242 ASTRONOMY. 



The eclipses of the sun and moon are the most 

 striking. They were formerly considered as 

 ominous, and have often excited the dread and 

 apprehension of the vulgar ; but the improvement 

 of science has shown, that they have no connexion 

 with future events; that they depend upon regular 

 and invariable causes, and may be calculated and 

 foretold with the greatest certainty. 



As the earth is an opaque body, enlightened 

 only by the sun, it will cast a shadow towards that 

 side which is farthest from the sun. If the sun 

 and earth were of the same size, this shadow would 

 be cylindrical, and would extend to an infinite dis- 

 tance; but as the sun is much larger than the 

 earth, the shadow of the latter must be conical, 

 and end in a point (Plate 24. Fig. 2). On the 

 sides of this conical shadow, there is a diverging 

 shadow, the density of which decreases in pro- 

 portion as it recedes from the sides of the former 

 conical shadow : this is called the penumbra. As 

 the moon revolves round the earth, sufficiently 

 near to pass through the shadow of the earth, an 

 eclipse must always take place when these three 

 are all in one straight line. An eclipse of the 

 moon can never happen but at the time of full 

 moon j but on account of the inclination of the 

 moon's orbit to that of the earth, an eclipse can- 

 not take place every full moon. When the moon 

 passes entirely through the earth's shadow, the 

 eclipse is total ; but when only a part of it passes 

 through the shadow, the eclipse is partial. The 

 quantity of the moon's disc which is eclipsed (and 

 the same thing is to be understood of that of the 

 sun, in a solar eclipse), is expressed by twelve 

 parts called digits : that is, the disc is supposed to 

 be divided by 12 parallel lines; then if half the 



