50 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



they would be entirely invisible except when 

 Mars was near perihelion. They have, how- 

 ever, been well observed at every opposition 

 since their discovery, including that of 1884. 

 It is now expected that in the most powerful 

 telescopes they will be visible even in the least 

 favorable oppositions, including those of 1886 

 and 1888. 



The Planet Jupiter. It has for some time been 

 suspected that the planet Jupiter bore a re- 

 markable resemblance to the sun, in that its 

 equatorial regions rotated in less time than 

 those nearer the poles. This view seems to 

 have been entirely confirmed by observations 

 upon the remarkable red spot which was visi- 

 ble from 1879 until 1883. This spot was in the 

 middle latitude, and from its motion the fol- 

 lowing periods of rotation of the planet on its 

 axis were obtained by Denning, of England : 



From a white spot near the equator the fol- 

 lowing times of rotation were obtained : 



It thus appears that there is a difference 

 of more than five minutes between the peri- 

 od of rotation at the equator and in the lati- 

 tude of the spot. This result is of great inter- 

 est, as showing how great the analogy is be- 

 tween the planet Jupiter and the sun. No sat- 

 isfactory explanation of this more rapid rota- 

 tion at the equator has yet been found. Since 

 the distance which the equatorial regions of 

 the planet must travel in order to make one 

 complete revolution is greater at the equator 

 than anywhere else, the time of its revolution 

 should, it might be supposed, be greater, in- 

 stead of less, as we actually find it. 



Saturn and its Satellites. Important and inter- 

 esting researches on the Saturnian system have 

 been made by Prof. Asaph Hall with the great 

 Washington telescope. One of these research- 

 es has led to a more certain value of the mass 

 of Saturn than any before attained. The 

 mean distance of the outer satellite was found 

 from the observations of 1875, 187G, and 1877, 

 which give the following separate values of 

 this element, when seen from a point distant 

 9'53885 astronomical units: 



1875; = 51.V504" 056". 57 observations. 



1*76; = 51.V4M" 0-O.W.. . 40 

 1877; = 515-517" 0-053" 31 " 



Mean; = 515-522" 



123 



By comparing the observations of Sir Will- 

 iam Herschel in 1789, one by Sir John Her- 

 schel at the Cape of Good Hope in 1837, and 

 his own observations, Prof. Hall found the 

 sidereal revolution to be performed in 79'- 

 3310152 days. From these results, the mass 

 of Saturn comes out -54*?.? of that of the sun. 

 This mass is one half per cent, greater than that 

 of Bessel, and more than one per cent, greater 

 than that found by Leverrier. It agrees, how- 

 ever, with other determinations from the mo- 

 tions of the satellites, and especially with the 

 conclusion of Dr. Meyer described in the 

 "Annual Cyclopedia" for 1883. The other 

 researches refer to the motion of Hyperion, 

 the minute satellite discovered by Bond in 

 1848, the orbit of which is next within that of 

 Titan. Some years ago Prof. Hall showed 

 that the line of apsides of the orbit of this 

 satellite was revolving in a retrograde direc- 

 tion. This _ was apparently in contradiction 

 to the received results of gravitation, since 

 the secular variation should be from west to 

 east. But in a paper published by the Royal 

 Astronomical Society in May last, it was shown 

 by the comparison of all the observations since 

 1852 that there could be no doubt of this seem- 

 ingly anomalous motion. This unexpected re- 

 sult led Prof. Newcomb to investigate the sub- 

 ject, and he found that the motion was due to 

 the fact that three times the period of revolu- 

 tion of Hyperion was very nearly equal to four 

 times that of Titan, the largest of the satellites, 

 which lay next within the orbit of Hyperion. 

 The correspondence was not perfectly exact, 

 and the result of its not being exact was that 

 the line of apsides was dragged around as it 

 were by Titan, in such a way that all conjunc- 

 tions of the two satellites take place at or near 

 the point when Hyperion was farthest from 

 Saturn. Thus there exists a third case of a 

 relation among motions in the solar system. 

 Those previously known were the rotation of 

 the moon on its axis, which we know to cor- 

 respond exactly to its time of revolution around 

 the earth, so that the same face is always pre- 

 sented to us, and the relation among three of 

 the satellites of Jupiter which prevents their 

 ever being in conjunction at the same time. 



In the spring of 1885 the ring of Saturn 

 reaches its greatest inclination to the line of 

 sight of an observer on the earth. The win- 

 ters of 1884, 1885, and 1886 are, therefore, the 

 the most favorable for studying the planet and 

 its rings which astronomers will enjoy until 

 another revolution is completed, which will not 

 be until 1914. It is understood that, under 

 favorable circumstances, Prof. Hall has suc- 

 ceeded in seeing the planet through the division 

 in the ring, thus showing beyond doubt that 

 the division is a real gap, and not merely a 

 dark portion of the ring. So far as known, 

 the observations made with the great Washing- 

 ton telescope do not confirm the view which 

 has sometimes been expressed by astronomers, 

 that the rings of Saturn change their aspect 



