THE MAJOR PLANETS. 



So rapid are these changes that we can conceive the gradual changes of the 

 phases of the moon to be actually visible as they proceed. The next satellite 

 makes its complete revolution in about eighty-five hours, or in about eight of Ju- 

 piter's days and a half. Such is the month of the second satellite. The third 

 satellite completes his revolution in one hundred and seventy hours, or in about 

 seventeen days of Jupiter. The fourth and most distant satellite, requires about 

 four hundred hours, to complete its revolution, and therefore has a month of 

 about forty of Jupiter's days. 



It appears, then, that upon Jupiter there are four different months, correspond- 

 ing to the four different moons ; one of about four days' duration, another about 

 eight days, a third about seventeen days, and the fourth about forty days. What 

 a complicated system of reckoning time is thus supplied ! 



The magnitude of the nearest of Jupiter's moons is about a quarter greater 

 than that of our own ; that of the second is equal to ours ; the diameter of the 

 third, however, is nearly double to that of our moon, and it is nearly equal to the 

 planet Mercury ; the diameter of the fourth satellite is about one half greater 

 than that of our moon. 



The distance of the nearest moon from the surface of Jupiter is somewhat 

 less than the distance of ours from the surface of the earth. Its apparent mag- 

 nitude, therefore, seen from Jupiter, will be greater than ours. The distance 

 of the second moon from Jupiter is about one half greater than the distance of 

 our moon, and as its diameter is nearly equal to that of our moon, its apparent 

 magnitude will be proportionally less. The distance of the third moon is 

 more than double the distance of ours, but as its magnitude is a little less than 

 double, its appearance to the inhabitants of Jupiter will be nearly the same as 

 that of ours. The appearance of the fourth moon will be somewhat less. 



Thus it appears that the four moons which attend Jupiter vary very little 

 in the apparent magnitude they present to its inhabitants from that which ours 

 presents to the inhabitants of the earth. 



One of the peculiarities in the motion of our moon which distinguishes it 

 in a remarkable manner from the planets, is its revolution upon its axis. It 

 will be remembered, that the planets generally rotate on their axes in times 

 somewhat analogous to that of the earth. Now, on the contrary, the moon re- 

 volves on its axis in the same time that it takes to revolve round the earth ; in 

 consequence of which adjustment of its motions it turns the same hemisphere 

 continually toward the earth. It would seem that this is a general character- 

 istic of all satellites ; for the observations of Sir William Herschel on those 

 of Jupiter, show that the same motion prevails among them ; that they, as 

 they revolve round their primary, turn constantly the same hemisphere towajd 

 Jupiter. 



The globe of Jupiter, though of considerable magnitude, is small compared 

 with that of the sun. In consequence of this it throws in the direction oppo- 

 site to that of the sun a conical shadow of Considerable length, the thickness of 

 which, at Jupiter, is equal to the diameter of the planet, but which diminishes until 

 it is reduced to a point in receding from Jupiter. As the satellites move round 

 Jupiter, in the plane of his equator, and as the plane of his equator is very { 

 nearly coincident with that of his orbit round the sun, it follows that the satel- 

 lites, every revolution, as they pass behind him, must move through his shadow. 

 The only exception to this is presented by the fourth, or most distant satellite, 

 which, owing to its great distance from the planet, and the obliquity of its 

 orbit, sometimes, in passing behind the planet, goes above or below its shadow 

 When the satellites get into the shadow of Jupiter they become invisible to us ; 

 and hence we know that they are opaque bodies, which shine, like the moon 

 by the reflected light of the sun. All the circumstances connected with their 



