be partially deprived of the sun's light ; but if it be so much nearer as to be in- 

 cluded between the lines a /and b / it will be wholly deprived of the sun's 

 light. Thus the causes of a partial or total eclipse of the moon are ex- 

 plained. 



If the plane of the moon's orbit coincided with that of the ecliptic, the moon 

 would pass behind the centre of the earth in the direction of the line E f form- 

 ing the axis of the shadow, every revolution, and consequently there would be 

 a total lunar eclipse every month ; but as the moon's orbit is inclined at an 

 angle of five degrees to the plane of the ecliptic, the distance of the moon from 

 that plane is greater than the distance of lines of a c and b d from E f, except 

 when the moon is near to that point where its orbit crosses the ecliptic, which 

 is called the moon's node.' 



No lunar eclipses happen, therefore, except when either of the moon's nodes 

 is nearly in opposition to the sun. 



When a lunar eclipse does happen, the moon will first enter the penumbra 

 at a c, and will be very slightly obscured. As it approaches a/, it is more and 

 more deprived of the sun's light, until finally it enters the shadow afb, where 

 it is altogether obscured. At the end of the eclipse, as it must pass through 

 the penumbra, it will recover the sun's light by slow degrees. 



The length of the line E f being about 800,000 miles, and the distance of 

 the moon from the earth being less than 250,000, the moon when it passes 

 through the shadow will be about 500,000 miles within the point / and will 

 consequently pass through the shadow at a part of considerable breadth. 



In expressing the magnitude of the eclipse, whether of the sun or of the 

 moon, it is customary to suppose the diameters of these bodies divided into 

 twelve equal parts, called digits, and the magnitude of the eclipse is ex- 

 pressed by stating the proportion of the diameter of the disk which is obscured. 

 Thus when half the disk is obscured, we say that the eclipse measures six 

 digits, and so on. 



From what has been stated, it is evident that an eclipse of the moon will 

 not be affected in its appearance by the position of the observer on the surface 

 of the earth. Wherever he may be, the eclipse will appear to him the same ; 

 but if it should happen that while the moon is passing through the shadow, the 

 person desirous to observe it is in a portion of the earth which at that time is 

 turned toward the sun, the eclipse will, of course, be invisible to him. In 

 short, it will only be visible from that hemisphere of the earth that is turned 

 from the sun at the time of its occurrence. 



The moon, like the earth, receiving the sun's light, projects behind it a conical 

 shadow and a diverging penumbra : if this shadow or penumbra fall upon any 

 portion of the earth's surface, they will deprive such portion wholly or partially 

 of the sun's light, and there will be a solar eclipse of a corresponding species. 

 When the moon is between the sun and earth, the length of its shadow is about 

 equal to its distance from the earth, and consequently the point of the shadow 

 would just reach the surface of the earth ; but as the moon's distance is subject 

 to a slight variation, it sometimes happens that the length of the moon's shadow 

 is a' little more and sometimes a little less than its distance from the earth. If 

 . the length of the shadow be greater than its distance from the earth, then the 

 ( shadow will cover a small portion of the earth's surface, to all places within 

 which there will be a total solar eclipse. The circumstances affecting a solar 

 eclipse are represented in the annexed figure, where S is the centre of the 

 sun's disk, W is its upper limb, and V its lower limb ; c d is the moon, and e 

 the point of its shadow ; d h and c g are the sides of its penumbra, and a b is 

 the portion of the earth on which the penumbra falls. An observer placed be- 

 tween e and g, will see the upper limb of the sun only, the lower limb being 



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