[Illustration: DIAGRAMS ILLUSTRATING THE THEORY OF ECLIPSES.]
The theory of lunar eclipses will be understood from Fig. 1, where _S_ represents the sun, _E_ the earth, and _M_ the moon. If the sun were a point of light there would be a sharply outlined shadow or umbra only, but since the luminous surface is so large, there is always a region in which the light of the sun is only partially cut off by the earth, which region is known as the penumbra (_P P_). Hence during a lunar eclipse the moon first enters the penumbra, then is totally eclipsed by the umbra, then emerges through the penumbra again.
An eclipse of the sun is an occultation of the whole or part of the face of the sun occasioned by an interposition of the moon between the earth and the sun; thus all eclipses of the sun happen at the time of new moon.
Fig. 2 is a diagram showing the principle of a solar eclipse. The dark or central part of the moon’s shadow, where the sun’s rays are wholly intercepted, is here the umbra, and the light part, where only a part of them are intercepted, is the penumbra; and it is evident that if a spectator be situated on that part of the earth where the umbra falls there will be a total eclipse of the sun at that place; in the penumbra there will be a partial eclipse, and beyond the penumbra there will be no eclipse.
As the earth is not always at the same distance from the moon, and as the moon is a comparatively small body, if an eclipse should happen when the earth is so far from the moon that the moon’s shadow falls short of the earth, a spectator situated on the earth in a direct line between the centers of the sun and moon would see a ring of light around the dark body of the moon; such an eclipse is called annular, as shown in Fig. 3; when this happens there can be no total eclipse anywhere, because the moon’s umbra does not reach the earth.
An eclipse can never be annular longer than twelve minutes twenty-four seconds, nor total longer than seven minutes fifty-eight seconds; nor can the entire duration of an eclipse of the sun ever exceed two hours.
An eclipse of the sun begins on the western side of his disc and ends on the eastern; and an eclipse of the moon begins on the eastern side of her disc and ends on the western.