Every one knows that when we look at the heavens we do not see the celestial bodies where they are, but where they _were_, and again, not where they were at any one moment of time, but some where they were a short time ago, others where they were very long ago. But it is not so generally known, or remembered by those who do know it, that if light were not so active as it is the result would be that utterly incorrect pictures of the celestial depths would continually be presented to us. As matters actually are no orb in space can appear very far from its true place. We see the sun, for instance, at any moment, not where he is, but where he was (or rather towards the direction in which he lay) about eight minutes before. But as the real velocity of the earth, and therefore the apparent velocity of the sun, amounts only to about eighteen miles per second, the sun is only thrown about 9000 miles out of his true position, which is but about the ninetieth part of his diameter: so that we see the sun very nearly in his right place. Now it might seem that a star whose light takes, say, twenty years in reaching us, must be seen very far from its true place, supposing the star to be travelling along very quickly; and, in one sense, this is true. If such a star is moving at the rate of fifty miles per second, athwart the line of sight, it will be out of place by so considerable a distance as 315,000,000,000 of miles. Yet the star will appear very nearly in its true position, simply because, at the star's enormous distance from us, even the great distance just named is reduced to a very small apparent amount. Such a star would, in fact, be displaced by only about the thirtieth part of the sun's or moon's apparent diameters, or by about a fifteenth part of the distance separating the middle star of the Great Bear's tail from its small companion, sometimes called Jack by the Middle Horse. Thus the stellar heavens present very truly to us the positions of the stars; for such athwart motion as I have just imagined would be very much larger than the motion of far the greater number of the stars. But we only thus see the heavens truly pictured because of the enormous velocity with which light travels. If light swept along only at the rate of a hundred miles in a second (a velocity still far beyond our powers of conception), there would be no believing what we should see, for every star, and our own sun, and all the planets, and even our own companion planet, the moon, would be thrown in appearance very far from their true positions. If they were all shifted in position by the same amount and in the same direction the picture would still be true, in a sense, just as we see a true picture of an object at the bottom of a clear lake, though the picture is displaced by the refractive action of the water on the rays of light. But, in the imagined case, the sun, and moon, and planets, and stars would be