Jupiter without Satellites. 145 



satellites are eclipsed at eacli revolution, which does not happen 

 to the fourth, as it can pass above or below the shadow. 

 Passing between Jupiter and the sun, they likewise produce 

 solar eclipses, and their shadows on the disk can be seen in the 

 form of round dark spots, and in powerful telescopes even the 

 satellites themselves are visible. Some very curious laws have 

 been detected by the combination of the motions of the three 

 interior satellites, and it is impossible that they can all be 

 eclipsed at the same time. On some extraordinarily rare 

 occasions, however, Jupiter has been seen without any satellites. 

 They are not necessarily eclipsed on that account, as they may 

 pass either before or behind the disk. The three interior 

 satellites return to nearly the same position in respect to 

 Jupiter in a period of 487 days 4 hours/'' 



Jupiter is composed of much lighter materials than our 

 earth, his density being 243 when that of the Earth is reckoned 

 at 1000 ; but from his great size he is able to exert a great 

 power of attraction, and a body which would weigh 100 lbs. 

 on the Earth would weigh 224 lbs. at Jupiter's equator, and as 

 much as 276 at his poles. Jupiter's diurnal rotation takes 

 place in a few minutes less than ten hours ; so that his atmos- 

 pheric currents resulting from rotation must be extremely 

 violent — a fact which helps to explain the occasional rapid 

 changes in his belts, which are supposed to be the body of the 

 planet seen through his clouds, but which makes it more 

 difficult to account for the frequent persistence of those belts in 

 nearly the same condition for considerable periods. The first 

 of Jupiter's satellites has a diameter of 2400, and is about the 

 density of water; the second satellite is somewhat less in 

 bulk, being 2190 miles in diameter, and thus resembles our 

 moon in size : its density is greater than that of the other 

 three, or of the planet himself; the third satellite is 3580 

 miles in diameter ; and the fourth 3060 miles. Their distances 

 from Jupiter's centre are, respectively, 278,542 miles, 442,904 

 miles, 706,714 miles, and 1,242,619. They revolve round their 

 primary in periods varying from 1 day 18h. 27m. 33'505s., the 

 time of the first, to 16 days 16h. 32m. ll'271s., the time of 

 the fourth. 



From the preceding facts, it will be seen why the positions, 

 of the satellites change so constantly and so quickly. On the 

 21st of August their motions were so curiously combined, that 

 three satellites were on the disk of the planet at the same time, 

 and a fourth behind it. The appearance of the first satellite 

 gliding on to the disk at 10.4 p.m. was extremely beautiful as 

 we observed it. The first contact, the gradual passage, the 

 slight addition to the planet's margin as the last portion of the 

 satellite's disk passed inwards, was beautifully shown in one of 



VOL. XII. NO. II. L 



