16. COMMISSION POUR LES OBSERVATIONS PHYSIQUES DES PLANETES, DES COMETES ET DES SATELLITES PRESIDENT: Rev. T. E. R. PHILLIPS. MEMBRES: MM. Andrea, Antoniadi, Armellini, Baldet, L. Campbell, Delporte, Donitch, Gastardi, Mile Harwood, MM. Jarry-Desloges, Lampland, Luplau- Janssen, Lyot, Maggini, Mascart, Nicholson, W. H. Pickering, Quenisset, Ross, E. C. Slipher, Steavenson, Stroobant, Wright. The following works embodying researches coming within the scope of this Commission have been published since the last meeting of the Union: i. "Recherches sur la polarisation de la lumiere des planetes et de quelques substances terrestres," by B. Lyot, Annates de VObservatoire de Paris, Section de Meudon, 3, Fasc. 1. Characteristic curves are found representing the fraction of light polarized in the case of the Moon and several of the planets for the angles Sun-Moon or planet-Earth and compared with curves obtained in the laboratory for light reflected by a number of terrestrial substances. Some of the conclusions arrived at are referred to in the notes below. 2. La Planete Mars 1656-1929, by E. M. Antoniadi. This is based mainly on the author's own telescopic observations with the 0-83 m. Meudon refractor in 1909 and the following years, and contains the author's conclusions therefrom. The work of numerous earlier and contemporary observers is also summarized and compared with the Meudon observations. 3. The Moon, by W. Goodacre. Contains a description of the lunar surface formations, numerous drawings, charts and photographs, and the author's revised Map of the Moon in 25 sections. 4. "Ricerche Polarigrafiche sui Pianeti. La Polarizzazione sul Disco di Giove," by M. Maggini, Mem. delta Soc. Astron. Italiana, 4. 5. La Constitution des Cometes, by F. Baldet. Paris, Hermann et Cie. 1930. 6. " Halley's Comet in its Apparition of 1909-1911," by Nicholas T. Bobrovnikoff, Pub. of the Lick Observatory, 17, pt 11, 1931. 7. " Absorptionsspektren und Atmospharen der grossen Planeten," by R. Wildt, Nach. von der Gesellschaft der Wissenschaften zu Gottingen, Math.-Phys. Klasse, 1932. THE MAJOR PLANETS Pluto. The outstanding event under this heading has been the discovery at the Lowell Observatory of a Trans-Neptunian planet to which the name Pluto has been given. The discovery telegram read: "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 con­ formed to Trans-Neptunian body at approximate distance he assigned. Fifteenth magnitude. Position March twelve days three hours G.M.T. was seven seconds of time West from Delta Geminorum, agreeing with Lowell's predicted longitude." The discovery was announced by the Lowell Observatory on 1930 March 13, the anniversary both of Lowell's birth, and of the discovery of Uranus by Herschel in 1781. The discovery of Pluto was a direct result of the search programme set going in 1905 by the late Dr Percival Lowell in connection with his theoretical work on the dynamical evidence of a planet beyond Neptune. The existence and orbit of such a planet had been predicted in 1915 by Lowell, who died in 1916, from a rigorous mathematical investigation of the discordances between the tabular and observed 87 Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.19, on 03 Oct 2021 at 17:35:28, subject to the Cambridge Core terms of use , available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0251107X00016436 longitudes of Uranus (Lowell Obs. Memoirs, i, "A Trans-Neptunian Planet"). A position and magnitude for the planet which proved to be more accurate had also been derived by a semi-graphical method by Professor W. H. Pickering, who had utilized the residuals of Neptune instead of those of Uranus, in 1919. Eventually it was with the Lawrence Lowell Camera of 13-inch aperture designed by the Lowell staff specially for the purpose and constructed by C. A. R. Lundin at the Alvan Clark Corporation's works at Cambridge, Mass. that the looked-for photographic images were detected by C. W. Tombaugh on plates taken on 1930 Jan. 21, 23, and 29. At first the real nature of the object appeared somewhat uncertain, but the de­ tection by E. Delporte of an image on a plate taken at Uccle in 1927 Jan. followed by the discovery of similar images on plates taken at Mt Wilson for that purpose in 1919, at Yerkes in 1921 and 1927, and at Konigstuhl in 1914 enabled the planetary character of its orbit to be established. It must, however, be remarked that despite the very close agreement between the predicted and actual orbits it is maintained by several astronomers that the outstanding discordances on which the predictions of Pluto's existence were based by Lowell cannot be attributed to perturbations by that planet—see e.g. Prof. E. W. Brown's paper in Proc. Nat. Acad. Sci. 16, 364 and " On a criterion for the prediction of an unknown Planet," M.N.R.A S. 92,101. Reference may also be made here to Dr A. C. D. Crommelin's note on "The Discovery of Pluto" in the Annual Report of the Council of the Royal Astronomical Society, and his address on the award of the Jackson-Gwilt Medal to C. W. Tom­ baugh, in M.N.R.A.S. 91, 380 and 434. It seems unlikely that the diameter and mass of Pluto exceed 07 and 0-4 those of the Earth respectively. Two determinations of its photographic magnitude give 15-76 (W. Baade, A.N. 5804) and 16-04 (W. Munch, Vierteljahrsschrift Astr. Gesellschaft, 66, 266). The visual values are approximately one magnitude brighter. Mercury. In addition to visual observations (Trans. I.A.U. 3, 108) much photo­ graphic work on this planet has been carried out in recent years. At Flagstaff numerous daylight photographs have been taken in yellow light between 1921 and 1931, supplemented by some in blue and red light in 1927 and 1928. Unmistakable markings were first detected on plates taken in 1925 July (P.A. 37, 129, 327, 1929). At Juvisy F. Quenisset, who had been the first to obtain photographs of the markings on Venus, secured with the Viennet photographic objective on 1931 April 11 plates showing very definitely surface features on Mercury consisting mainly of a bright region with dusky areas on its N. and S. sides, that on the S. being especially dark (P.A. 39, 430,1931, and M.N.R.A.S. 91, 94). These markings were also observed visually on the same evening with the 0-24 m. equatorial. Attempts have been made by N. Donitch to find evidence of absorption by a possible gaseous layer surrounding Mercury at the time of its transit across the Sun's face on 1907 Nov. 14 (at Assouan), and on 1924 May 7 and 1927 Nov. 10 (at Starya Dubossary). On these occasions spectrograms were secured of the region of the "black ligament" formed at the moments of the interior contacts showing almost the whole of the visible portion of the spectrum. Calculation and laboratory experiments had led to the belief that, assuming favourable conditions, new absorption lines would be found on the plates if the depth of the atmosphere producing them were 15 km. and their intensity from 8 to 10 on Rowland's scale. The spectrograms of 1907 yielded an entirely negative result. On the two later 88 Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.19, on 03 Oct 2021 at 17:35:28, subject to the Cambridge Core terms of use , available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0251107X00016436 occasions the linear dispersion employed was three times as great as before, but the results obtained were again negative (Bull, de I'Acad. Imp. des Sciences de St Petersbourg, 1908, p. 233 and 1912, p. 1011; and Memoires de la Section Scientifique de I'Acad. Roumaine, Serie 3, T. 6, Mem. 3, pp. 81-3). The polarimeter observations of B. Lyot appear to indicate that if the planet has an atmosphere similar to that of the Earth its effective thickness cannot exceed 2-1 per cent, of the latter: and that if the white patches sometimes recorded are clouds they must be composed of dust and not water (Annates de I'Obs. de Paris, Section Meudon, 3,1). The surface of Mercury is found to give polarization results remarkably similar to those derived from observations of the Moon and Mars, the three curves resembling that yielded by volcanic ash. Venus. It will be recalled that photographs by F. E. Ross in 1927 showed belt­ like markings suggesting a rotation in a considerably shorter period than that of the planet's revolution (P.A. 35, 492). A long series of ultra-violet photographs secured at Flagstaff in 1927-8 show bright and dark markings of variable character which frequently appeared and disappeared between the daily observations. These would also seem to suggest a relatively quick rotation and are significant in con­ nection with the results of radiometric measures. Parts 1 and 11 of a study of the recorded observations of surface markings in 1927 in relation to the rotation axis and period of 68 hours suggested by W. H. Pickering have been published by H. McEwen in B.A .A J. 40-42. From observations made at Setif in 1913, 1920 and 1922 by M. G. Fournier, it is concluded that the rotation period is 22h2im, and that the planet's north pole is in longitude 3i7°-5, latitude + 4i°-5 (C.R.