34 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



the almost absurd assumption of a density rapidly m- 

 emiMii" 1 from the center toward the surface. 



It has seemed to the writer quite possible that the 

 difference of illumination of the limbs of the planet, 

 caused by phase, may lie at the bottom of the difficul- 

 ty. Except on rare occasions there is phase enough, 

 even at the moment of opposition, to produce a no- 

 table difference of appearance between the fully illu- 

 minated edge of the planet's disk and that opposite, 

 a difference which can hardly fail to be felt in micro- 

 metric measurements. Unexceptionable observations 

 for determining the polar compression can therefore be 

 made only when the planet reaches opposition and its 

 node together. This was so nearly the case last sea- 

 son that, on the night of November 12th, an observer 

 on the planet would have witnessed a transit of the 

 earth. At this time, and for a few days before and 

 after, the phase was extremely small, and an oppor- 

 tunity was presented for determining the planet's 

 ellipticity such as will not be available again for nearly 

 half a century. 



An elaborate discussion of Professor Young's 

 observations gives, as a final result of the work 

 detailed, an ellipticity of F f^. In other words, 

 the polar diameter is to the equatorial in the 

 ratio of 218 to 219. This agrees very closely 

 with Professor J. C. Adams's recent estimate 

 of the compression (jfa) which the planet 

 ought to have if it follows the same law as 

 the earth in the variation of its density. The 

 ellipticity assigned by Professor Young is con- 

 siderably greater, it will be observed, than that 

 of the earth. 



Minor Planets. Since the date of our last 

 report, nine minor planets have been added to 

 the cluster between Mars and Jupiter, making 

 the present number 219. The dates of discov- 

 ery, names of discoverers, per.ods and mean 

 distances, are as follows : 



No. 216 has the greatest eccentricity (0'29) 

 of those discovered during the year, and the 

 last in the catalogue the greatest apparent 

 magnitude. 



The 219 minor planets now known have 

 been detected by thirty-five discoverers. First 

 in this list is Dr. C. H. F. Peters, Director of 

 the Observatory of Hamilton College, at Clin- 

 ton, New York, who has discovered forty-one 

 nearly one fifth of the whole number. The 

 next is Professor Palisa, of Pola, now credited 

 with twenty-seven. This successful observer 

 discovered five of those announced in 1880. 

 The third is the lamented James C. Watson, 

 late Director of the Observatory at Ann Ar- 

 bor, Michigan, who detected twenty-two. Of 

 the whole number, seventy have been discov- 

 ered by American astronomers. 



Jupiter's Period of Rotation. In the month- 

 ly notices of the Royal Astronomical Society 



for January, 1880, II. Pratt, Esq., of England, 

 discusses a series of observations of the great 

 red spot of Jupiter. These observations were 

 continued from July 26 to December 6, 1879, 

 including 321 rotations of the planet. Mr. 

 Pratt's resulting period of rotation is 9 h- 55 m - 

 33'91 8 - This exceeds the period found by the 

 Astronomer Royal in 1835 by 12'91 8- A series 

 of observations by T. D. Brewin, Esq., cover- 

 ing a period of 437 rotations, gave 9 h - 55 ra - 

 34*1 8- exceeding the value found by Mr. Pratt 

 by only 0-19 8 - 



Evidence that the Light of Jupiter is partly 

 intrinsic. Among the papers read at the 

 meeting of the Royal Astronomical Society, 

 on May 14, 1880, was one of great value, by 

 Dr. Henry Draper, of New York. For several 

 years it has been held, by Mr. Proctor and 

 others, that Jupiter's light is not wholly re- 

 flected. The facts now obtained by spectro- 

 scopic investigation seem clearly to indicate, 

 as Dr. Draper remarks, that "Jupiter is still 

 hot enough to give out light, though perhaps 

 only in a periodic or eruptive manner." It is 

 proper to remark, however, that in the discus- 

 sion which followed the reading of Dr. Draper's 

 paper, some doubt was expressed in regard to 

 his conclusions. The question can not be re- 

 garded as definitively settled without further 

 investigation. 



Jupiter's Bed Spot. Observations of Jupi- 

 ter's great red spot in 1879 and 1880 indicate 

 but little change in its appearance between the 

 last two oppositions of the planet. 



Probable Existence of Ultra - Neptunian 

 Planets. In February, 1880, Professor George 

 Forbes, of the University of Glasgow, read a 

 memoir before the Royal Society of Edinburgh, 

 designed "first, to show reasons for a belief in 

 the existence of two planets, whose orbits are 

 greater than that of Neptune ; and, secondly, 

 to indicate the probable positions of these 

 planets." 



The whole of this research is founded upon the the- 

 ory of the introduction of comets as permanent mem- 

 bers of the solar system, which is now generally held. 

 According to this theory, comets are bodies of size, 

 composition, and character, which we need not at 

 present discuss, but which move through interstellar 

 space subject to the laws of gravitation. Every time 

 that such a comet becomes sensibly attracted by any 

 star, such as we have reason to suppose our sun to be, 

 it is attracted toward it, and tends to describe about 

 it an orbit sensibly parabolic. 



Let us call the distance of the Earth from the sun 

 one Earth's radius. The orbits of comets thus at- 

 tracted into the sun's neighborhood, though on the 

 numerical average parabolic, may have their orbits 

 transformed, by planetary perturbation, either into 

 ellipses^ or hyperbolas. If the comet approach a 

 planet in such a manner that its velocity is increased, 

 it then will describe an hyperbolic orbit in future, 

 and will never again return to the sun. But if the 

 action of the planet be such as to reduce the velocity 

 of the comet, it will then for the future revolve in 

 an elliptic orbit round the sun, which may have its 

 elements altered by planetary perturbations, and rnay 

 eventually be actually in this way driven out of the 

 solar system altogether, but which, in default of these 

 accidental occurrences, must become a permanent mem- 

 ber of the solar system. 



