530 



SCIENTIFIC RECREATIONS. 



Galileo that the satellites sometimes appeared on one side of the planet, and 

 at other times on the opposite side. 



From the diagram of the Jovian System we shall understand the orbits of 

 the moons, which are all of nearly equal size, two 

 thousand miles in diameter, and cause eclipses of the 

 sun to Jupiter. If the earth be in the same direction 

 as the sun the moons are lost to view. The satel- 

 lites disappear into the shadow, and are eclipsed at 

 l '"> 2/ "> 3"', 4"', respectively, but they do not always 

 come into view again immediately they have passed 

 through the planet's shadow, because the earth is a 

 little at one side of the sun. So when the satellite 

 gets behind the edge of Jupiter, his shadow being on 

 the opposite side to the satellite's, it is said that the 



" moon " is in "occupation" ; when it disappears in the shadow it is " eclipsed." 

 Cassini discovered the " transit " of Jupiter's moons. The annexed diagram 

 illustrates the eclipses, etc., very clearly. At the four points, A B c D, we have 

 the earth ; J is Jupiter with his moons ; I 2 3 4 is their orbits. At a moon 

 No. i enters his shadow, and emerges at b. From the earth at D a will be 

 visible, but not b, because Jupiter is in the way. So at B, the coming out, 

 or emersion, will be visible, but not the entrance into the shadow, or immer- 

 sion. At A the satellite is in transit d y on the disc of the planet, J. 



Fig 588. Eclipses of Jupiter's Moons. 



From the observation of the eclipses of Jupiter's moons the rate of the 

 transmission of light was discovered by Roe'mer in 1675, and its progressive 

 motion was calculated. The eclipses were noticed to take place later than 

 the calculated time, when the planet was approaching conjunction. Roe'mer 

 suggested that the delay was owing to the greater distance the light had 

 to travel a distance equal to the diameter of the earth's orbit, or about 

 1 90,000,000 of miles. The time was about sixteen minutes. Light was 

 found to travel at the rate of nearly 1 2,000,000 of miles a minute. 



Let us now endeavour to picture Jupiter himself. Here we have an 



