night and destruction to the world. But, thanks to the advancement of science, which, while it has delivered us from the foolish fears and idle apprehensions of the ancients, leaves us in possession of their representative knowledge, enables us to explain the appearances on which it was founded, and points out the perversion and abuse of it.

the

Any opake body, that is exposed to the light of sun, will cast a shadow behind it. This shadow space deprived of light, into which, if another body comes, it cannot be seen for want of light; the body thus falling within the shadow, is said to be eclipsed.

The earth and moon being opake bodies, and deriving their light from the sun, do each of them cast a shadow behind, or towards the hemisphere opposed to the sun. Now, when either the moon or the earth passes through the other's shadow, it is thereby deprived of illumination from the sun, and becomes invisible to a spectator on the body from whence the shadow comes; and such spectator will observe an eclipse of the body which is passing through the shadow; while a spectator on the body which passes through the shadow, will observe an eclipse of the sun, being deprived of his light.

Hence there must be three bodies concerned in an eclipse; 1. the luminous body; 2. the opake body that casts the shadow; and, 3. the body involved in the shadow.

1

OF ECLIPSES OF THE MOON.

As the earth is an opake body, enlightened by the sun, it will cast a shadow towards those parts that are opposite to the sun; and the axis of this shadow will always be in the plane of the ecliptic, because both the sun and the earth are always there.

The sun and the earth are both spherical bodies; if they were, therefore, of an equal size, the shadow of the earth would be cylindrical, as in plate 8, fig. 5; and would continue of the same breadth at all distances from the earth; and would, consequently, extend to an infinite distance, so that Mars, Jupiter, or Saturn, might be eclipsed by it; but, as the planets are never eclipsed by the earth, this is not the shape of the shadow; and, consequently, the earth is not equal in size to the sun.

If the sun were less than the earth, the shadow would be wider the farther it was from the earth, see plate 8, fig. 6; and would, therefore, reach to the orbits of Jupiter and Saturn, and eclipse any of these planets when the earth came between the sun and them; but the earth never eclipses them, therefore this is not the shape of its shadow; and, consequently, the sun is not less than the earth.

As we have proved that the earth is neither larger nor equal to the sun, we may fairly conclude that it is less; and, that the shadow of the earth is a cone, which ends in a point at some distance from the earth, see plate S, fig. 7.

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.

The

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. 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