ASTRONOMICAL NOTES.

Press, Volume LXVII, Issue 20225, 1 May 1931, Page 6

ASTRONOMICAL NOTES.

FOR MAY, 1931

j (specially written yoa the press.) [By E. G. Hogg, M.A., F^R.A.S.j The planet Mercury is too close to the sun at the beginning of the month.to bo visible; it will rise on May loth at 5.20 a.m. and on May 25th at 5.4 a.m.; its apparent magnitudes on the days in question will be 1.6 and 0.9 respectively. Venus rises on May Ist at 3.55 a.m. and on May 15th at 4.25 a.m.; Mars sets on these dates at 11.8 p.m. and 10.40 p.m. respectively. Jupiter sets on May Ist at 8.5(3 p.m. and on May 15th at 8.11 p.m.; Saturn- rises on these dates at 9.33 p.m. and 5.38 p.m. respectively. Pluto. Although the planet Pluto is now a duly accredited member of the solar family, our information about it is still scanty. The observations first made allowed a rough orbit to be constructed; with this as a &uide, photographic plates of the region through which the body must have passed in recent years were eagerly searched and several earlier records of it were secured and a second and more accurate orbit was obtained. Both of these paths were derived n the assumption that Pluto was the only planet revolving about the sun, i.e., the action of the other planets on it was neglected. The next step to secure a more correct orbit by allowing partially for the disturbing action of the other planets has now been taken by two astronomers, Nicholson and Mayall, who have recalculated the orbit on the supposition that the sun is replaced by a body whose total mass is the same as that of the sun and all the planets except Pluto. This orbit is a little more accurate than the earlier ones, but it is still only an approximation, and the final and definitive orbit cannot be derived until the gravitative action of each planet on the new body ha;s been allowed for separately, and as this, involves heavy labour it cannot be known for some considerable time. Its Period of Revolution. The results attained by Nicholson and Mayall are interesting and important, as they have reduced Pluto's time of revolution about the sun, which, according to the first calculations, was 249 years, by about one and a half years. It now appears that the period of Pluto is even more closely related to that of Neptune than was at first thought. ' Neptune takes about 164.78 years to move once round the sun and therefore performs three journeys in 494.34 years; if the period of Pluto be 247.5 years, it will pass round the sun twice in 495 years, an interval of time differing by only one year from that taken by Neptune in these revolutions. This close relation between the periods of the planets means that each body will exercise a powerful disturbing influence upon, the other and therefore bring about a correspondingly great distortion of their orbits.

The Mass of Pinto. Nicholson and Mayall have also made a preliminary investigation of the mass, of Pluto. If that body possessed a satellite which could be seen from the earth, the determination of its mass would be a comparatively simple flatter, but as this mode;o£ attacking tlie problem .is quite out of the question, the mass of . Pluto must be ascertained by the disturbances that body causes in the orbits of Neptuno and Uranus by its gravitational attraction. From the positions of Neptune in its orbit since 1847, the co-workers find, as ' a first estimate, that the mass, of Pluto is about three-quarters of that of the earth. There is, however, another; and rather curious> fact to bo taken into consideration. When sufficient observations of Neptune had been made after its discovery in 1846 to enable a fairly good orbit to be calculated, astronomers started to traco back its path in tho heavens with a view to seeing if they could pick up any earlier observations of it; this was a most laborious procedure, there being, of course, no photographic maps of the,heavens in those days, and'star catalogues were the only material available. At last it was ascertained that Neptune had been seen by Lalande as a star on May Bth and May 10th, 1795, but that he had rejected the latter of his two observations, by reason of its non-agreement with the former—a consequence of the motion of tho planet in its orbit during the interval of two days. Here we see how close Lalande came to being the discoverer of Neptune; had he interpreted correctly the discrepancy between his two observations he would hive anticipated Adams and Leverrier by half a century. This observation of Neptune by Lalande is not of the same degree of accuracy as the modern ones, but still it. cannot be neglected altogether, and Nicholson and Mayall find that, if they take it into account, the mass of Pluto is slightly greater than that of the earth. Hence for the present Sve shall not go far wrong in pier turing Pluto and the earth as nearly eqiial in mass.

Its Volume. At present it seems unlikely that we shq.ll ever know much about the size of Pluto; if it were equal in volume to tlje earth its disc would have a diameter of only half-a-second of arc and would therefore be impossible "to observe even in the greatest telescope we have, so we can make no direct measurement of its size. Its reflecting power may be as low as that of Mercury or as high as that of Venus; by assuming a reflecting power intermediate between these two, and knowing its distance, we might conclude that Pluto exceeds the earth slightly in volume, but we should not be able to attach any great weight to this conclusion. The Illusion of the Martian Canals. Our knowledge of the topography of the planet. Mars is duo in a largo measure to three astronomers, the late M. Schiaparelli, of the Milan Observatory, the late Dr. Percival Lowell, of the Flagstaff Observatory, Arizona, and M. Antionadi, of the Mendon Paris, • who has just published "La Planete Mars 1656-1929," in which are incorporated the results of 22 years of research on this body—a work likely to remain for decades the standard authority on this subject.. • _ AT. Antionadi deals exhaustively with the .question of the so-called "canals," and his attitudo thereto may be judged from the title of Chapter IV., "The Illusion of the Canals." It may be recalled that during the favourable opposition of Mars in September, 1877, M. Schiaparelli, observing with a SJin Merz refractor, detected a series of faint lines crossing the Martian surface, so as to form thereon a complicated geometrical pattern; these delicate'markings be called canali or channels, but the term "caufl"' was used in translating hiß word and hence arose tie suggestion of their artificial origin and consequently of an intelligent mind directing their construction. Dr. Lowell, using a 27in reflector in the remarkably clear atmosphere at Flagstaff (altitude 7300 ft) confirmed the observations of Schiaparelli and greatly extended the number of the canals. In many attractive volumes he advocated their artificial origin; with facile pen he

drew an alluring picture of the denizeus of a dying world developing an j intricate system of irrigation to utilise | to the full the Bcanty supply of water with which Nature had provided them. j His skilfully presented case caught the public imagination and plans for communicating with our planetary neighbours were discussed, though, perhaps, not in the serious manner worthy of so important a matter. There were, however, some who doubted the real existence of the "canals",, it was remarked that their visibility was better in small' instruments than iii large. A story was current to the effect that when a seeker after truth enquired of the Director of one of ; the leading American observatories if the canals could, be seen at his establishment, lie received the reply "Canals not visible here: telescope - too big," and a considerable hpdy of opinion favoured the idea that ! the so-called canals wore the product of some form of optical illusion. Mr E. W. Maunder, of the Greenwich Observatory, held that the canals resulted from the joih-ing-up by the eye of separate and irregular markings at the limit of vision with the instruments employed. - M. Antionadi tells us that the veil was lifted and the mystery of the canals solved by the wonderful views he had of Mars in tho Mendon refractor -in 1909, when the geometrical and artificial appearance so often reported by observers was resolved into a number of perfectly natural and irregular surface features in accordance with Maunder ; s theory. Among other facts to which our author draws attention is the curious dark band which is so often seen as an edging to the polar snow-caps; this he is satisfied is a contrast illusion. He also states that there is an apparent relation between the melting of the snows and the variations of the solar activity. It is regarded by some solar physicists as well established that the intensity of the heat radiated out by the sun is subject to periodic changes, the amount of heat poured out when the sun-spots are at a maximum being greater than when they are at a minimum, and M. Antionadi finds a confirmation of this in his observation that the Martian winters are far more vigorous at a time of minimum sunspot activity thair at a maximum, the most striking instance of this being furnished by the great extent of the northern snow-cap at the time of the exceptional minimum of 1913. The sun is now passing towards a state of minimum spot-activity, and it will be interesting to gee whether the Tigours of the next two or three Martian winters confirm the conclusion M. Antionadi has reached on this point.