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 wedth 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 infinite space, and accordingly must sometimes involve in it the other planets, and eclipse them, which is contrary to fact. Therefore, since the earth is neither larger than, nor equal to the sun, it must be the lesser body. We will now proceed to the eclipses of the sun. Charles. How are these occasioned? Tutor. An eclipse of the sun happens when the moon м, passing between the sun s and the earth T (Plate v11. Fig. 15.) intercepts the sun's light. James. The sun then can be eclipsed only at the new moon. Tutor. Certainly; for it is only when the moon is in conjunction that it can pass directly between the sun and earth. Charles. Is it only when the moon at her conjunction, is near one of its nodes, that there can be an eclipse of the sun? Tutor. An eclipse of the sun depends upon this circumstance: for unless the moon is in, or near one of its nodes, she cannot appear in the same plane with the sun, or seem to pass over his disk. In every other for in the former case the moon's shadow will reach the earth, and in the latter, on account of the earth's spericity, it will not. Charles. Have not eclipses been esteem. ed as omens presaging some direful cala mity? Tutor. Till the causes of these appearances were discovered, they were generally beheld with terror by the inhabitants of the world, which fact is beautifully alluded to by Milton in the 1st book of Paradise Lost, line 594: -As when the sun, new risen, Looks through the horizontal misty air |