The Moon. 315 



centre, and observe what portion of the illu- 

 minated portion of the moon comes below that 

 tangent, with regard to the earth. 



About the first octant and the fourth, the en- 

 lightened portion of the moon is in the form of 

 a crescent. The rest of the body of the moon 

 is then seen pretty distinctly. This results from 

 the light which is reflected upon the moon from 

 the surface of the earth. For, as we have the 

 light of the moon, so the moon has the light of 

 die earth. In other words, the earth is a moon 

 to the moon, and with similar phases. 



The revolution of the moon round the earth 

 measured by any fixed point in the heavens is 

 27 days 7 hours 43 minutes and 11 seconds. 

 This is called a periodical month. But the time 

 which intervenes from one conjunction with the 

 sun to another is 29 days 12 hours 44 minutes 

 and 3 seconds, and this is called a synodical 

 month or lunation. The reason of this dif- 

 ference is that, during the synodical revolution 

 of the moon, the earth advances on an average 

 about 27 degrees on the ecliptic. 



To render this sufficiently intelligible we must 

 have recourse to a diagram. In fig. 123. (PL 

 XXIX.) let S represent the sun, FC a part of 

 the earth's orbit, or ecliptic, M D a diameter 

 of the moon's orbit when the earth is at A, and 

 m d the same diameter when the earth is at B. 

 While the earth is at A, if the moon is at D 3 she 

 will be in conjunction, and if the earth were to 



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