At length the sun had fully gained his rightful position in the minds of men as center and controller of the Solar System, and earth was fully dethroned from her old false position. In point of fact, the sun had gained _more_ than his rightful position; since he was now looked upon very much as the earth had been formerly looked upon; since he was regarded practically as the motionless Universe-Center. The earth might and did move--people had grown used to that idea--but the sun, at all events, was fixed. The sun, beyond turning upon his axis, was motionless. Still, as to the true distance of the sun from ourselves, ignorance reigned. Tycho Brahe had made a great advance on earlier notions by placing the light-giver 4,000,000 miles away in imagination. Kepler, a quarter of a century later, increased this to 14,000,000. Galileo afterwards reverted to the 4,000,000. All this, however, was sheer guesswork. Not till well on in the seventeenth century did Cassini make a definite attempt at actual measurement of the sun’s distance, and this attempt gave as a result some 82,000,000 miles--not far removed from the truth. But at the same period other observers gave results of 41,000,000 and 136,000,000; so for years the question still remained swathed in mist. Better modes and better instruments were required before it could receive settlement. The measurement of the distance of objects a good way off is by means of their “parallax,” or the apparent change in their position when viewed from two different points at some distance from each other. To this method of calculating distance we have already referred. This kind of measurement of distances is not confined to our earth’s surface alone. The distances of bright bodies in the heavens may be calculated after just the same method, provided only that the heavenly body is not too far distant to be made to change its seeming position in the sky. Suppose you wished to find out how far off the moon is. You would have to make your observation of the moon’s exact position in the sky--of just that spot precisely where she is to be seen among heavenly scenery at a particular moment; and you would have to get somebody else to make a similar observation, at the same time, from some other part of earth, at a good distance from your post. There are no hills or woods in the sky for the moon to be seen against; but there are plenty of stars, bright points of light far beyond the moon. If you look at her from one place, and your friend looks at her from another place a good way off, there will be a difference in your two “views.” You will see her very close to certain stars; and your friend will see her not quite so close to those stars, but closer to others. This difference of view would give the moon’s parallax.