... "Build, unbuild, contrive To save appearances, to gird the sphere With centric and eccentric scribbled o'er, Cycle and epicycle, orb in orb."
It was to the study of Mars, the wildest wanderer of all, that we owe the removal of all these perplexities. The idea had occurred to the great astronomer, Copernicus, that the complexities of the planets' paths are not real, but are caused by the constant moving about of the place from whence we watch the planets. If a fly at rest at the middle of a clock face watched the ends of the two hands, they would seem to go round him in circles; but if, instead, he was on the end of one of the hands (and was not knocked off as the other passed), the end of this other hand would not move round the fly in the same simple way. When the two hands were together it would be near, when they were opposite it would be far away, and, without entering into any particular description of the way in which it would seem to move, you can easily see that the motion would seem much more complicated than if the fly watched it from the middle of the clock face. Now, Copernicus _did_ enter into particulars, and showed by mathematical reasoning that nearly all the peculiarities of the planets' motions could be explained by supposing that the sun, not the earth, was the body round which the planets move, and that they go round him nearly in circles.
[Illustration: FIG. 3. THE PATHS OF MARS, THE EARTH, VENUS, AND MERCURY.]
But Copernicus could not explain _all_ the motions. And Tycho Brahe, another great astronomer, who did not believe at all in the new ideas of Copernicus, made a number of observations on our near neighbor Mars, to show that Copernicus was wrong. He gave these to Kepler, another great astronomer, enjoining him to explain them in such a way as to overthrow the Copernican ideas. But Kepler behaved like Balaam the son of Beor; for, called on to curse (or at least to denounce) the views of Copernicus, he altogether blessed them three times. First, he found from the motions of Mars that the planets do not travel in circles, but in ovals, very nearly circular in shape, but not having the sun exactly at the center. Secondly, he discovered the law according to which they move, now faster now slower, in their oval paths; and thirdly, he found a law according to which the nearer planets travel more quickly and the farther planets more slowly, every distance having its own proper rate. These three laws of Kepler constitute the Magna Charta of the solar system.