60 A Short History of Astronomy [CH. n. 



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 



M 



FIG. 3i.-j-Parallax 



an ideal observer at the centre of the earth. If, in 

 fig. 31, M denote the moon, c the centre of the earth, 

 A a point on the earth between c and M (at which therefore 

 the moon is overhead), and B any other point on the earth, 

 then observers at c (or A) and B see the moon in slightly 

 different directions, c M, B M, the difference between which 

 is an angle known as the parallax, which is equal to the 

 angle BMC and depends on the distance of the moon, 

 the size of the earth, and the position of the observer 

 at B. 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 i and 

 could not be neglected. 



