Page:A short history of astronomy(1898).djvu/104

60 see the moon at all; for example, to possible inhabitants of other planets, just as we on the earth can see precisely similar eclipses of Jupiter's moons. An eclipse of the sun is, however, merely the screening off of the sun's light from a particular observer, and the sun may therefore be eclipsed to one observer while to another elsewhere it is visible as usual. Hence in computing an eclipse of the sun it is necessary to take into account the position of the observer on the earth. The simplest way of doing this is to make allowance for the difference of direction of the moon as seen by an observer at the place in question, and by an observer in some standard position on the earth, preferably

an ideal observer at the centre of the earth. If, in fig. 31, denote the moon,  the centre of the earth,  a point on the earth between  and  (at which therefore the moon is overhead), and  any other point on the earth, then observers at  (or ) and  see the moon in slightly different directions,  the difference between which is an angle known as the parallax, which is equal to the angle  and depends on the distance of the moon, the size of the earth, and the position of the observer at. In the case of the sun, owing to its great distance, even as estimated by the Greeks, the parallax was in all cases too small to be taken into account, but in the case of the moon the parallax might be as much as 1° and could not be neglected.