THE PLANETS, AEE THEY INHABITED ? 



apparent diameter in rather less than four minutes. Such an 

 object would serve the purpose of the hand of a stupendous 

 celestial clock. 



The second satellite completes its revolution in about eighty- 

 five terrestrial hours, or about eight and a half Jovian days. It 

 passes, therefore, from quarter to quarter in twenty-one hours, 

 or about two Jovian days, its apparent motion in the firmament 

 being at the rate of about 4'25 per hour, which is as if our moon 

 were to move over a space equal to nine times its own diameter 

 per hour, or over its own diameter in less than seven minutes. 



The movements and changes of phase of the other two moons 

 are not so rapid. The third passes through its phases in about 

 170 hours, or seventeen Jovian days, and its apparent motion is 

 at the rate of about 1 per hour. The fourth and last completes 

 its changes in 400 hours, or forty Jovian days, and its apparent 

 motion is at the rate of little less than 1 per hour, being double 

 the apparent motion of our moon. 



Thus the inhabitants of Jupiter have four different months, of 

 four, eight, seventeen, and forty Jovian days respectively. 



16. Jupiter's moons differ from that of the earth, inasmuch as 

 all of them move in the plane of the planet's equator, from which 

 plane the sun can never depart further than about 3. At and 

 for a considerable time before and after the Jovian equinoxes, the 

 sun is so very near the planet's equator that each of the moons, 

 which never leave that equator, must necessarily pass between 

 the sun and the planet every revolution. It follows, therefore, 

 that for a long interval before and after each of the equinoxes, 

 solar eclipses will be produced by each of the four moons every 

 revolution. These eclipses, however, will be visible only at 

 certain low latitudes. The inhabitants of the higher latitudes 

 in either hemisphere will be so far removed from the common 

 direction of the moons and sun, or what is the same, from the 

 plane of the Jovian equator, that the visual line directed to the 

 sun will be clear of the moons. 



The shadow of this vast globe is so prodigious in its dimen- 

 sions that the three inner moons never pass behind Jupiter 

 without passing through it. They are therefore invariably 

 eclipsed every revolution ; and since at the time these moons 

 would appear full they are in direct opposition to the sun, they 

 are then plunged in the shadow, and therefore eclipsed. The 

 Jovians consequently never see any of these three moons when 

 they are full. 



The fourth or most remote of the moons is, like the others, 

 generally eclipsed every revolution ; but at the Jovian seasons 

 of midsummer and midwinter, for a certain interval, the sun, aiid 

 44 



