reflection, we should find the object immersed in the shadow to be almost entirely invisible. Reasoning thus in the case of the Moon and the Earth, we find a strict analogy: a flood of light is shed by the Sun on the Moon at m, by which its front surface is wholly illuminated, and its hinder surface kept in the shade: there is also a shadow cast behind the opaque body of the Moon, the extent of which shadow depends conjointly on the sizes of the Sun and Moon, and the distance between them. This shadow is of two kinds, the umbra* and the penumbra; the former being the darker of the two: the reason of this difference is, that the umbra covers a portion of space wholly deprived of the Sun's light, while the penumbra covers that portion of the Earth's surface which has a view of part of the Sun's disk. Were the Sun but a luminous point, this distinction would not occur; but, as he has a very considerable diameter, and the Moon is very small compared even with the Earth, it follows that there will be no eclipse to places out of the penumbra; that places in the penumbra will have the Sun partially eclipsed; and that places in the umbra will see a total, or an annular, eclipse of the Sun; the latter depending upon the distance of the Moon from the Earth. The first of the two subsequent figures represents a total, the second an annular eclipse.

The Moon m being exactly between the Earth and the Sun, throws her dark conical shadow m a towards the Earth, which shadow, in the first case, reaches the Earth, and in the second falls short of it.

Umbra is the Latin for a shadow; penumbra implies a partial shadow.

If, now, we suppose an inhabitant of the Earth to be situated at the point a, (fig. 19,) he would witness a total eclipse of the Sun: he will be situated in the shadow of the Moon, and the body of the Moon will completely hide the Sun from his sight: this will be evident, when it is seen that he will be in the shadow, between the lines da, e a. If another inhabitant of the Earth were situated at any point between b and a, or a and C, he would see a partial solar eclipse; that is, the black disk of the Moon would appear to cut off a portion of the luminous surface of the Sun. The parts of the Earth beyond b and c, on either side, are out of the effects of the eclipse: the people beyond b and c see the whole disk of the Sun, as is plain by observing the direction of the lines be and c d.

If the Moon were at a constant distance from the Earth, and the Earth at a constant distance from the Sun, a central eclipse of the Sun would always present the same features to an inhabitant of the Earth; but such is not the case: the Earth is about three millions of miles nearer to the Sun in December than in June, on account of her being at that end of her elliptical orbit which is nearest to the Sun; consequently the Sun appears larger, or subtends a larger angle, at the former period than at the latter. Again, the Moon's orbit round the Earth is likewise elliptical; and although we are accustomed to say that the Moon is about 240,000 miles distant from the Earth, yet, when we wish to determine the distance at different times, it is found that the Moon's greatest distance from the Earth is about 251,000 miles, and her least about 227,000. Now, if the Moon were in perigee (that part of her orbit nearest to the Earth), and the Earth were in

aphelion (that part of her orbit furthest from the Sun), at the time of a central solar eclipse, the eclipse would be total, because the apparent diameter of the Moon, as seen from the Earth, would be greater than that of the Sun. But if the reverse were the case; if the Moon were at her greatest, and the Sun at his least, distance from the Earth, at the time of a central eclipse, then the eclipse would be annular*; that is, a ring of the Sun's light would seem to surround the dark body of the Moon, in consequence of the Sun having at that time a larger apparent diameter than the Moon. These differences would not occur, were it not that the apparent dimensions of the Sun and Moon, as seen from the Earth, are so nearly equal.

Hence it will be seen, by inspecting figure 20, that, in an annular eclipse, no part of the Earth can be enveloped in the Moon's umbra, owing to its not reaching the Earth; and the solar ring can be seen only by that part of the Earth where the umbra would have fallen, if the Moon had been nearer the Earth; that is, under the point a, where a straight line would reach which had passed through the centres of the Sun and Moon. The part of the Earth out of this spot, but between b and c, sees only a partial eclipse, and those beyond b and c see none at all.

In order to obtain some means for estimating the magnitude of an eclipse, the diameter of the Sun or Moon is supposed to be divided into twelve equal parts, called digits; so that, when it is said that such a number of digits was eclipsed, the meaning is at once known: the Moon was not in a node at the time of the eclipse,

* So termed from annulus, the Latin for a ring.