INTRODUCTION TO ASTRONOMY. li 



earth, and she will then appear horned, as in the figure 6 : when she has 

 performed one quarter of her orbit, she shows us one half of her 

 enlightened side, as at C, and appears as in the figure c; at D she is said 

 to be gibbous, and at E the whole of the enlightened side appears to us, 

 and the moon is at full. As she proceeds in her orbit she becomes again 

 gibbous, and her enlightened hemisphere turns gradually away from us 

 till she completes her orbit and disappears, and then again resumes her 

 form of a new moon. The small exterior figures, o, 6, c, d, &c., it will 

 be seen, represent the phases corresponding to the situations A, B, C, D, 

 &c. : the light part of the figures, , 6, c, d, &c., alone being supposed 

 visible. 



When the moon is at full, she is said to be in opposition ; when a new 

 moon, to be in conjunction with the sun. At each of these times, the 

 sun, the moon, and the earth are in "the same right line ; but in the first 

 case, the earth is between the sun and the moon ; in the second, the moon 

 is between the sun and the earth. An eclipse can take place only when 

 the sun, moon, and earth are in a right line. When the moon passes 

 between the sun and the earth, she intercepts his rays, or, in other words, 

 casts a shadow on the earth : then the sun is eclipsed, and the daylight 

 gives place to darkness, while the moon's shadow is passing over us. 

 When, on the contrary, the earth is between the sun and the moon, it is 

 we who intercept the sun's rays, and cast a shadow on the moon : she 

 then disappears from our view, and is eclipsed. 



Why then have we not a solar and a lunar eclipse every month? 



The planes of the orbits of the earth and moon do not exactly coincide, 

 but cross or intersect each other ; and the moon generally passes either on 

 one side or the other when she is in conjunction with, or in opposition to, 

 the sun, and therefore does not intercept the sun's rays, or produce an 

 eclipse ; for this can take place only when the earth and moon are in con- 

 junction near those parts of their orbits which cross each other (called the 

 nodes of their orbits), because it is then only that they are both in the 

 same plane, and in a right line with the sun. A partial eclipse takes 

 place when the moon, in passing by the earth, does not entirely escape 

 her shadow. When the eclipse happens precisely at the nodes, they are 

 not only total, but last for some length of time. 



When the sun is eclipsed, the total darkness is confined to one parti- 

 cular part of the earth. In fig* 17 a solar eclipse is exhibited : S is the 

 sun, M the moon, and E the earth ; and the moon's shadow is not large 

 enough to cover the earth. 



Fig. 17. 



The lunar eclipses, on the contrary, are visible from every part of the 

 earth, where the moon is above the horizon. In Jig. 18, S represents the 

 sun, which pours forth rays of light in straight lines in every direction ; 

 E is the earth, and M the moon. Now a ray of light coming from one 

 extremity of the sun's disk in the direction AB will meet another coming 



E 2 



