The axis of the earth's shadow falls always upon that point of the ecliptic that is opposite to the sun's geocentric place; thus, if the sun be in the first point of Aries, the axis of the earth's shadow will terminate in the first point of Libra. It is clear, therefore, that there can be no eclipse of the moon but when the earth is interposed between it and the sun; that is, at the time of its opposition, or when it is full; for, unless it is opposite to the sun, it never can be in the earth's shadow: and, if the moon did always move in the plane of the ecliptic, she would, every full-moon, pass through the body of the shadow, and then it would be a total eclipse of the moon.

We have already observed, that the moon's orbit is inclined to the plane of the ecliptic, and only coincides with it in two places, which are termed the nodes. It may, therefore, be full-moon* without her. being in the plane of the ecliptic; she may be either on the north or the south side of it; in either of these cases she will not enter into the shadow, but be above it in the one, and below it in the other.

To illustrate this, let HG, plate 10, fig. 1, represent the orbit of the moon; EF the plane of the ecliptic, in which the centre of the earth's shadow always moves; and N the node of the moon's orbit; ABCD four places of the shadow of the earth in

A planet may be in opposition to, or conjunction with, the sun, without being in a right line that passes through the sun and the earth. Astronomers term it in conjunction with the sun, if it be in the same part of the zodiac; in opposition, if it be in a part of the zodiac, 180° from the sun.

the ecliptic. When the shadow is at A, and the moon at I, there will be no eclipse; when the full moon is nearer the node, as at K, only part of her globe passes through the shadow, and that part becoming dark, it is called a partial eclipse; and it is said to be of so many digits as there are twelfth parts of the moon's diameter darkened. When the fullmoon is at M, she enters into the shadow C; and, passing through it, becomes wholly darkened at L, and leaves the shadow at O: as the whole body of the moon is here immersed in the shade, this is called a total eclipse; but, when the moon's centre passes through that of the shadow, which can only happen when she is in the node at N, it is called a total and central eclipse. There will always be such eclipses, when the centre of the moon and axis of the shadow meet in the nodes,

The duration of a central eclipse is so long, as to let the moon go the length of three of its diameters totally eclipsed, which stay in the earth's shadow, is computed to be about four hours; whereof the moon takes one hour, from its beginning, to enter the shadow, till quite immersed therein; two hours more she continues totally dark; and the fourth hour is taken up from her first beginning to come out of the shadow, till she is quite out of it.

In the beginning of an eclipse, the moon enters the western part of the shadow with the eastern part of her limb; and, in the end of it, she leaves the eastern part of the shadow with the western part of her limb. All the intermediate time, from

her entrance to her quitting the shadow, is reckoned into the eclipse; but only so much into the total immersion, as passes while the moon is altogether obscured.

From the magnitude of the sun, the size of the earth, their distance from each other, the refraction of the atmosphere, and the distance of the moon from the earth, it has been calculated that the shadow of the earth terminates in a point, which does not reach so far as the moon's orbit. The moon is not, therefore, eclipsed by the shadow of the earth alone. The atmosphere, by refracting some of the rays of the sun, and reflecting others, cast a shadow, though not so dark a one as that which arises from an opake body: when, therefore, we say that the moon is eclipsed, by passing into the shadow of the earth, it is to be understood of the shadow of the earth, together with its atmosphere. Hence it is that the moon is visible in eclipses, the shadow cast by the atmosphere not being so dark as that cast by the earth. The cone of this shadow is larger than the cone of the earth's shadow, the base thereof broader, the axis longer, There have been eclipses of the moon, in which the moon has entirely disappeared: Hevelius mentions one of this kind, which happened in August 1647, when he was not able to distinguish the place of the moon, even with a good telescope, although the sky was sufficiently clear for him to see the stars of the fifth magnitude.

All opake bodies, when illuminated by the rays

of the sun, cast a shadow from them, which is encompassed by a penumbra, or thinner shadow, which every where surrounds the former; growing larger and larger as we recede from the body: in other words, the penumbra is all that space surrounding the shadow, into which the rays of light can only come, from some part of that half of the globe of the sun which is turned towards the planet, all the rest being intercepted by the intervening body.

Let S, plate 10, fig. 2, be the sun; E the planet; then the penumbral cone is FGH. The nearer any part of the penumbra is to the shadow, the less light it receives from the sun; but the further it is, the more it is enlightened; thus the parts of the penumbra, near M, are illuminated by those rays of light which come from that part of the sun near to I, all the rest being intercepted by the planet E. In like manner, the parts about N can only receive the light that comes from the part of the sun near to L; whereas the parts of the penumbra at P and Q are enlightened in a much greater degree: for the planet intercepts from P only those rays which come from the sun near L, and hides from Q only a small part of the sun near I.

The moon passes through the penumbra before she enters into the shadow of the atmosphere. This causes her gradually to lose her light, which is not sensible at first; but as she goes into the darker part of the penumbra, she grows paler. The penumbra, where it is contiguous to the shadow, is so dark, that it is difficult to distinguish one from the other. If

the atmosphere be serene, every eclipse of the moon is visible at the same instant to all the inhabitants of. that side of the earth to which he is opposite.

The moon in a total eclipse, generally appears of a dusky reddish colour, especially towards the edges; but of a darker towards the middle of the shadow.

OF ECLIPSES OF THE SUN.

The moon, when in conjunction, if near one of her nodes, will be interposed between us and the sun, and will consequently hide the sun, or a part of him, from us, and cast a shadow upon the earth: this is called an eclipse of the sun; it may be either partial or total.

An eclipse of any lucid body is a deficiency or diminution of light, which would otherwise come from it to our eye, and is caused by the interposition of some opake body.

The eclipses of the sun and moon, though expressed by the same word, are in nature very different; the sun, in reality, loses nothing of its native lustre in the greatest eclipses, but is all the while incessantly sending forth streams of light every way round him, as copiously as before. Some of these streams are, however, intercepted in their way towards our earth, by the moon coming between the earth and the sun: and the moon having no light of her own, and receiving none from the sun on that half of the globe which is towards our eye,