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

Tutor. You must at present content yourself with knowing that it is not so; and it shall be my business to prove it.

Pupil. May I beg the favour of the information you intended respecting the planets?

Tutor. I will grant it with pleasure. The planets are spherical bodies, which appear like stars, but are not luminous; that is, they have no light in themselves; though they give us light; for they shine by reflecting the light of the sun.

Pupil. You say, Sir, that they appear like stars; if so, how am I to know them from stars?

Tutor. Very easily: for the stars, or as they are more properly called fixed stars, always keep the same situation with respect to each other; whereas the planets, as they move round the sun, must be continually changing their places among the fixed stars, and with one another.

Pupil. Is there any other method of distinguishing them besides what you have mentioned?

Tutor. Yes. The planets never twinkle like the fixed stars, and are seen earliest in the evening and latest in the morning.

Pupil. How is the twinkling of the stars in a clear night accounted for?

Tutor. It arises from the continual agitation of the air or atmosphere through which we view them; the particles of air being always in motion, will cause a twinkling in any distant luminous body, which shines with a strong light.

Pupil. Then, I suppose, the planets not being luminous, is the reason why they do not twinkle.