342 POPULAR SCIENCE MONTHLY. 



astronomical speculation, and make of the striped and spotted 

 sphere of Jove just such a world as may please his fancy — for a 

 world of some kind it certainly is. 



For many observers the satellites of Jupiter possess even greater 

 attractions than the gigantic ball itself. As I have already re- 

 marked, their movements are very noticeable and lend a wonderful 

 animation to the scene. Although they bear classical names, they 

 are almost universally referred to by their Roman numbers, begin- 

 ning with the innermost, whose symbol is I, and running outward 

 in regular order II, III, and IV. The minute satellite much nearer 

 to the planet than any of the others, which Mr. Barnard discovered 

 with the Lick telescope in 1892, is called the fifth, although in the 

 order of distance it would be the first. In size and importance, 

 however, it can not rank with its comparatively gigantic brothers. 

 Of course, no amateur's telescope can show the faintest glimpse 

 of it. 



Satellite I, situated at a mean distance of 261,000 miles from 

 Jupiter's center — about 22,000 miles farther than the moon is 

 from the earth — is urged by its master's overpowering attraction 

 to a speed of 320 miles per minute, so that it performs a complete 

 revolution in about forty-two hours and a half. The others, of 

 course, move more slowly, but even the most distant performs 

 its revolution in several hours less than sixteen days. The plane 

 of their orbits is presented edgewise toward the earth, from which 

 it follows that they appear to move back and forth nearly in straight 

 lines, some apparently approaching the planet, while others are 

 receding from it. The changes in their relative positions, which 

 can be detected from hour to hour, are very striking night after 

 night, and lead to a great variety of arrangements always pleas- 

 ing to the eye. 



The most interesting phenomena that they present are their 

 transits and those of their round, black shadows across the face 

 of the planet ; their eclipses by the planet's shadow, when they dis- 

 appear and afterward reappear with astonishing suddenness; and 

 their occultations by the globe of Jupiter. Upon the whole, the 

 most interesting thing for the amateur to watch is the passage of 

 the shadows across Jupiter. The distinctness with which they can 

 be seen when the air is steady is likely to surprise, as it is certain 

 to delight, the observer. "When it falls upon a light part of the 

 disk the shadow of a satellite is as black and sharply outlined as 

 a drop of ink; on a dark-colored belt it can not so easily be seen. 



It is more difficult to see the satellites themselves in transit. 

 There appears to be some difference among them as to visibility 

 in such circumstances. Owing to their luminosity they are best 



