The Study of Astronomy, adapted to the capacities of youth / In twelve familiar dialogues, between a tutor and his pupil: explaining the general phænomena of the heavenly bodies, the theory of the tides, &c.

by teacher of astronomy John Stedman

Pupil. The shadow is a strait line.

Tutor. You now see that no other body than that of a globe can in all positions cast a circular shadow.

Pupil. I do, Sir.

Tutor. The darkness on the disc of the moon at the time of an eclipse is the shadow of the earth, which in all situations is circular; the earth, therefore, which casts the shadow, must be a globe.

Pupil. It must be so.—But——

Tutor. The earth is mountainous.—It is so: but remember that the highest mountain bears no greater proportion to the bulk of the earth than the small irregularities on the peel of an orange bears to that fruit: that objection therefore is soon removed. And yet it is not a true sphere.

Pupil. What then?

Tutor. A spheroid, that is, it is a little flattened at the poles, and is in shape not unlike an orange or a turnip. This you will not be surprized at when I tell you that the equatorial parts are about four thousand miles from the center of motion.

Pupil. I suppose then you infer that as the centrifugal force is greater the farther it is removed from the center, that the parts near the poles have a tendency to fly off towards the equator.

Tutor. I do. And as we have finished this part of our subject, I shall take leave of you.