OBLATE FORK AND SATELLITES. 



This is just the proportion which would be produced by a 

 rotation like that which Jupiter is ascertained to have. 



14. However agreeable may be the light of the moon in the 

 absence of the sun, that attendant is not indispensable to the 

 well-being of the inhabitants of the earth ; and of the inner 

 group 1 of planets the earth alone has been supplied with such a 

 supplement to the solar illumination. 



The planets constituting the outer group are, however, much 

 more munificently provided with this convenience, each being 

 supplied with so many moons that their nights must be per- 

 petually moonlit. 



When Galileo directed the first telescope to the examination 

 of Jupiter, he observed four minute stars, which appeared in the 

 line of the equator of the planet. He took these at first to be 

 fixed stars, but was soon undeceived. He saw them alternately 

 approach to and recede from the planet, observed them pass 

 behind it and before it, and oscillate, as it were, to the right 

 and left of it, to certain limited and equal distances. He soon 

 arrived at the obvious conclusion that these were bodies which 

 revolved round Jupiter in orbits, at limited distances, and that 

 each successive body included the orbit of the others within it ; 

 in short, that they formed a miniature of the solar system, in 

 which, however, Jupiter himself played the part of the sun. As 

 the telescope improved, it became apparent that these bodies 

 were small globes, related to Jupiter in the same manner exactly 

 as the moon is related to the earth ; that, in fine, they were a 

 system of four moons, accompanying Jupiter round the sun. 



15. But connected with these appendages there is perhaps 

 nothing more remarkable than the period of their revolutions. 

 That moon which is nearest to Jupiter completes its revolution in 

 forty-two hours. In that brief space of time it goes through all 

 its various phases ; it is a thin crescent, halved, gibbous, and 

 fulL It must be remembered, however, that the day of Jupiter, 

 instead of being twenty-four hours, is less than ten hours. This 

 moon, therefore, has a month equal to a little more than four 

 Jovian days. In each day it passes through one complete 

 quarter ; thus, on the first day of the month it passes from the 

 thinnest crescent to the half moon; on the second, from the 

 half moon to the full moon ; on the third, from the full 

 moon to the last quarter ; and on the fourth returns to con- 

 junction with the sun. So rapid are these changes that they 

 must be actually visible as they proceed. 



The apparent motion of this satellite in the firmament of 

 Jupiter is at the rate of more than 6 per hour, and is the same 

 as if our moon were to move over a space equal to her own 



