ECLIPSES OF THE SUN. 109 



contrary direction. When it so happens that the moon, in 

 the course of her revokition about the earth, falls into this 

 shadow, she loses the sun's light, and appears to us eclipsed. 

 If we suppose two straight lines drawn from the opposite 

 parts of the solar disk, touching the surface of the earth on 

 opposite sides ; these lines will represent the limits of the 

 shadow, and as the sun is much larger than the earth, they 

 will meet at a point and cross each other behind the earth, 

 and the shadow will thus take the form of a cone. The 

 moon can come within the shadow of the earth only when 

 it is full, or in opposition to the sun. But the moon is not 

 eclipsed every time it is full, because its orbit does not coincide 

 with the plane of the earth's orbit, one half being about five 

 degrees and a third above it, and the other half as much below 

 it ; and unless the full moon, therefore, happen in or near 

 one of the nodes, that is, in or near the points in which the 

 two orbits intersect each other, she will pass above or below 

 the shadow of the earth, in which case there can be no 

 eclipse. If the moon be within twelve degrees from the 

 node, at the time when she is full, there will be a partial or 

 total eclipse, according as a part, or the whole of her disk 

 falls within the earth's shadow. As the shadow is consi- 

 derably wider than the moon's diameter, an eclipse of the 

 moon lasts sometimes three or four hours. It is by knowing 

 exactly at what distance the moon is from the earth, and of 

 course the width of the earth's shadow at that distance, that 

 eclipses are calculated with the greatest accuracy, for many 

 years before they happen. Lunar eclipses are visible over 

 every part of the earth that has the moon at that time above 

 the horizon ; and the eclipse appears of the same magnitude 

 to all from the beginning to the end. That faint reddish 

 colour, which the moon exhibits in the midst of an eclipse, 

 is supposed to proceed from the rays of light, which are re- 

 fracted by the earth's atmosphere, and fall upon the surface 

 of the moon. 



An eclipse of the sun is caused by an interposition of the 

 moon between the sun and the earth. This can happen 

 only at the new moon, or when the moon at her conjunction 

 is near one of her nodes ; for unless the moon is in or near 

 one of her nodes, she cannot appear in the same plane with 

 the sun, or seem to pass over his disk. In every other part 

 of her orbit she will appear above or below the sun. If the 

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