as the country of Greece," and that he was persecuted for saying so. Having these things before us, I am not much surprised that the Greek astronomers considered the sun as completely subordinate to the earth, and therefore supposed it to revolve round the earth; and when they had once adopted this idea, they were compelled to take the complicated and unnatural explanation which I have given of the motion of the planets.

The motions of the planet Mars, however, still presented some discordance, and there were some smaller discordances with regard to all the other planets. Then were invented those things, known by the name of epicycles, deferents, &c. of which the nature may be thus explained. By the contrivance of which I have previously spoken, (and which is represented in Figure 26), they found that the movement of the point Ma at the end of the rod n Ma would nearly, but not exactly, represent the motion of Mars. To make it represent the motion more exactly, they supposed that another small rod MN was carried by the longer rod, jointed at M, and turning round in a different time. To make it still more exact, they supposed another shorter rod carried at N, and that its extremity carried the planet Mars; and so for the other planets. Of all the complications of systems that ever man devised, there never was one like this Ptolemaic system. The celebrated King of Castile, Alphonse, the greatest patron of Astronomy in his age, alluding to this theory of epicycles, said "If he had been consulted at the Creation, he could have done the thing better." It was merely expressing his absolute inability to receive, as a possible explanation of nature, such a complexity of things. But there was one consideration so simple, that it

seems astonishing that it did not occur to people before. When we suppose the earth fixed as at E, Figure 27, and take Venus (for instance) revolving

FIG. 27.

round a centre, we may alter the place of that centre and its distance from the earth as much as we please, and we shall then get the same appearances, provided we alter the dimensions of the orbit of Venus in the same proportion. As, for instance, in Figure 27, suppose E to be the earth, and suppose the small circle in which V is to be the orbit of Venus, the sun being at S; then, in revolving in her orbit, Venus appears to go to a certain extent to the right and to the left of the sun. But we might take any other point on the bar, even the point S itself, for the centre of the orbit of Venus, provided we give Venus a larger circle to revolve in. In the large orbit in which V' is seen, Venus will appear (as viewed from the earth) to move to the right or left of the sun; and if we do but make the orbit large enough, it will, as viewed from E, appear to move just as much to the right or left of the sun as if it moved in the small orbit. We may then fix the centre of the orbit of Venus where we please. When we have got thus far, we may easily make another step. Suppose we

assume the centre of the orbit of Venus to be the same point as the centre of the sun: we shall not have so much complexity. Suppose now we assume also that the centres of the orbits of the other planets are in the centre of the sun: we have seen that we can thus account for the motion of Venus, by giving proper dimensions to her orbit; and we can do the same thing for Mercury, and for Mars, and for every one of the planets. Just observe the state of things we have got to, as in Figure 28; instead of having

ма

FIG. 28.

different centres of motion for different planets, we have got them all in the centre of the sun; and the sun turns round the earth, carrying the orbits of the planets with it. That is a considerable simplification. In this state, I believe, the theory was received by the great Danish Astronomer, Tycho Brahé.

But now, instead of supposing the sun to be travelling, being itself the centre of the other orbits, and by some imaginary power causing the planets to revolve round itself as their travelling centre, suppose

H

we say that the earth revolves round the sun, and that the sun is a fixed, or nearly fixed body, and that all the planets, including the earth, go round the sun; that is, in Figure 28, instead of supposing S with the whole system of orbits to travel round E, suppose Me, V, E, Ma, and others, to travel in separate orbits round S. The appearances of the planets, as viewed from the earth, will be represented exactly as well as before. How it could then happen that a theory like that of the Greek astronomers could still be received as true, after the publication of the simple explanation which I have now given, is beyond my conception. It did, however, very much change the relative importance of the sun and the earth; it made the sun the most important body of all, and the earth one of the least important; and perhaps it was this which really occasioned the difficulty of receiving it. This great step in the explanation of the planetary motions was made by Copernicus, an ecclesiastic of the Romish Church, a Canon of Thorn, a city of Prussia. The work in which he published it is dedicated to the Pope. At that time it would appear that there was no disinclination in the Romish Church to receive new astronomical theories. But in no long time after, when Galileo, a philosopher of Florence, taught the same theory, he was brought to trial by the Romish Church, then in full power, and he was compelled to renounce the theory. How these two different courses of the Romish Church are to be reconciled I do not know, but the fact is so.

Soon after the time of Copernicus the telescope was invented by Galileo. One of the most important discoveries made with it was, that the planets do not always appear to be round, and that they obviously are not fully illuminated at all times. The

planet Venus puts on all the phases of the moon. When the planet Venus is at that part of her orbit at which, in conformity with our theory, she is beyond the sun, as at V', Figure 28, we then see her as round as the full moon; and when the planet Venus is at those parts at which, in conformity with our theory, she is almost between us and the sun, as at V”, it is found, by observation with the telescope, that she then puts on the phases of a young moon. These are precisely the appearances that would be seen if the theory is true. This is a most important confirmation, which was wanting in the time of Copernicus, and which with us is so convincing, that any one who has seen Venus will not doubt the truth of the theory.

The great step made by Copernicus was the assumption that the sun is the centre of the motion of all the planets (including the earth). But he could not get rid of the epicycles. As in Figure 26, where Mars is carried at N, at the extremity of a small arm, jointed on to a longer arm and revolving round the joint; so it was still necessary to suppose that each of the planets, as well as the earth, was carried by a similar apparatus; and even this did not represent the movements with perfect accuracy. This was reserved for Kepler to explain, who-not so much from his own observations as by examining accurately the observations which Tycho Brahé had made of the planets, and especially the planet Mars, and comparing them with his own-ascertained that the whole would be represented to the utmost accuracy by supposing that Mars moves in an ellipse. It is impossible now to explain in a few words how Kepler came to that conclusion; generally speaking, it was by the method of trial and error. The number of