Page:Popular Astronomy - Airy - 1881.djvu/221

Rh in one position with respect to Jupiter, we see the satellites go into the shadow; that is, we see them disappear without any apparent cause. In another position we see them come out of the shadow; that is, we see them begin to appear in the dark space at a short distance from Jupiter. Figure 55 is adapted



to the supposition that the satellites are coming out of the shadow. Suppose that C is the sun; E' ,E", the earth in two positions; J', J″, Jupiter in two corresponding positions. The time which is most favourable for the observation of Jupiter's satellites is that when the earth is nearly between the sun and Jupiter, as at E', because then Jupiter is seen nearly the whole night. In a short time after the invention of telescopes, Galileo and other astronomers observed the satellites, and found that their eclipses could be observed with great accuracy, and registered them with great care. They were able in no long time to form tables and calculations of the eclipses of Jupiter's satellites. These occurred principally when the earth