PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC. Vol. 47 San Francisco, California, February, 1935 No. 275

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1 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC Vol. 47 San Francisco, California, February, 1935 No. 275 THE AWARD OF THE BRUCE GOLD MEDAL TO DR. VESTO MELVIN SLIPHER 1 By S. Einarsson In 1897 Miss Catherine Wolfe Bruce of New York founded and endowed a gold medal, to be awarded not oftener than once a year by the Astronomical Society of the Pacific, for distinguished services to Astronomy. Nomination of astronomers to receive the medal is made each year by the Directors of the following observatories: Harvard, Lick, Yerkes, Paris, Greenwich, and Cordoba. A special meeting of the Board of Directors of the Astronomical Society of the Pacific is devoted to selection of an astronomer from the names submitted, for the award of the medal for the ensuing year. The thirtieth award of the Bruce Gold Medal, by unanimous vote of the Board, has been made to Dr. Vesto Melvin Slipher, Director of the Lowell Observatory, Flagstaff, Arizona. The award is eminently satisfactory in that it recognizes distinguished services to astronomy in two fields : one of administration, the other of advancement of knowledge in the science. The question of what to attack and when is often as important as how to attack. In the former, the judgment of the director of an observatory plays an important part, not only in his own work but at times in regard to the choice of problems by his colleagues. Looking over Dr. Slipher s work of even thirty years ago, one is impressed with his good judgment as to choice of problems. Slipher has been associated with the Lowell Observatory for over a third of a century. Between 1901, the year of his graduation from Indiana University, and 1909, he not only carried 1 Address of the retiring president of the Astronomical Society of the Pacific, January 26,

2 6 PUBLICATIONS OF THE on his work as astronomer, but also found time to satisfy the formal requirements for the degree of M.A. (Indiana Univer- sity), 1903, and Ph.D. (Indiana University), He was appointed Assistant Director of the Observatory in His ability as an administrator led to his appointment as Director of Lowell Observatory in One of Slipher s earliest published investigations had to do with determination of the rotation period of Venus. Early attempts were based on observations of surface markings on the planet s disk. Owing to the fact that these surface markings are not at all conspicuous, estimates of the period of rotation differed widely from approximately 24 hours to 225 days with the 24-hour period more generally accepted. A spectroscopic investigation of the rotation velocity of Venus was made by Slipher in He placed the slit of the spectrograph perpendicular to the terminator, on the assump- tion that the equator of the planet is approximately parallel to the plane of its orbit. With the slit in this position the lines of the spectrum of the planet will be inclined to the lines of the comparison spectrum in case of fairly rapid rotation. Careful measurement and reduction of twenty-six plates led Slipher to the following conclusion: The results... show that the errors of observation were small, and that there is no evidence that Venus has a short period of rotation. Rotation of twenty-four hours would incline the planetary lines about one-third of a degree, a quantity too large in comparison with the errors of observation to have escaped detection. 2 In order to test this method of determination of planetary rotation, spectrograms were taken of the planet Mars. In the case of Mars, the equatorial velocity is 0.24 kilometers per second, or just half what Venus would show with rotation in twenty-four hours. Measurement of the inclination of the lines of the planet s spectrum yielded a rotation period of 25 hours 35 minutes, or just one hour longer than the true period. This result removed all doubt as to the reliability of this method of attacking the problem. The period of rotation of Venus is still 2 Lowell Observatory Bulletin No. 3, 1903.

3 ASTRONOMICAL SOCIETY OF THE PACIFIC 7 not definitely known, but Slipher s early investigation proved a short period impossible. The disk of Uranus is practically devoid of surface mark- ings, but the equatorial diameter is greater than the polar, from which we may infer that the planet rotates. Spectroscopic Dis- covery of the Rotation Period of Uranus 3 was published by Lowell and Slipher in They found that Uranus rotates once in 10.8 hours, retrograde, i.e., in the same direction as that in which the satellites revolve. The rotation period of Neptune, determined by Moore and Menzel in 1928, is about 16 hours. Thus we have fairly definite information concerning the periods of rotation of six of the nine planets of the solar system, namely, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Mercury's period is assumed to be equal to its period of revolution about the Sun, and that of Venus is still uncer- tain, but undoubtedly is two weeks or more. Nothing is known concerning the rotation of Pluto. Interpretation of the band spectra of the atmospheres of the major planets has long been in great part an unsolved problem in astronomy. Slipher has played a leading rôle on this problem. His spectrograms of the major planets taken under most fa- vorable conditions, and discussed by him in great detail, awaited duplication in the laboratory. Within the last year the researches of Slipher and of Arthur Adel of the University of Michigan, Rupert Wildt of Göttingen, and Theodore Dunham, Jr., of Mount Wilson Observatory, have shown that the atmospheres of Jupiter and Saturn consist largely of the gases menthane and ammonia. I shall not dwell on this subject in more detail, owing to the fact that two excellent addresses on the subject have recently appeared in print: one by our medalist, on the occasion of the award to him of the Gold Medal of the Royal Astronomical Society, and the other by Russell. 4 3 Lowell Observatory Bulletin No. 53, V. M. Slipher, Spectroscopic Studies of the Planets, Mon. Not. R.A.S., 93, , 1933; H. N. Russell, The Atmospheres of the Planets, address of the retiring president of the American Association for the Advancement of Science, in Science, 81, 1 9, 1935.

4 8 PUBLICATIONS OF THE A type of investigation which Slipher may be said to have initiated is his method of examination of the source of light of the diffuse nebulae. There are many nebulae in our galactic system of enormous extent and irregular form. Early spectroscopic investigations of the brighter diffuse nebulae revealed a bright line spectrum, which apparently proves that they are incandescent gases under low pressure. The source of the light by which these nebulae shine has long been a puzzle to astronomers. In 1912, Slipher made the discovery that some of the diffuse nebulae have dark line spectra. 5 He found that the nebulosity surrounding Merope in the Pleiades has a dark line spectrum, similar to that of Merope. He suggested that these nebulae shine by the reflected light of the stars of the Pleiades group. Photometric measures of the Pleiades by Hertzsprung in 1913 strengthened the plausibility of this suggestion. Slipher later found other examples elsewhere. His discovery led to a new point of view concerning the light of the diffuse nebulae, and much additional research has been carried on in this field, notably by Hubble at Mount Wilson Observatory. In the eighteenth century, prevailing opinion among astron- omers in regard to all nebulae was that there was no distinction between nebulae and star-clusters except in regard to distance, the nebulae being only clusters too remote to show the separate stars. They considered a nebula, therefore, as a universe of stars like our own galactic cluster to which the Sun belongs, but as far beyond the star-clusters as these were believed to be beyond the isolated stars. In some respects this old belief strikes one as grander than the truth even. It made our vision penetrate more deeply into space than we now dare think it can. 6 The spiral form of many of these objects had not then been recognized. This discovery was made by the Earl of Rosse, with his six-foot reflector. The spiral character of 5194 (M 51) was first noted in 1845, and announcement was made in However, during the latter half of the nineteenth century, writ- 5 Lowell Obs. Bull. No. 55, C. A. Young, General Astronomy, Article 897, H. D. Curtis, Handbuch der Astrophysik, p. 833, 1933.

5 ASTRONOMICAL SOCIETY OF THE PACIFIC 9 ers on cosmogony favored the belief that the spiral nebulae were a part of our stellar system. But, during this period, observa- tions began to accumulate which set the spiral nebulae apart from all others. To mention only three : their number is counted in the hundreds of thousands, they are most numerous near the poles of our galaxy, and their spectra are continuous. Slipher s important contribution to this subject was the discovery of still another type of abnormal behavior of the spiral nebulae, namely, high radial velocity. The choice of spiral nebula for the first attempt at radial velocity determination naturally fell to the nebula in Andromeda, the brightest in the sky. The one obstacle in the way of success of this undertak- ing is the faintness of these nebulae. The extreme feebleness of their dispersed light is difficult to realize by one not experi- enced in such observing, and it no doubt appears strange that the magnificent Andromeda Spiral, which under a transparent sky is so evident to the naked eye, should be so faint spectrograph- ically. The contest is with the low intrinsic brightness of the nebular surface, a condition which no choice of telescope can relieve. 8 The observations were as follows : 1912 September 17 Velocity 284 km. November Velocity 296 December 3-4 Velocity 308 December Velocity -301 Mean velocity 300 km. Tests for determining the degree of accuracy of such observations have not been completed, but in rounding off to 300 kilometers in taking the mean one is doubtless well within the accuracy of the observations. The measures extended over the region of the spectrum from F to H. 8 Slipher s observations of other spiral nebulae revealed radial velocities much greater than that of Andromeda. In 1914 Slipher announced the detection of nebular rotation. A spectrogram of the Virgo Nebula N.G.C. 4594, made a year ago, showed the nebular lines to be inclined The slit of the spectrograph was placed over the long axis of the 8 Lowell Obs. Bull. No. 58, 1913.

6 10 PUBLICATIONS OF THE nebula which is of the spindle type and hence the observation shows clearly that such nebulae are as previous evidence tended to show spirals seen edgewise. 9 According to Curtis, who for many years had maintained that the spiral nebulae are extra-galactic, it was not until 1924 that all doubts as to the island universe character of the spirals were finally swept away by Hubble s discovery of Cepheid variables in these objects. 10 The work of Slipher undoubtedly had much to do with final acceptance of the spiral nebulae as extra-galactic. The globular star clusters are in rapid motion relative to the Sun, and to one another. Slipher measured the radial velocities of eighteen clusters and found some as high as 300 kilometers per second. In closing this brief account, it is evident that no attempt has been made to give a complete outline of Slipher s work. Nothing has been said concerning his researches on the planet Mars, his work on the radial velocities of the stars, his pioneer work on stationary calcium lines in the spectra of stars in Scorpio and other regions of the sky, and other investigations. In his long list of honors may be mentioned the Lalande prize, Paris Academy, 1919 ; the honorary degree of Sc.D., Uni- versity of Arizona, 1923 ; the honorary degree of LL.D., Indiana University, 1929 ; the Gold Medal of the Royal Astronomical Society, 1933 ; and election to membership in the National Academy of Sciences. The Astronomical Society of the Pacific takes pride in adding the name of Dr. Vesto Melvin Slipher to its distinguished list of Bruce Gold Medalists. 9 Lowell Obs. Bull. No. 62, H. D. Curtis, Handbuch der Astrophysik, p. 837, 1933.