138 OUTLINES OF NATURAL PHILOSOPHY'. 



Eclipses of the Moon. 



141. The length of the earth's shadow varies, 

 according to the distance of the sun and earth, 

 between the limits of 212.896 and 220.238 se- 

 midiameters of the earth ; its mean length be- 

 ing 216.531. 



Half the angle of the cone formed by the shadow of 

 the earth, = semid. parallax ; and there- 

 fore if r be the radius of the earth, S being the ap- 

 parent semidiameter, and p the horizontal parallax 

 of the sun, the length of the shadow reckoned from 



the earth's centre = 



sin (S p ) 



142. Hence half the angle subtended at the 

 earth's centre, by the section of the shadow at 

 the distance of the moon, is P S + p. 



P is the horizontal parallax of the moon, S and p as 

 above. 



The apparent semidiameter of the shadow, may 

 therefore vary from 37' 42" to 45',52. 



See WOODHOUSE, Astron. p. S40. 



As 



