Page:Popular Astronomy - Airy - 1881.djvu/102

88 I then spoke of the apparent movement of the sun amongst the stars; and in speaking of their movements, I endeavoured to impress upon you how much you can observe for yourselves. You can learn more by your own observations than by the Lectures I can deliver, or by all the books you can read. In speaking of the apparent motion of the sun amongst the stars, I told you that in summer the sun is longer above the horizon and goes higher than in winter; the sun also rises more to the north in summer than in winter. Thus it describes a daily circle nearer to the North Pole of the heavens in summer than in winter; or, in other words, its place among the stars is nearer to the North Pole in summer than in winter. But there is also another set of facts, though not quite so familiarly known as these, which you can observe for yourselves. If you watch the appearance of the stars at a certain hour every night, you will find that these stars are to be found in a position on the night of one month, different from the position they are in on the corresponding night of the following month. You will observe from one month to another (if you always look at the same hour of the night) that they will travel away to the west. These motions are referred to the sun, by our habit of using solar time; that is to say, at the same solar hour, or when the sun is at the same distance from the meridian, the stars are travelling away to the right; or, in other words, the sun travels away to the left amongst the stars. And at the same time, as I have mentioned, it changes its distance from the North Pole.

Now, I will endeavour to point out to you how this is more accurately observed. I remarked that the Mural Circle is used to determine how far the