Page:Popular Astronomy - Airy - 1881.djvu/52

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38
POPULAR ASTRONOMY

LECTURE II.

Recapitulation of Lecture I.—Investigation of the form and dimensions of the earth.—Proof that the earth really revolves.—Apparent motion of the Sun among the stars, or real motion of the earth round the Sun.—Permanence of an axis of Rotation.

IN the last lecture, I endeavoured to point out to you the principal phenomena of the motions of the stars, as observed on any fine night. And I called your attention to the fact, that these motions are performed in such a way, as to give us the idea of rotation round an axis inclined to the horizon; that some of the stars move very little; that others describe larger circles; that others just touch the horizon and descend below it; that others descend on one side and rise on the other side. I mentioned the names of two or three stars admitting of easy observation, as I am desirous that you should observe a little for yourselves, because you will acquire more knowledge from personal observation than from my lectures. The first of these is the Polar Star, which everybody ought to know: the second is the constellation of the Great Bear, which most people know as Charles' Wain; the third is the bright star Capella; the fourth is the bright star of Lyra. I described their motions: and I then pointed out to you that the observations were rendered more accurate by means of the instrument named the Equatoreal, which makes a telescope turn on an axis parallel to