86
ECLIPSES.
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The Earth and the Moon , being opaque bodies, must cast ashadow into space, though of course, owing to its larger size,the Earth ’s shadow is much the largest. If the Moon shouldhappen to pass through either node, at or near the time ofopposition or Full Moon , it will be again, as before, in thesame straight line; only the Moon , in the present case, willbe involved in the shadow of the Earth , and therefore will bedeprived of the Sun ’s light: this is an Eclipse of the Moon .
Fig.18.
In Fig. 18, S, represents the Sun : E, the Earth , and m n,the orbit of the Moon : that the Moon becomes involved inthe Earth ’s shadow in passing from m to n is obvious.
If the orbits of the Earth and the Moon were in the sameplane, an eclipse would happen at every nodal passage, orabout 25 times every year; but as such is not the case,eclipses are of less frequent occurrence. According to themost recent investigations, in order that an eclipse of theSun may take place, the greatest possible distance of the Sun or Moon , from the true place of the nodes of the Moon ’s orbit,is 18° 36', whilst the latitude of the latter body must notexceed i° 34' 52". If, however, it be less than i° 23' 15",an eclipse must take place, though between these limits theoccurrence of the eclipse at any one station is doubtful, anddepends upon the horizontal parallaxes and semi-diameters ofthe two bodies at the moment of conjunction. In order thata lunar eclipse may take place, the remark we have just madewill equally hold good, provided only that 12 0 24', 63' 45"and 51' 57", be substituted for the numbers given above.