ASTRONOMY. 139 



As the diameter of the shadow may be greater than 

 three times the diameter of the moon, the moon 

 may be totally eclipsed for as long a time as she 

 takes to describe twice her own diameter, that is, 

 for about two hours. 



143. The phenomana of a lunar eclipse may 

 be understood, by conceiving two circles of gi- 

 ven magnitudes, the disk of the moon, and the 

 section of the earth's shadow, as moving in the 

 same plane with given velocities, and given di- 

 rections ; the moments when they touch, when 

 they cease to touch, and when their centres are 

 nearest to one another, determine the begin- 

 ning, the greatest obscuration, and the end of 

 the eclipse. 



As these circles move in the same plane, and are in 

 contact with one another, the place of the specta- 

 tor makes no difference as to the preceding phe- 

 nomena, which must happen at the same instant 

 of absolute time to all observers, in whatever part 

 of space they are situated. Hence the pheno- 

 mena of a lunar eclipse are in themselves far sim- 

 pler, and more easily calculated, than those of a 

 solar eclipse, where the body that obscures the o- 

 ther is distant from it, so that their apparent 

 places are much affected by the place and the mo- 

 tion of the observer. 



144. Considering 



