Page:Astronomy for Everybody.djvu/55

It is well known that the earth not only turns on its axis, but makes an annual revolution round the sun. The result of this motion—in fact, the phenomenon by which it is shown—is that the sun appears to make an annual revolution around the celestial sphere among the stars. We have only to imagine ourselves moving round the sun and therefore seeing the latter in different directions, to see that it must appear to us to move among the stars, which are farther than it is. It is true that the motion is not at once evident because the stars are invisible in the daytime. But the fact of the motion will be made very clear if, day after day, we watch some particular fixed star in the west. We shall find that it sets earlier and earlier every day; in other words, it is getting continually nearer and nearer the sun. More exactly, since the real direction of the star is unchanged, the sun appears to be approaching the star.

If we could see the stars in the daytime, all round the sun, the case would be yet clearer. We should see that if the sun and a star rose together in the morning the sun would, during the day, gradually work past the star in an easterly direction. Between the rising and setting it would move nearly its own diameter relative to the star. Next morning we should see that it had gotten quite