one big adventure. But (ii) its axis is inclined by 2°30 with respect to the nor- nowadays the of general interest has completely mal of the ecliptic (more exactly by 1°31 only); shifted away from dealing with the complicated motion (iii) the direction of the Moon’s axis, the normal to of the Moon; it is now concerned with the physical con- the ecliptic, and the normal to the Moon’s orbit lie in stitution of our companion in space. one plane. Nevertheless, the astronauts of the Apollo program In 1764 Lagrange won the prize of the French Acad- did some important work that is directly related to the emy in the competition to explain the libration of the Moon’s motions. They left behind three ‘‘retroreflec- Moon, but he succeeded only in explaining the equality tors’’ that form a triangle with sides of 1250, 1100, and of the mean motions in translation and rotation. He 970 km. The laser light that is sent to the Moon with the came back to the problem in 1780 to explain the cou- help mostly of the McDonald Observatory’s 2.7-meter pling of the axes. Modern versions of this theory have telescope (Silverberg, 1974) illuminates a spot of about 5 been offered by Koziel (1962) and Moutsalas (1971), km in diameter. The reflectors send this light back ex- and further work presented in the volume edited by Ko- actly where it came from so that their signal is 10 to 100 pal and Goudas (1967), as well as that edited by times stronger than the reflected intensity from the lunar Chapront, Henrard, and Schmidt (1982). surface. By pulsing the light at rates of nanoseconds the With several reflecting telescopes on Earth sending distance of the reflectors can be measured with an accu- laser pulses, not only the distance but also the relative racy of a few centimeters, a 10Ϫ10 fraction of the lunar orientations of the Earth and of the Moon can be deter- distance. Two reflectors that were left on the Moon by mined. The centers of mass can be found and the figure unmanned Soviet vehicles returned the light only for a of the Earth can be checked out. The Moon is almost a few days, possibly because of the from their vehicle. in contrast to the Earth. Not surprisingly, the The reports from this program, such as the one by rotational motion of the Earth is more complicated and Bender et al. (1973), make fascinating reading. Among of greater interest for us earthlings. A vast amount of the scientific objectives that were perceived in 1964 and work is reported in the scientific literature that ties in that led to this Lunar Laser Ranging (LLR) experiment, with LLR and with the dynamics of the Earth-Moon is listed in first place ‘‘a much improved lunar orbit,’’ in system generally [see the collections of articles edited by third place the ‘‘study of the lunar physical ,’’ McCarthy and Pilkington (1979); Fedorov, Smith, and and in fifth place ‘‘an accurate check on gravitational Bender (1980); and Calame (1982)]. theory.’’ This report already mentions a number of im- All these articles deal with problems of great technical portant advances in these three areas. French scientists sophistication and are quite different in spirit from dis- were involved in the whole endeavor from the beginning cussions of the Earth-Moon system like those collected (see Calame, 1973, and Orszag, 1973). by Marsden and Cameron (1966). Although the rotation This section will give a very short summary of some of the Earth is again the central topic, the aim is an results related to these scientific objectives. The most answer to questions concerning the long-term history obvious improvement concerns the lunar parallax be- and the constitution of our planet. At the same time, cause its earlier measurements were always less direct different views on the are proposed, than those of the longitude and latitude, and also more but the jury is still out on this issue after more than 30 sensitive to the refraction of the Earth’s atmosphere. La- years of deliberation. The Moon looks like an exception ser ranging avoids the triangulation that used to be the rather than a representative of the rule among the sat- foundation for all distances in the solar system. The re- ellites in the solar system. port by Bender et al. (1973) already quotes corrections The last argument in favor of LLR was to check on to the lunar eccentricity, the mean longitude of the peri- gravitational theory. Indeed, the effects of general rela- gee, and the mean longitude of the lunar center of mass. tivity on the motion of the Moon were discussed by de Everybody knows from first-hand experience that the Sitter (1916) immediately after solving Einstein’s equa- Moon always turns the same side toward the Earth. tion for an isolated point-mass; Kottler (1922) gives an Physically, the Moon rotates around its own axis at the early review in the Encyclopaedie der Mathematischen same rate as it moves around the Earth. But the motion Wissenschaften, where Pauli’s review of general relativ- around the Earth is not uniform because of the eccen- ity was published. Among the many papers on general tricity ␧Х1/18 and the variation of Tycho Brahe. The relativity in celestial mechanics, let me cite those of Moon’s orientation, however, is not coupled so strongly Finkelstein and Kreinovich (1976) and Mashhoon and that it follows the direction of the Moon’s center of Theiss (1991), who are particularly concerned with the mass. Moreover, we profit from the inclination of the Moon. Without trying to discuss their results, however, lunar orbit and large parallax. The resulting changes in let me just make two comments of some historical inter- the Moon’s appearance were well understood by the as- est. tronomers in the 17th century; they reveal almost an A glance at a standard textbook on general relativity additional third of the remaining lunar surface. like that of Pauli (1921, 1958) shows that the Schwarzs-

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 630 Martin C. Gutzwiller: The oldest three-body problem child metric adds a short-range force with an inverse- resent the lunar motions from 4000 B.C. to A.D. 8000 fourth-power of the distance to the usual inverse-square although with some significant loss of accuracy far away force of gravitation. That is exactly what Clairaut tried from today. to do in order to fudge the motion of the lunar perigee For more than ten years, however, the ephemerides and get over Newton’s frustrations (see Sec. VII.B). In a used in the various national and for the work completely different context, L. H. Thomas had studied of the space agencies have been based on the numerical the effect of general relativity on the rotation of the integrations from JPL and MIT/CFA. They have been Moon in the early 1920s, a problem he considered rather developed in many steps of refinement ever since the difficult. When he became interested in the spin of the 1970s, with the most recent version under the name DE electron, he remembered his work on the Moon; his fa- 200/LE 200 (for development ephemeris and lunar mous factor 1/2 in the formula for the spin-orbit cou- ephemeris) in a series of papers by Standish (1982), Ne- pling of an electron then appeared without much effort, whall (1983), Stumpff and Lieske (1984), and their col- as he used to say (see Misner, Thorne, and Wheeler, laborators. Some of this work is given in the list of ref- 1972). erences; it deals with difficult issues such as the practical definition of the inertial coordinate system, which can- not possibly be discussed in this review with any kind of depth. Nevertheless, some conclusions have to be men- tioned because they show some of the essential differ- I. The modern ephemerides of the Moon ences between classical and modern astronomy. The Fourier (epicycle) expansions for the polar coor- Progress has not been as fast as one might think, how- dinates in the solar system were invented to make pre- ever, since Calame (1982) still reported substantial dis- dictions before there was any physical understanding. agreements between different numerical ephemerides. That method continued to be useful even after Newton On the other hand, Kinoshita (1982) used numerical in- had shown that the problem is equivalent to integrating tegration for the main problem of lunar theory to check some ordinary differential equations. But the advent of on the Fourier series that were obtained by the theore- electronic computing made it possible to integrate New- ticians. It turned out that ELP, and to a slightly lesser ton’s equations of motion without worrying about degree SALE, essentially live up to their nominal accu- whether the solution has a good Fourier expansion or racy, thus confirming their mutual agreement. Lieske not. Moreover, the size of the problem could be en- (1968) analyzed 8639 observations of the larged to include many more than three bodies without Eros from 1893 to 1966 to get a better value for the mass much increasing the technical difficulties. The main limi- of the Earth-Moon system as well as for the solar paral- tation is the length of time over which the numerical lax. Soma (1985) went through thousands of lunar occul- integration is valid, whereas the Fourier expansion is not tations from the years 1955 to 1980 to check up on ELP so limited. 2000. Standish (1990) examined many sets of optical ob- The space agencies have come to rely on numerical servations to check on the accuracy of DE 200 for the integration because they allow us to handle the realistic outer planets. circumstances, for example, to include the mass distribu- There is no doubt in Standish’s mind, however, that tion in the Earth and the Moon in addition to the effect modern ephemerides for the Moon are best based on of the other planets and relativistic corrections. The re- the LLR measurements (see and Standish, sult is a complete mathematical description of the lunar 1989, 1990). Optical observations are tied to the star trajectory, exactly as it can be observed, not only as a catalogues, which depend on the definition of the equa- mathematical model like the main problem of lunar tor, the ecliptic, and their intersection in the equinox. theory. The various corrections to the main problem had The error in laser ranging depends on only the pulse been worked out to some extent ever since the 18th cen- width and the noise, but does not depend on the orien- tury, but it seems that there are so many complicated, tation of the reference frames. The Moon’s longitude although small, effects that the analytical solutions are with respect to the Earth is good to 0.001Љ, and its mean motion to 0.04Љ/century where one Julian century covers unable to produce the required precision. 9 ´ 1.73 10 Љ. But in the long run there remains an uncer- Jean Chapront and Michelle Chapront-Touze (1982, 2 1983) have expanded their semianalytic solution of the tainty of 1Љ/cty in the secular acceleration. The agree- main problem (see Sec. XI.F) into a full-fledged ephem- ment with the optical observations also suffers from a , ELP 2000, for the Moon. The perturbations due to seemingly irreducible discrepancy of 1Љ. the planets, the shape of the Moon and the Earth, gen- eral relativity were all included, and the result was com- J. Collisions in gravitational problems pared with the of LE 200 from the Jet Propulsion Laboratory (JPL). The same authors Starting with Lagrange and through the first three (1991) also constructed a modern version of lunar tables quarters of the 19th century, it looked as if all the work that are still in the form of trigonometric series, but with in celestial mechanics could be reduced to one general the mean motions corrected with terms up to third and method. Moreover, the source of all the difficulties ap- fourth power in time going beyond Eq. (97). Fewer peared to be hidden in the three-body problem, of which terms are needed in the series, which is still able to rep- the system Moon-Earth-Sun was the best-known ex-

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 631 ample. There was hope that the general methods would belong in this class. The problem is usually treated in provide some very fundamental insights and yield a sat- two dimensions, so that Hill’s motion of the lunar peri- isfactory account of the general many-body problem. gee becomes a special limiting case. This grand illusion was almost realized in the work of The main problem of lunar theory occupies a special (1913). He first showed that triple collisions niche in the space of all three-body problems, and there are not possible in the three-body problem if the total are many other niches of this kind. Since the Moon- angular momentum does not vanish. Then he used the Earth-Sun system is important to us and we have many regularization of the double in the Kepler prob- precise observations, we have learnt a lot about this spe- lem to show that such events do not destroy the smooth cial niche and we have started to explore the others. But behavior of the three-body trajectory and its analytic it is hard to carry our experience from the Moon-Earth- dependence on time as well as on the other parameters. Sun corner, say, into one of the corners. When Finally he was able to construct a general solution that is the mass ratios, the main frequencies, and the initial analytic, i.e., it has a power-series expansion as a func- conditions are modified so as to get from one niche to tion of a timelike parameter (see Siegel and Moser, the other, we shall find out that our solutions have only 1971). A tacit agreement seems to prevail among celes- tial mechanicians, however, that this result is useless be- limited validity. cause the relevant series converge very poorly (Diacu, The most dramatic event in a three-body system is the 1996; Barrow-Green, 1996). ejection of one mass, e.g., the Earth could get a little When two mass points have a near collision they closer to the Sun and thereby provide the Moon with the make U turns around each other. The smooth limit of required energy for escape, from the Earth if not from such an event, in which the two masses head straight for the solar system. Such ejection trajectories are hard to each other, is equivalent to the two bodies’ bouncing off come by, but some people believe that they are every- each other. Three-body collisions are quite different be- where dense in phase space, like a of very low cause there is no way to consider them as a smooth lim- dimension. Indeed, the proliferation of terms in the Fou- iting case. When three bodies nearly collide at the same rier expansion of the lunar motion is not only a big nui- time, their kinetic energy and their (negative) potential sance, but could cover up what is known as dif- energy are very large, although their sum is small. The fusion. scaling properties allow the problem to be reduced to If the motion of the Moon is restricted to the ecliptic, the case where the total energy vanishes. The problem her trajectory can be viewed as filling a two-dimensional gets simplified, but the possibilities are still enormous torus in a three-dimensional phase-space, as long as the (McGehee, 1974, 1975). Earth-Moon planet goes around the Sun on a circle (see Three-body collisions can no longer be excluded in a Sec. X.F.) If this circle is replaced by a more realistic gravitational problem with four or more bodies. When- Kepler ellipse, the torus will breathe with the yearly ever one tries to classify the motions in a many-body rhythm. Every invariant torus divides the phase space system, it is crucial to understand what happens in a into separate pieces, and any chaotic trajectory that collision. If initial conditions or other parameters are starts inside a torus cannot escape. If the Moon is al- changed continuously, the system may run into a colli- lowed to move at a right angle to the ecliptic, however, sion. The two situations on either side of the collision her invariant torus is three-dimensional, while the phase have very little in common if more than two masses col- space gets five dimensions. Now, a chaotic trajectory is lide. Even the two-body collisions in the three-body no longer caught, although the escape in this kind of problem are sufficiently complicated to prevent a com- Arnold diffusion always takes a very long time. plete classification of all three-body trajectories. Poincare´ was the first to make us think along these lines. We have been able to amass a large store of indi- K. The three-body problem vidual investigations in the last hundred years, most of them coming from computers in the last few decades. Surveys of the general three-body problem are very But we are still missing a general approach that allows bulky, e.g., Hagihara’s Celestial Mechanics (1970–1976) us to understand all the different kinds of behavior from in five volumes, where the last four volumes have two one common point of view. parts, each bound separately, and volume V with over The three-body problem teaches us a sobering lesson 1500 pages is entitled ‘‘Topology of the Three-Body about our ability to comprehend the outside world in Problem.’’ More recent monographs like The Three- terms of a few basic mathematical relations. Many physi- Body Problem of Marchal (1990) are heavy on numeri- cists, maybe early in their careers, had hopes of coordi- cal calculations. The motion of the Moon has a signifi- nating their field of interest, if not all of physics, into cant overlap with the much smaller class of the restricted some overall rational scheme. The more complicated three-body problem, where two large masses move situations could then be reduced to some simpler models around each other on a fixed circular orbit, whereas the in which all phenomena would find their explanation. third mass is assumed to be so small that it does not This ideal goal of the scientific enterprise has been pro- interfere with the motion of the two large masses (Con- moted by many distinguished scientists [see Weinberg’s topoulos 1966; Szebehely, 1967). Asteroids, space travel (1992) of a Final Theory, with a chapter ‘‘Two between Earth and Moon, and satellites of binary stars Cheers for Reductionism’’].

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 632 Martin C. Gutzwiller: The oldest three-body problem

Newton saw the motion of the planets in the solar L IX.B action variable conjugate to l system as instances of the two-body problem. The ob- L៝ V.B angular momentum of Moon with respect to served changes in their aphelia and eccentricities as well Earth as in the nodes and inclinations of their orbital planes M V.C Moon ͑mass or location͒ could only be detected after collecting data with instru- M(␨) X.D function in the Moon’s variational equation ments over many years. The Moon is different: any alert individual could watch her motion through the sky and N II.B north pole become aware of her idiosyncracies. Both her varying N(␨) X.D function in the Moon’s variational equation speed and the spread of moonrises and moonsets on the O IV.E center of the planet’s orbit horizon proceed at their own rhythm, which is most P II.B north pole clearly displayed in the schedule of lunar and solar P IV.F perihelion or perigee eclipses. These rather obvious features are the most im- Q II.C reference point on equator or eclipse ͑equi- portant manifestations of the three-body problem and nox͒ became the first objects of Newton’s attention. R VI.E distance from Sun to center of mass Earth- Many physicists may be tempted to see in Newton’s Moon equations of motion and his universal gravitation a suf- S IV.E Sun mass or location ficient explanation for the three-body problem, with the ͑ ͒ details to be worked out by the technicians. But even a SЈ IV.E equant point for planetary motion close look at the differential equations (29) and (30) T V.C synodic month ͑Sun to Sun͒ does not prepare us for the idiosyncracies of the lunar T0 V.C tropical ͑sidereal͒ year ͑equinox to equinox͒ motion, nor does it help us to understand the orbits of T1 V.C tropical ͑sidereal͒ month ͑equinox to equi- asteroids in the combined gravitational field of the Sun nox͒ and Jupiter. This review was meant to demonstrate the T2 V.C anomalistic month ͑radial motion͒ long process of trial and error, including many interme- T3 V.C draconitic month ͑motion vertical to eclip- diate stops, that finally led to the modern lunar theory. tic͒ The reader is expected to make a choice among the V X.B quadrupole potential of the Sun near the many different pictures and explanations. Earth Since the equations of motion (29) and (30) can be integrated on a computer nowadays, an option that was W VIII.C perturbing solar potential not available a generation ago, they are sufficient for X៝ VI.E vector from S to ⌫ some special purposes. The expansions (93) and (94) Z II.B local zenith give the whole story, but only if they are interpreted in a V.C semi-major axis of Moon with respect the light of some specific question or in order to carry to Earth out some particular task. If we want to appreciate the aЈ V.C semi-major axis of Earth-Moon with respect tremendous amount of information in these series, we to Sun are almost forced to fall back on the earlier treatments c X.D Hill’s lowest-order motion of the perigee of the problem. They used a limited and perhaps primi- 0 c XI.B Julian centuryϭ36525 days tive, but also more explicit and direct approach that Jul made them more easily understood. f V.A Moon’s true anomaly ͑perigee to Moon͒ f Ј IV.G planet’s true anomaly ͑perihelion to planet͒ LIST OF SYMBOLS g V.A angle from Moon’s ascending node to peri- A IV.F aphelion or apogee gee D V.C ␭Ϫ␭Јϭelongation ͑Moon from Sun as seen g0 X.E Hill’s lowest-order motion of the node from Earth͒ gЈϩhЈ VIII.G angle from Q to perihelion D X.C operator d/id␶ in Hill’ theory h V.A angle from reference Q to ascending node E IV.F Earth ͑mass or location͒ k VII.D Clairaut’s semi-major axis parameter F V.B lϩgϭargument of latitude k IX.C ϭ f ЈϩgЈϩhЈ ͑mean longitude of the Sun͒ F VII.D angular momentum parameter in Clairaut’s l V.B Moon’s mean anomaly with respect to Earth theory lЈ IV.G planet’s mean anomaly with respect to Sun F IX.C generating function for canonical transfor- l0 VIII.B Moon’s mean anomaly at tϭ0 ͑epoch͒ mation m VII.C nЈ/nϭratio of solar to lunar mean motion F៝ V.B Runge-Lenz vector for Moon’s motion mЈ X.C nЈ/(nϪnЈ)ϭratio of solar to synodic mean around Earth motion G IX.B action variable conjugate to g n V.C lunar mean motionϭ2␲/T1 G0 II.J gravitational constant nЈ V.C solar ͑Earth’s͒ mean motionϭ2␲/T0 H IX.B action variable conjugate to h nϪnЈ X.C synodic mean motionϭ2␲/T K II.C pole of the ecliptic r VI.E distance from Earth to Moon K IX.C action variable conjugate to k rЈ VI.E distance from Sun to Earth

Rev. Mod. Phys., Vol. 70, No. 2, April 1998 Martin C. Gutzwiller: The oldest three-body problem 633 s VII.C lunar parallax ͑inverse distance͒ in Clair- Aaboe, A., 1980, ‘‘Observation and theory in Babylonian as- aut’s theory tronomy,’’ Centaurus 24, 14. t0 IV.G time of perigee ͑or perihelion͒ passage Adams, J. C., 1853, ‘‘Theory of the secular acceleration of the u IV.D xϩiyϭcomplex coordinate in ecliptic Moon’s mean motion,’’ Philos. Trans. R. Soc. London 397. v IV.H eccentric anomaly ͑angle from the center of Adams, J. C., 1860, ‘‘Reply to various objections which have the orbit͒ been brought against his theory of the secular acceleration of the Moon’s mean motion,’’ Mon. Not. R. Astron. Soc. 3. x៝ VI.E (x,y,z) Moon’s coordinates in the ecliptic Adams, J. C., 1877, ‘‘On the motion of the Moon’s node in the with respect to E case when the orbit of the Sun and the Moon are supposed to *x Ј VI.E (xЈ,yЈ,zЈ) Earth’s coordinates in the ecliptic be indefinitely small,’’ Mon. Not. R. 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