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

The Georgium Sidus cannot be readily perceived without the assistance of a telescope.

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DIALOGUE V.

Tutor.

Before I proceed to explain what I promised you, it is necessary you should be informed that the earth as seen from the sun, in its periodical revolutions, will describe a circle among the stars which astronomers call the ecliptic, and sometimes the sun’s annual path, because the sun, as seen from the earth, always appears in that line.

Pupil. Do not all the planets move in the ecliptic?

Tutor. No.—On account of the obliquity of their orbits, they are, in every revolution, one half of their periods above the ecliptic, and the other half below it.

Pupil. I think I comprehend your meaning; but shall be obliged to you, Sir, if you can make it clearer to me.

Tutor. I have here a little design, (Plate II. Fig. 1.) which will answer our purpose: where S represents the sun; ABCD, the orbit of the earth; and EFGH, the orbit of one of the inferior planets, suppose Venus.