A Short History of Astronomy 



[Cn. II. 



phases are those which would be assumed by a spherical 



tbody of which one half only is illuminated by the sun. 



'Thus the visible portion of the moon is bounded by two 



planes passing nearly through its centre, perpendicular 



respectively to the lines joining the centre of the moon to 



those of the sun and earth. In the accompanying diagram, 



which represents a section through the centres of the sun 



FIG. 8. The phases of the moon. 



(s), earth (E), and moon (M), A B c D representing on a 

 much enlarged scale a section of the moon itself, the 

 portion DAB which is turned away from the sun is dark, 

 while the portion ADC, being turned away from the 

 observer on the earth, is in any case invisible to him. The 

 part of the moon which appears bright is therefore that of 

 which B c is a section, or the portion 

 represented by F B G c in fig. 9 (which 

 represents the complete moon), which 

 consequently appears to the eye as 

 bounded by a semicircle F c G, and a 

 portion F B G of an oval curve (actually 

 an ellipse). The breadth of this bright 

 surface clearly varies with the relative 

 positions of sun, moon, and earth ; so 

 that in the course of a month, during 

 which the moon assumes successively the positions relative 

 to sun and earth represented by i, 2, 3, 4, 5, 6, 7, 8 in 

 fig. 10, its appearances are those represented by the cor- 

 responding numbers in fig. n, the moon thus passing 



FIG. 9. The phases 

 of the moon. 



