Astronomical Society of the Pacific Publications of The

ASTRONOMICAL SOCIETY OF THE PACIFIC

PUBLICATIONS OF THE

Vol. XLIV San Francisco, California, August, 1932 No. 260

THE ASTRONOMICAL ROMANCE OF PLUTO*

By A. O. Leuschner

More than two years have passed since announcement was made by the Lowell Observatory, on March 13, 1930, the anni- versary of the birth of Percival Lowell, of the discovery by Tombaugh on January 23, 1930, of a trans-Neptunian planet. You all remember the intense interest which this announcement aroused throughout the world. I quote its exact words : Systematic search begun years ago supplementing Lowell's investigations for Trans-Neptunian planet has revealed object which since seven weeks has in rate of motion and path consistently conformed to Trans- Neptunian body at approximate distance he assigned. Fifteenth magnitude. Position March twelve days three hours GMT was 7 seconds of time west from Delta Geminorum, agreeing with Lowell's predicted longitude.

In the Scientific Monthly for January, 1932, Mr. Roger Lowell Putnam, a trustee of the Lowell Observatory, and Dr. V. M. Slipher, director of the Observatory, have published the fascinating and truly romantic story of Percival Lowell's keen mathematical work in attempting to predict the existence of an extra-Neptunian planet which he designated as "Planet X" from unexplained differences of the computed and observed motion of the planet Uranus; and of his systematic and persistent efforts until his death in 1917 and, after that, of the staff of the Observatory, to verify the prediction by photographic search of the heavens. Lowell's "Memoir on a Trans-Neptunian Planet" published in 1915, represented laborious investigations covering a period

* Address of the retiring President, delivered at the Sixteenth Annual Meeting of the Pacific Division, American Association for the Advancement of Science, June 15, 1932, at the State College of Washington, Pullman, Washington.

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of ten years. Preliminary observational tests began as early as 1905 when he found indications of the existence of a new planet and had been continued with truly remarkable faith and per- severance to the time of the announcement in 1930 of the discovery of a trans-Neptunian object. Of the many names suggested for the new object, Pluto was adopted by the Lowell Observatory. It was first suggested by Miss Venetia Burney, aged 11, of Oxford, England. The suggestion was cabled to the Lowell Observatory by the late Professor H. H. Turner, then Savilian Professor of Astronomy at Oxford. Two reasons made the choice of this name particularly appropriate, first, the place of the object in the outer dark regions of space, and secondly, the identity of its first two letters with the initials of Percival Lowell. The object now known as Pluto was actually photographed at the Lowell Observatory in 1915, but the faint images were not recognized until after its discovery in 1930. Until identity of Pluto and "Planet X" is really established, Pluto should not be referred to as ^Planet X." It is not my purpose today to repeat the authoritative account by Putnam and Slipher of the prediction and discovery of Pluto, but rather to supplement the story in the light of developments of the past two years. These developments are discussed by the authors named only in so far as they re- late to the closeness of the orbit predicted by Lowell to the now well-known orbit of Pluto, and to the consequent identification of Pluto with "Planet X." But the developments in the gradual determination of the orbit of Pluto with its now well- known accuracy present many romantic features and the astronomical world is still divided on the question of whether Pluto actually was predicted by Lowell or whether its discovery was an accidental by-product of the remarkable search instituted at the Lowell Observatory for "Planet X." Foremost astronomers are still discussing the latter question. I hope to be able to show that the scientific arguments pro and con appear to be almost equally valid and that we are not in a position to render a final verdict at the present time. An ultimately unfavorable verdict on the identity of Pluto with the predicted "Planet X" would in no way detract from LpwelFs brilliant work and the glory of the

Lowell Observatory in discovering a trans-Neptunian object. You may observe that with extreme conservatism I am still referring to Pluto as an object rather than as a planet. There is every probability that it is a planet, as is now universally concluded, from available material, but so far no mass sufficiently large has been established to show reliable differences of the observed and computed positions of its nearest neighbors Uranus and Neptune, particularly if we grant that their predicted positions are somewhat imperfect. So far only an upper limit for the mass of Pluto, about one- half that of the Earth, has been established, and such a mass is believed from gravitational considerations to be too small to affect the motions of Uranus and Neptune sufficiently during the period of available observations at their distance from Pluto to permit the prediction of a trans-Neptunian planet. While later determinations of Pluto's mass may make it barely sufficient to have caused adequate inequalities, there is also a remote chance that later investigations will render its mass comparable to that of comets. As I have indicated, our discussion this evening will deal on the one hand with the incidents connected with the determination of the orbit of Pluto and on the other hand with certain arguments for the view that at the present time it is hardly possible to render a final verdict on whether or not the great achievement of the Lowell Observatory represents veri- fication of a prediction or an accidental discovery. Immediately following the announcement of the discovery, observatories all over the world undertook systematic photographic and visual observations of the position of Pluto, and theoretical astronomers attacked the problem of determining its orbit in accordance with methods based on Newton's law of gravitation. At the Students' Observatory, Berkeley, in a period of twenty days, from March 16 to April 5, seventeen precise observations were received by telephone, cable, telegraph, and letter, from the Lick, Mount Wilson, Yerkes, and Babelsberg, Germany, observatories. From a 17-day arc of Pluto's motion, the computation of an orbit was undertaken under my general direction by Bower and Whipple, graduate students. An orbit is characterized, as is well known, by six so-called elements.

Two of these, the inclination and the node, determine the position of the plane of the orbit in space, with reference to an assumed fundamental plane, generally the equator or the ecliptic. Two other elements, the semi-major axis and the eccentricity, determine the size and shape of the conic in which an undis- turbed body would move according to Kepler's laws. A fifth element, called the argument of perihelion, represents the angle in the orbit between the axis of the conic and the line of inter- section of the orbit plane with the reference plane. The sixth and last element is generally chosen as the date of perihelion, when the body is nearest the Sun. These elements, when once determined from observations, make it possible to compute the position of the body for the past or future. The semi-major axis, generally called the mean distance from the Sun, is related to the period of revolution about the Sun by Kepler's harmonic law that the squares of the periods of revolution about the Sun of any two planets are proportional to the cubes of their mean distances. By dividing the 360 degrees, corresponding to a revolution, by the period expressed in days, the mean or average daily motion about the Sun results, so that it is customary, for an ellipse, to use any of the equivalent elements : mean distance, period, or mean daily motion. On account of the unavoidable errors of observation, the percentage errors of the motion and of its variation, on which an orbit solution chiefly depends, are greater for short than for long observed arcs and for slow than for rapid geocentric motion. For this and other reasons, observations over a short period of time often may be represented by any conic from a circle to a hyperbola, including ellipses of various periods, and parabolas. It would therefore not be safe to adopt a computed orbit as a reliable result without testing its uncertainty. Bower and Whipple found that no definite result could be obtained from the limited observations available to April 5 without an ingenious assumption entirely justified by the slow motion of the body, if located in the outer regions of the solar system. The observed average daily motion for Pluto was * approximately only about 14 seconds of arc per day. The assumption made by them was that the principal part of the motion of the body toward or away from the Earth was due to the

component of the Earth's motion in the line of sight in its orbit around the Sun. This led to a general direct solution which resembled closely the now well-kno\^n orbit. The period came out to be 229 years, not very far from the now established period of about 248.5 years. However, one of the observations was not represented exactly by the direct solution, although the departure was only six-tenths of a second of arc. A perfect adjustment changed the period to 390 years ; thus the six-tenths of a second of arc—less than the uncertainty of the observations themselves in this case—account for a change in period of about 161 years. For this reason four arbitrary solutions were tried, inclusive of a parabola, in addition to the general and corrected solutions. All of these represented the observations within their presumable uncertainty. In any of these solutions, the first quantity to be established ordinarily is the distance of the body from the Earth. All of these different solutions, however, gave the same distance and the same position of the orbit plane. For these reasons the following was released on April 5, 1930, to Science Service and to the Harvard College Observatory, American centers for the distribution of astronomical news :

Preliminary investigation of the orbit of the Lowell Observatory object completed at the Students' Observatory of the University of California by E. C. Bower and F. L. Whipple under the general direction of Professor A. O. Leuschner has resulted in a group of solutions giving a fairly well-determined distance from the Earth of approximately forty- one astronomical units. With the assumption justified by this distance that the principal part of the motion of the body in the line of sight is due to the Earth's annual motion, Messrs. Bower and Whipple found the inclination of the orbit to the ecliptic to be 17 degrees and the longitude of the node 109 degrees. Other solutions accurately representing observations from March 16 to April 4 gave approximately the same inclination, node, and distance from the Earth, with orbits varying from a near-circle to a parabola, so that no definite conclusions concerning the eccentricity and the period can be drawn until observations covering a considerably longer interval shall have become available.

These results agree quite well with Lowell's prediction and with the approximate distances assumed by the Lowell observers from their own observations since January 23. The details

of Bower and Whipple's computations were published at once in a Lick Observatory Bulletin, inclusive of predicted positions of the body to June 5, to aid observers in locating the body. For reasons which involve technical details and therefore must be omitted, these predictions were based on what appeared the most probable of the six solutions, which corresponded to a period of 264 years. This judgment is now verified. Any of the solutions, however, would have located the body, for on June S positions predicted on parabolic solutions differed from those on the adopted orbit by only one and one-half seconds of arc in right ascension and declination. A week later, on April 12, the Lowell Observatory released, by telegram to the Harvard College Observatory, a preliminary orbit computed by its staff with the collaboration of Dr. John A. Miller, director of the Sproul Observatory, using observations from January 23 to March 23, an interval of two months, as compared with the interval of 17 days at that time available at Berkeley. This single solution from the data used proved to be very nearly parabolic, and corresponded closely to one of the parabolic solutions obtained by Bower and Whipple. For a time the announcement of this and other very nearly parabolic orbits caused consternation, for it practically eliminated the chances that the discovery of Pluto had anything to do with Lowell's predictions. I trust that . I have, however, sufficiently explained that the Lowell results represented a perfectly legiti- mate orbit, considering the complexities of the problem. A discussion of the Lowell calculations was released by printed circular on May 1, following their original circular of March 13. The details of the discussions in these circulars are included in the account published by Putnam and Slipher in Scientific Monthly. Observations accumulated rapidly all over the world. In- cluding certain pre-discovery observations to be mentioned immediately, and the early 1930 observations released in the meantime by the Lowell Observatory, the total number reached 136 by June 1, practically every important observatory in the world participating. At this point it is appropriate to refer to the intensive search at once instituted for pre-discovery images

on plates taken partly in search of the planet and partly for other purposes, in previous decades. The first such position announced was that by Delporte of the Uccle Observatory, Bel- gium, who found an image on a plate taken on January 27, 1927. Crommelin of England used this for a more accurate orbit determination. It is impossible here to make due reference to all of the theoretical investigations of the orbit which have been undertaken. One of the most exhaustive of these is by Zagar and his colleagues of the Padua Observatory, Italy. In this country, Nicholson and Mayall of the Mount Wilson Observatory applied themselves at once not only to securing new positions of Pluto and to searching photographic plates of former years, but also to making exhaustive theoretical investigations. The account of their work, published in Contributions from the Mount Wilson Observatory No. 417, represents as romantic a story as the account of Putnam and Slipher. The only Mount Wilson plates that gave promise of containing images of Pluto were a set taken by Humason late in Decem- ber, 1919. Humason's search in 1919 was stimulated by an article by W. H. Pickering, in which he gave a new solution of the problem, which included the observed deviations of Neptune from its predicted positions. Humason's search covered the positions predicted by Pickering as well as those by Lowell, but the planet was not found at that time because the now known high inclination of the orbit plane to the ecliptic was not suspected. Since the preliminary orbits of Pluto gave the position of the orbit plane with considerable accuracy, the search of the plates was resumed by Nicholson with remarkable in- genuity. He used computed positions based on the theoretical work of Bower and Whipple and of Crommelin, with the result that Pluto was discovered on four plates. Predicted pre-discovery positions had also been furnished by us to the Yerkes Observatory, where Ross and Van Biesbroeck found the object on plates taken in January, 1921, and in January, 1927. But this is not the whole story of the pre-discovery positions. In addition, we now have positions in 1914, January and No- vember, found on Heidelberg, Germany, and Harvard plates, respectively; in 1915 on Lowell plates, as mentioned earlier this

evening; in 1925 on Mount Wilson plates. In all, fifteen pre- discovery positions well distributed for improving the orbit are available. There is very little hope for more on account of the faintness of the object. Every plate of the regions of the sky where Pluto may possibly have been, taken at any observatory since the application of photography, about 1890, has been searched, particularly those taken in connection with the well- known astrographic chart, an undertaking made possible by international co-operation. Computers, particularly at Berkeley, who found difficulty with certain observations, have referred them back for re- measurements of the plates. It is clear that without international co-operation such problems as these could not be successfully attacked. I have already made reference to the sensitiveness of the orbit of Pluto to small uncertainties of observation. Much time and effort were lost by computers in identifying the less reliable positions. Thus, unfortunately, the first pre-discovery observation caused many complications until it was re-referred and remeasured. The near-parabolic solution of the Lowell Observ- atory is chiefly due to a slight and unavoidable inconsistency in its January 23, 1930, position from a small-scale plate, amounting to less than three seconds of arc in each of the two «co-ordinates. On the other hand, apparent inconsistencies in the 1919 and 1921 observations were removed when the action of the major planets on Pluto was taken into account. In the meantime the plates had been remeasured without appreciably chang- ing the original positions. Hope ran high for a time that the available positions would be pushed back as far as 1903, when Crommelin announced that, among the Franklin-Adams plates taken in South Africa and stored at the Greenwich Observatory, there was one of the region where Pluto had been in 1903.. He sent us the approximate positions of two possible images, promising to have the one nearest to Pluto's position in that year accurately measured if it did not turn out to be a faint star. Unfortunately, however, the plate having been taken by an amateur, the observer had recorded only the date but not the time of the day when it was

taken, which is needed to a certain degree of accuracy. It then occurred both to Crommelin and to Bower that, if some minor planet trails could be found on the plate, their measured positions could be used by comparison with those computed from their known orbits to establish the time of the day. Thus these minor planets were to be used as clocks. The experiment was successful and the time of the day established with sufficient accuracy. One of the pçsitions was established by Bower as close to Pluto, but to make sure that it actually was Pluto it was necessary to rephotograph the region of the Franklin-Adams plate, so as to ascertain whether the image was still there or had disappeared. If the image was found to be in the same position this year as in 1903, it would belong to a star and hopes would be shattered. Such comparison plates were taken by Bower this spring at the Lick Observatory and forwarded to Greenwich for comparison with the Franklin-Adams plate. To the disap- pointment of all concerned, the faint Franklin-Adams image suspected to belong to Pluto was found on the Lick plates and therefore did not belong to Pluto but to a faint fixed star. Pluto was too faint to make an impression on the 1903 plate. With the mass of material now available, including the 15 well-distributed pre-discovery positions, it has been possible nevertheless to determine the orbit with great accuracy. The outstanding results are those by Nicholson and Mayall of the Mount Wilson Observatory, and those by Bower. Little more needs to be added now concerning the results of these exhaustive orbit determinations, which, as a next step, were based on selected observations including the Mount Wilson observations of 1919 and later observations. The results of Nicholson and Mayall and of Bower and Whipple, based on this eleven-year arc, were released almost simultaneously. The full details of the work at Berkeley are contained in Lick Observatory Bulletins 421, 427, and 437. It might be mentioned here, however, that Bower, Mayall and Nicholson, and Zagar have further improved their results by taking into consideration the effect on the motion of Pluto caused by the separate attractions of all the major planets of the solar system. Bower has included pre-discovery observations as far back as 1914. On these latest results he has based

predictions which have served astronomers all over the world. Although the last position used by him was taken on October 1, 1930, about a year and a half ago, recent observations show no appreciable departure of the planet from the predicted positions. The orbit probably is not in need of any further correction for approximately ten years. We now come to the perplexing question of the identity of Pluto with "Planet X/' which had been predicted on the basis of the supposed irregularities of motion of Uranus. The orbit of Neptune, with a mean distance of thirty astronomical units, is nearer to that of Pluto. Newcomb had used the difference between observed and predicted positions of Neptune to test the feasibility of predicting an extra-Neptunian planet and had come to a negative conclusion. He published a set of modified dis- crepancies, so-called "residuals in longitude," for the period that Neptune had been under observation, from 1795 to 1896. The position of 1795, however, is somewhat doubtful, and there are no positions between that year and 1846, the year of Neptune's discovery. Jackson of England continued the comparison of Newcomb's theory with observations of Neptune from 1896 to 1928. He used all of the residuals from 1795 to 1928 to im- prove the orbit of Neptune, without regard to the existence of an extra-Neptunian planet. He thereby reduced the outstanding differences so successfully that attraction by an extra-Neptunian planet did not seem to be involved. Nicholson and Mayall repeated the solution, but introduced the presumptive effects of Pluto on the residuals, on the assumption that Pluto's mass was equal to that of the Earth and then solved for the most probable mass of Pluto. By including the doubtful 1795 position, the mean of two observations, they obtained 0.94 for the mass of Pluto, a little less than that of the Earth (Earth =1). By excluding that observation, no definite mass determination could be made. A zero mass would leave a residual of —6^2, a mass of 1.5 Earth +3^5 for the Lalande position. This mass investigation was repeated and continued by Bower, after he had discovered that the Newcomb series of residuals to 1896 and the Jackson residuals after that were not quite comparable. To make them strictly comparable would have necessitated recourse

to Newcomb's original computations. These were searched for with the assistance of his daughter, Dr. Anita McGee, that of the Librarian of the Congressional Library, and of members of the staff of the United States Naval Observatory, but have not been located either at the Naval Observatory or in the Library of Congress. Nevertheless, Bower was able to make the residuals approximately comparable. He also made a general revision of the residuals, resulting in minor changes and a significant correction to one of the Jackson residuals. Bower's solutions were similar to those of Jackson, on the one hand, including the 1795 observation and disregarding the existence of an extra-Neptunian planet, and, on the other hand, to the Nicholson and Mayall solutions for the determination of the mass, with and without the 1795 observation. These more precise calculations practically verify the previous results. We can now follow Neptune on the basis of several theories : that it is unaffected by Pluto, inclusive or exclusive of the 1795 position ; and, that it is affected by Pluto, inclusive or exclusive of the 1795 position. Predictions for the position of Neptune on the basis of three of these theories have been made until 1952, and have been compared with predictions on Newcomb's theory. Observed future departures of Neptune may be compared with the differences in position given by Newcomb's and the three new theories. Such comparisons evidently will reveal which of the various theories is the most reliable, and throw light on the mass of Pluto and on whether the 1795 observation is sufficiently dependable. You have noticed my frequent reference to the 1795 pre- discovery position of Neptune. Neptune was not recognized in 1795 as a planet, but thought to be a star, and was included by Lalande in a survey of star positions for catalogue purposes. He estimated his transits of Neptune only to the nearest half- second of time, or seven and one-half seconds of arc, too rough for the purposes in hand. Newcomb has revised the two existing 1795 observations of Lalande by determining corrections to his clock and to the instrument from modern positions of the basic stars. Using these corrections for the Neptune positions, he found them to be in agreement to about two seconds of arc, but

both doubtful to within probably four seconds. This represents some more of the detective work involved in our problem. Thus we see how troublesome is the problem of determining the mass of Pluto by gravitational methods. Crommelin of England has pointed out that the perturbations of Pluto on Jupiter and Saturn, though extremely small, have the advantage over those of Uranus and Neptune, in that the orbits of the former are better known and many synodic revolutions are available. Synodic revolutions of two planets correspond to intervals between times when they are both in the same line with the Sun. Qualitative comparison of the known perturbations by Neptune on Jupiter and Saturn with the deviations of Neptune supposed to be due to Pluto were then made by Crommelin. On revision of this work, Bower found that Crommelin would get a mass for Pluto eleven or five times that of the Earth, according to whether Jupiter or Saturn is taken—results quite comparable with Lowell's prediction. When the action of a disturbing body is not known or suspected, an orbit is made to fit as closely as possible to observations, by the theory of probability. E. W. Brown of Yale has explained that, when the interval of observation is nearly the same as the synodic period, this method produces a sym- metry in the finally outstanding residuals, the use of which would lead to fictitious elements of a suspected body, and would produce the results given by Lowell, whether or not the sought- for body really exists. Brown therefore considers the discovery of Pluto an accidental by-product of the Lowell search. I venture to observe here that the accidental feature of Lowell's discovery might be interpreted to be that the circum- stances outlined by Brown fortunately prevailed for a correct prediction on the part of Lowell. This, of course, would imply a reality of the available residuals of Uranus, including that of the 1795 position, doubted by Brown, Bower, and others. Any possible error in the theory of Uranus, of short- or long-period term, may completely mask the admittedly small effect of Pluto on Uranus. Bower points out that, inasmuch as the orbit of Pluto and that of "Planet X" agree, Lowell's work may be regarded as a

very rough determination of Pluto's mass, about seven times that of the Earth. Recently, Brown has published theoretical considerations which tend to strengthen his position. These are too intricate to be referred to here in detail. He developed certain criteria which show that from the then available residuals of Uranus, the predictions of the existence of Neptune made by Adams and Leverrier were practicable, but that the similar prediction of Pluto from the Uranus residuals outstanding after the discovery of Neptune was impracticable and that the source of these residuals must be sought in the imperfection of the theory of Uranus. He finds a striking similarity of longitude- residual curves for various bodies of the solar system for a considerable range of mean distances from the Sun, to which the corresponding residuals of Uranus do not fit. Incidentally, he confirms that the longitude component "of the perturbations is not maximum at conjunction, at the time of maximum gravitational action. Time does not permit me to comment specifically on various other deductions made by Brown. No criticism can be made of his careful analysis as far as it goes, but it is admittedly somewhat approximate and qualitative, and in view of the complexity of the problem, not all factors involved may have been considered. His final conclusions are essentially the same as Bower's. According to Bower, the most favorable conjunction data of Neptune now available for a.gravitational mass computation of Pluto involving all components are those of the recent 1892 conjunction covering available precise observations. The most favorable, the close conjunction of less than three astronomical units, will occur in about 8,000 years. Our next chance will be the 1968 conjunction of Uranus with Pluto. Brown, however, points out that the maximum longitude perturbation does not occur near conj unction, and that other positions are more favorable in longitude.. As is well known, and has been pointed out by several writers, there are two other possible methods of determining Pluto's mass and its effect on other planets besides the gravitational method which we have just discussed. The first of these is by observation of apparent magnitude, with estimated albedo

and density. The second is by observing the apparent diameter with a powerful telescope and assuming the density. Albedo is the reflecting power of the surface. Apparent or visual magnitude is the amount of light received from a body. Without going into further details let me merely say here that within the uncertainty of the visual magnitude, estimated by Nicholson and Mayall as between 14 and 14.5, and assuming the most favorable values of density and albedo known in the solar system, the mass of Pluto would lie between 0.7 and 0.4 that of the Earth. To furnish material for the last method, every attempt has been made by observers with the largest and most powerful telescopes available in the world, at Mount Wilson, at Lick, at Lowell Observatory, and others, to measure the diameter of Pluto, all without success. There is no visible diameter with these' instruments. Aitken and Wright of the Lick Observatory estimate that it must be less than 073, which would give a mass of about one-third that of the Earth. If the diameter should come out to be about 071 the mass could not exceed 0.01 that of the Earth. From all the data available, Bower concludes that the upper limit of the mass is 0.7 times that of the Earth and that its most probable value known at present is about 0.1 that of the Earth. The more intensively Pluto is studied, the smaller the value of its mass becomes. An ingenious gravitational determination of the mass of Neptune's satellite has recently been made by Nicholson, van Maanen, and Willis of the Mount Wilson Observatory. They find the mass of this satellite, Triton, to be from 0.06 to 0.09 of the Earth's mass. If Triton were moved from its distance at 30 astronomical units to Pluto's distance of about 40 astronomical units, it would be diminished in visual magnitude so as to become comparable with that of Pluto. Hence, if both bodies have the same albedo and density, they would have about the· same mass. This may be regarded as further sub- stantiation of Bower's conclusion that, until further evidence is forthcoming, the most probable value of Pluto's mass may be taken to be about one-tenth that of the Earth.. In view of the facts which I have presented I trust that I have justified in your opinion the position that a final verdict

on the mass of Pluto and its connection with Lowell's work may well await future researches. It is necessary to revise the orbits of the planets on which Pluto is supposed to have an effect in order to obtain more dependable residuals. This in itself is a tremendous task, but even with the material now available, factors may not have been considered which might assist in the solution of the puzzle. In illustration, I may refer to the fact that our judgment at Berkeley, in the selection for prediction purposes from our six preliminary orbits by Bower and Whipple of one of the two which are nearest to the final orbit, proved correct in spite of the fact that the meager data on which the orbits were based, mathematically, apparently did not permit of choice. Many similar cases may be cited. Just recently Whipple and Cunning- ham at Harvard produced the first nearly correct elliptic orbit for the recently discovered Delporte object, which approaches close to the Earth, from such meager material that the difference of the orbit from a parabola could not well be established by comparison of the observations with predictions. In 1910, it was established at Berkeley that a comet discovered by Cerulli of Italy, for which only parabolas appeared to be calculable, was identical with the lost periodic Faye comet. The computations for the ellipse from only a four-day arc were made by Dr. Meyer and Dr. Sophia Levy, then graduate students. The announcement of the identity was severely questioned, particularly abroad, on the assumption that it was not possible to make the identification from the available material, but the identity proved correct and the results were not accidental. Many other instances might be cited, where it has been possible to obtain dependable orbits from apparently insufficient material. If this is true in the comparatively simple problem of orbit determination, what position should we take in the extremely complicated problem of Pluto's identity with "Planet X," which involves the complex theory of mutual perturbations throughout the solar system and the reliability of observations, and many other factors ? In the case of the discovery of Neptune, the residuals of Uranus were large, with a range of 31 and were only slightly

affected by the complications just referred to. It was a fairly definite problem, but in the case of Pluto, with a range of 0r/60, we are dealing with a borderline case. Jackson points out that the existence of a disturbing body would be shown by the residuals (after his correction of the orbit without assumption of an extra-Neptunian body) running through two complete cycles in the interval covered by the observations and concludes from the absence of such cycles that the effect of such outside bodies as may exist (such as Pluto) does not exceed the error of observation. Nicholson and May all, however, clearly show the existence of the cycles on the basis of a solution involving a mass of 0.94 Earth's for Pluto with the use of the 1795 position. To be sure, the amplitude of the comparison curve of the Jackson and Nicholson-May all residuals is less than the error of observation and is therefore masked in the Jackson but not in the Nicholson-Mayall residuals. The point I wish to make is that, although different solutions may give residuals within the errors of observation, a solution including a correct assumption may give more correct results in spite of a seeming indeterminateness of the problem. To illustrate my point let us consider the case of preliminary orbits of the Trojan group of minor planets. They are at nearly the same distance from Jupiter as they are from the Sun. Jupiter's gravitational action on them is 1/1000 that of the Sun, as the mass of the Sun is one thousand times that of Jupiter. If preliminary orbits are computed for the Trojans from arcs of several weeks both with and without consideration of Jupiter's mass in the direct (original) solution, the two orbits will differ but both may satisfy the observations within their error. Ex- perience, however, definitely shows that of the two solutions the one with considerations of Jupiter's mass describes the future motions of the Trojans far more accurately. The special perturbations by Jupiter during the interval of the several weeks of observation on which the two orbits are based are masked in the residuals. And yet, a more dependable solution is obtained by making a correct assumption at the start, namely that' the observed positions are the result of the action of the Sun and Jupiter and not of the Sun alone.. A comparison of the two

sets of residuals should give a curve similar to the special perturbation curve by Jupiter during the interval. Thus, in the rejection of Lowell's orbit of "Planet X" and of its identity with Pluto, undue emphasis has been placed on the fact that the perturbations are masked in the residuals of Uranus during the period of observation. By making the assumption that the residuals of Uranus include the action of an extra-Neptunian body, Lowell proceeded along logical lines and the similarity of the orbit of his "Planet X" with that of Pluto may be the result of the fact that his assumption may prove correct. Much light will be thrown on the question of the residuals of Uranus and Neptune when the orbits shall have been revised on^the basis of more accurate thories of motion in the solar system. What shall we conclude on the other hand if the future shows that Pluto, as is the case with comets, has no mass sufficiently appreciable to affect other bodies of the solar system? It may then have to pass into the class of objects known as minor planets, the largest of which, Ceres, has a mass considerably less than one one-thousandth that of the Earth. But we are not quite so sure any more whether we should call an object a comet or a minor planet, even if the object shows all the characteristics of the former and none of the latter. Thus, an object was discovered in 1913 by Neujmin in Russia, with plan- etary appearance, but with what was then considered as dis- tinctly cometary motion. Barnard, at the Yerkes Observatory, actually identified it as a comet by observing a faint coma around the nucleus and a diminutive tail. Positions for its return last year were predicted by both Van Biesbroeck and Crommelin. It was actually found by Nicholson at the Mount Wilson Observatory, but even with the most powerful telescopes there remained no vestige of coma or tail. If it had been discovered for the first time last fall instead of in 1913, it would have been identified as a planet and not as a comet, but owing to its cometary appearance in 1913, it will remain known as a comet. I think the implications I have in mind are clear. The wisest attitude at the present time is to have an entirely open mind on the mass and consequent gravitational effects of Pluto. It may cause some wonder that I have paid considerable

attention to the question of identity of Pluto with "Planet X/' without helping you to an answer, but my purpose in all I have said was to illustrate the methods of international co-operation in vogue in astronomical science, the general complexity of certain astronomical problems, and the necessity of further far- reaching researches on the motions of the bodies of the solar system. Whatever results the future may bring in regard to Pluto, it cannot affect the brilliant record which Lowell and his staff have made for themselves in attempting prediction of, and in observational search for, a hitherto unknown body, which resulted in the definite discovery of a new member of the solar system visible beyond the orbit of Neptune.