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 magni- tude. Position March twelve days three hours GMT was 7 seconds of time west from Delta Geminorum, agreeing with Lowell's predicted longi- tude. 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. 197 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 198 PUBLICATIONS OF THE 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 ap- propriate, 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 identifi- cation 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 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System ASTRONOMICAL SOCIETY OF THE PACIFIC 199 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 pre- cise 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. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 200 PUBLICATIONS OF THE Two of these, the inclination and the node, determine the position of the plane of the orbit in space, with reference to an as- sumed 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 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System ASTRONOMICAL SOCIETY OF THE PACIFIC 201 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.

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