LUNAR ECLIPSE. The phenomenon of a lunar eclipse is occa sioned by the passage of the moon through the earth's shadow, and the consequent interception of a portion of the solar light which usually falls upon the moon's surface. In the annexed figure, let A represent a section of the sun drawn through a great circle on its surface, and c D a similar section of the earth's surface, both sections being supposed to be in the same plane. Let the common tangents to these sections, A x, B Y, A n N, be drawn. In consequence of the great distance of the sun these tangents may be supposed to intersect each other at A, n and c, D, the opposite extremities of two pandlel diameters. It is plain that the space OD Z is 11 section of a cone, and that within its limits there will be a total interception of the sun's light. Hence, when the moon in the course of her monthly revolution passes into this cone, she will undergo an eclipse, which will be total or partial, according as she plunges wholly or partially into the terrestrial shadow. Within the region enclosing the shadow represented by Y c Z, Z D X, there will be a partial interception of the solar light. This region is termed the penumbra. If the moon revolved in the plane of the ecliptic, she would pass through the terrestrial shadow every titne that the earth was interposed between her and the sun, and conse quently she would undergo an eclipse in the course of each monthly revolution ; but, in point of fact, the lunar orbit is iuclined to the plane of the ecliptic at a mean angle of somewhat more than 5'.
It happens, from this circumstance, that an eclipse of the moon CAD occur only when the moon is near either of the nodes of her orbit. It is found, by calculation, that if the distance of the node from the point of the ecliptic opposite to the sun should exceed 11° 25' 40", there will bo uo eclipse. On the other hand, if the distance of the node from the same point should fall short of 20' 29", there must be an eclipse. Within these limits there may or may not be an eclipse, the question of the occurrence of such a phenomenon depending on the relative distances of the sun and moon from the earth at the time of opposition.
It has been found by observation that even when the moon Is totally immersed in the earth's shadow she still continues to be visible, her surface being distinguishable by a dull copper colour. This is occa sioned by the light passing through the terrestrial atmosphere being refracted so as to fall on the moon's surface. The ruddy colour of the moon is due to the absorption of the violet rays by the terrestrial atmosphere. The colour of the moon's surface during an eclipse is subject to considerable variations. Sometimes the degree of obscura tion is very slight; on other occasions it is so great that the moon ceases to be visible. Generally, it. may lei stated that the colour of the moon's surface will depend on the condition of the earth'. atmosphere at the time of the eclipse.