. Discovery. Science. 330 DISCOVERY foundations for the great modern advances. Yet even Copernicus was not entirely free from the influence of ancient superstition. He kept the perfect curve, the circle, for his planetary orbits, but he stated that the sun was displaced very slightly from the centre ; he also accounted for a few irregularities by retaining one or two epicycles. With the excep- tion of these minor points, however, the Copernican system was much the same as the one we now adopt. Kepler completed the work of his great predecessor some sixty-five years later by showing that the pa
. Discovery. Science. 330 DISCOVERY foundations for the great modern advances. Yet even Copernicus was not entirely free from the influence of ancient superstition. He kept the perfect curve, the circle, for his planetary orbits, but he stated that the sun was displaced very slightly from the centre ; he also accounted for a few irregularities by retaining one or two epicycles. With the excep- tion of these minor points, however, the Copernican system was much the same as the one we now adopt. Kepler completed the work of his great predecessor some sixty-five years later by showing that the paths of the planets are ellipses, but ellipses that do not depart very greatly from the circular form. Let us now pass on to consider how the loops in the planetary' paths can be explained. Suppose the inmost circle represents the orbit of the earth (Fig. 3), and the next one that of one of the outer planets, say Mars. Let the outer circle, shown in part, represent. the background of the sky, and let s be the sun. At equal intervals of time the earth will be at points marked i, 2, 3, 4, 5, 6, spaced out equally on its path. At the same time Mars will be at corresponding points on its orbit, marked I to 6 on the larger circle. When the earth and Mars are at their points i, Mars will appear to us to be at the corresponding point i on the sky-circle. Similarly for all the other points. The diagram shows us that we get a forward move- ment from I to 3, followed by a backward from 3 to 4, and then a forward from 4 to 6, and onwards until the next loop is commenced. If we imagine the circles as lying slightly out of the plane of the paper, and inclined to it at different angles, the path traced out becomes a loop. The outer planets alwavs appear brightest when they are in the middle of the backward portion of the loop, for then they are always in a line with the earth and the sun, or exactly opposite the sun as seen by us—a configuration to which we give the technical name " op
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