Charles. Will you, sir, explain to us the nature and causes of eclipses?

Tutor. I will, with great pleasure. You must observe, then, that eclipses depend upon this simple principle, that all opaque or dark bodies when exposed to any light, and therefore to the light of the sun, cast a shadow behind them in an opposite direction.

James. The earth being a body of this kind must cast a very large shadow on its side which is opposite to the sun.

Tutor. It does: and an eclipse of the moon happens when the earth T (Plate VII. Fig. 12.) passes between the sun s and the moon м, and it is occasioned by the earth's shadow being cast on the moon.

Charles. When does this happen?

Tutor. It is only when. the moon is full or in opposition, that it comes within the shadow of the earth.

James. Eclipses of the moon, however, do not happen every time it is full: what is the reason of this?

Tutor. Because the orbit of the moon does not coincide with the plane of the earth's orbit, but one half of it is elevated about five degrees and a third above it, and the other half is as much below it: and therefore, unless the full moon happen in or near one of the nodes, that is, in or near the points in which the two orbits intersect each other, she will pass above or below the shadow of the earth, in which case there can be no eclipse.

Charles. What is the greatest distance from the node, at which an eclipse of the moon can happen?

Tutor. There can be no eclipse, if the moon, at the time when she is full, be more than 12 degrees from the node; when she is within that distance, there will be a partial or total eclipse, according as a part, or the whole disk or face of the moon falls within the earth's shadow. If the eclipse happen exactly when the moon is full in the node, it is called a central eclipse.

James. I suppose the duration of the eclipse lasts all the time that the moon is passing through the shadow.

Tutor. It does: and you observe that the shadow is considerably wider than the moon's diameter, and therefore an eclipse of the moon lasts sometimes three or four hours. The shadow also you perceive is of a conical shape, and consequently, as the moon's orbit is an ellipse and not a circle, the moon will, at different times, be

eclipsed when she is at different distances from the earth.

Charles. And according as the moon is nearer to, or farther from the earth, the eclipse will be of a greater or less duration, for the shadow being conical, becomes less and less, as the distance from the body by which it is cast is greater.

Tutor. It is by knowing exactly at what distance the moon is from the earth, and of course the width of the earth's shadow at that distance, that all eclipses are calculated with the greatest accuracy, for many years before they happen. Now, it is found that in all eclipses, the shadow of the earth is conical, which is a demonstration, that the body by which it is projected is of a spherical form, for no other sort of figure would, in all positions, cast a conical shadow. This is mentioned as another proof, that the earth is a spherical body.

James. It seems to me to prove another thing, viz. that the sun must be a larger body than the earth.

Tutor. Your conclusion is just, for if the two bodies were equal to one another (Plate vII. Fig. 13.) the shadow would be cylindrical; and if the earth were the larger body, (Plate VII. Fig. 14.) its shadow would be of the figure of a cone, which had lost its vertex, and the farther it were extended the larger would it become. In either case the shadow would run out to an infi