LECTURE XXIV.

ASTRONOMY.

ECLIPSES.

1

In the 20th lecture I observed that, in a period somewhat shorter than a month, the moon was found once in conjunction, and once in opposition to the sun. In conjunction, when the moon is at N (Vol. i. Pl. XXVIII. fig. 119) between the sun S, and the earth T; and in opposition, when the moon is at L, in such a situation that the earth T is between it and the sun. In the first case we should naturally expect that the moon would hide from us the light of the sun; and in the second, that the earth would prevent the sun's light from reaching the moon; and, consequently, that every such occasion would produce an eclipse of one of those bodies. Notwithstanding this, we find the new and full moons but very seldom produce eclipses; and when they happen, it is not in the same proportions or in the same manner as you might expect on a cursory view of the figure.

If the plane of the moon's orbit were perfectly coincident with that of the earth, what we have been supposing would actually take place, and

there would be a total eclipse at every new and full moon; but Providence has wisely ordered it better, and I think I shall be able in few words to explain the reasons to your satisfaction. The moon's orbit is inclined about five degrees to the plane of the ecliptic, or the orbit- of the earth. When, therefore, the moon in the moment of her conjunction with the sun, happens to be in any point of her orbit a little distant from those points in which this orbit cuts the ecliptic, and which are called the nodes, there is sufficient latitude to admit of the light passing to the earth either above or below the moon, and there can therefore be no eclipse.-Or, when, in a similar case, the moon happens to be in opposition, the light of the sun will pass to her either above or below the earth, and the moon will not be eclipsed. But if the moon happens to be in the node, or very near it, at the time of her conjunction, she will then hide from us the light of the sun, and that luminary will be eclipsed: if, on the contrary, she happens to be in or near her node, while in opposition to the sun (being then in the plane of the earth's orbit), the earth will intercept from the moon the sun's light, and she will suffer an eclipse.

If you consider that it is only when the sun and moon happen to be in conjunction or opposition in or near one of the nodes that an eclipse can take place, you will see sufficient reason for the infrequency of the phænomenon. When, in

fact, the sun and moon are more than seventeen degrees from either of the nodes at the time of conjunction, the moon is then too high or too low in her orbit for any part of her shadow to fall on the earth; and when the sun is more than twelve degrees from either of the nodes at the time of opposition, the moon is too high, or too low, to pass through any part of the earth's shadow. Her orbit contains 360 degrees; of which seventeen, the limit of the solar eclipses on either side of the nodes, and twelve, the limit of lunar eclipses, are but small portions; and as the sun passes by the nodes but twice in a year, it is no wonder that we have several new and full moons without eclipses.

An eclipse of the moon then can only take place at its full, and when the moon is in opposition to the sun, and the moon is found either in one of his nodes, or near it. Suppose EE (Pl. III. fig. 6) to be a portion of the ecliptic at the distance of the moon, along which the shadow of the earth is moving. This shadow is represented by the black circular spots ABCD, which may be conceived to be sections of the cone DCE in fig. 7, where S is the sun, and T the earth. Suppose again the line LL (fig. 6) to be a portion of the orbit of the moon, which cuts the ecliptic EE in the point N, called the node, making with it an angle of a little more than five degrees. If, in the moment of her opposition, the moon be found in the point F in her

orbit, she will be too far from her node, which is in N, and will have too much latitude to be able to reach the shadow. But if she be found in the point G, having less latitude, a portion of her disc will be plunged in the shadow. There will then be a partial eclipse, and if the moon be still nearer to her node, as at the point H, the greater the obscuration will necessarily be.

In fine, if in the moment of opposition the moon be found precisely in her node N, the eclipse will not only be total, but central, and even will remain so for some time; for the centre of the moon coincides with the axis of the conical shadow formed by the earth; and this conical shadow DEC (fig. 7) occupying, in the orbit of the moon, a space FG, or fg, greater than the diameter of the moon L or M, it shades this planet for a time proportioned to the length by which the diameter of the shadow exceeds that of the moon, and this it is that causes the continuance of this planet in the shadow. The moon remains thus obscured the longest time when the sun S is in apogé, and the moon L in perigé ; for then the conical shadow is the greatest that can be; and the moon being in the point L of her orbit, which is the nearest the earth, is found also traversing the shadow in the place where this shadow has the greatest diameter, FG, that the moon can reach; whereas, when the moon M is in apogé, she traverses the conical shadow nearer the summit C, and conse

quently in a place, fg, where the shadow is nar

rower.

When the moon is even totally eclipsed, she does not cease to be visible, for it is only the shadow of the earth which is cast upon her. She appears of a copper colour, or like a heated iron half extinguished. This effect arises from the scattered rays of light from the moon, which are refracted by the terrestrial atmosphere, and, crossing each other, afford a faint picture of the moon. This light is feeble, because it is small in quantity, and it is of a reddish colour, because only the red rays can penetrate our atmosphere in these circumstances. This is more obscure in proportion as the moon is nearer the earth at the time of the eclipse: there have even been eclipses when the moon has totally disappeared, but this is a rare phænomenon.

An eclipse of the sun can only take place when the moon is in conjunction with the sun, and when she is in one of her nodes, or very near it. Suppose the line EE (fig. 8) to be a portion of the ecliptic, and LL a portion of the orbit of the moon, cutting the ecliptic in the point N, at an angle of a little more than five degrees. Then, if in the moment of her conjunction the moon is found in the point F of her orbit, she will be too far from her node to intercept the sun's light, and cause an eclipse. But if she is in the point G, having less latitude, she will hide a portion of the disc of the sun; and