THE MAJOR PLANETS. 243 



earth, the apparent diameter of the sun as seen from Jupiter will be one fifth 

 of its apparent diameter from the earth. It will, therefore, measure about six 

 minutes, since the diameter of the earth measures about thirty minutes. The 

 apparent magnitude of the sun as we see it, is very nearly that which a cent 

 piece would have if seen at the distance of one hundred and twenty feet from 

 the eye. The apparent magnitude of the sun as seen from Jupiter would then 

 be the same, or nearly so, as that of a cent piece seen at six hundred feet dis- 

 tance. 



. It is proved in those branches of physics in which the laws of heat and 

 ) light are developed, that the density of these principles is diminished in pro- 

 portion as the square of the distance from the body from which they emanate 

 is increased. It follows, therefore, that the heat and light of the sun at Jupiter 

 will be about twenty-five times less than at the earth. 



JUPITER'S SATELLITES. 



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 he was soon undeceived. 

 He saw them alternately approach and recede from the planet. He observed 

 them pass behind it and before it ; and, in fact, to oscillate, as it were, to the 

 right and the left of the planet, to certain limited distances ; each of the four 

 stars receding to equal distances east and west of the planet. He soon arrived 

 at the obvious conclusion that these objects were not fixed stars, but that they 

 were bodies which revolved round Jupiter in circular orbits, at limited dis- 

 tances ; 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, how- 

 ever, 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 cortege of four moons, attending Jupiter round the sun in the same 

 manner, and subserving the same purpose, as our moon does in reference to 

 the earth. v 



Thus, then, it seems that the population of Jupiter are favored by four moons 

 in their firmament. Since the examination of the motion of these bodies has 

 been carried to a greater extent of accuracy, it has been found that there is a 

 singular law prevailing among their motions, in virtue of which it is impossible 

 that the four satellites can ever be at the same time on the same side of Jupiter ; 

 one, at least, must be on the contrary side from the other three. Thus it fol- 

 lows that there must always be one moon full, or nearly so ; for if three of the 

 four satellites be on the same side of Jupiter with the sun, and therefore in 

 the condition of new or waning moons, the fourth must be on the opposite side, 

 and therefore nearly a full moon. 



But, connected with these appendages to Jupiter, there is perhaps nothing 

 more remarkable than the period of their revolutions round him. 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 ; 

 it is halved, gibbous, and full. It must be remembered, however, that the day 

 of Jupiter, instead of being twenty-four hours, is about ten nours. This moon, 

 therefore, has a month equal to a little more than four of Jupiter's days. In 

 each day it passes through one complete quarter ; thus the first day of the 

 month it passes from the thinnest crescent to the half moon ; in the second day, 

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

 the last quarter ; and on the fourth day returns to conjunction with the sun. 



