67. Centuries roll on, and the apparent motions of the heavenly bodies have now been gradually reduced to system. The stars in particular are found to move, just as if they were attached to the roof of a great hollow vault which revolves round the earth once in twenty-four hours. But even amongst them there are five exceptions—namely, Mercury, Venus, Mars, Jupiter, and Saturn—which perform a sort of wandering or zigzag motion in the midst of their stationary brethren, and have in consequence received the name of planets. All, however, are supposed to move round the earth, which forms the centre of the universe. 68. In process of time, this superiority of the earth over the heavenly bodies comes to be questioned. There is a rising tendency to regard our earth as a somewhat insignificant member of a great system, rather than as something apart by itself. These tendencies are, however, strongly opposed by the authorities of a large section of the Christian Church, on the ground that the language employed in the Jewish Scriptures is against such a method of regarding the universe. Nevertheless the Copernican system ultimately prevails, and the planets and the earth are associated together as stars which travel round the sun; while the diurnal motion of the heavenly bodies is attributed to a motion of the earth round its axis. And we cannot help thinking that philosophers of the present day are too much disposed to undervalue the absolutely enormous stride that was made when the Copernican system was fully established. 69. But the planets are still supposed to move in perfect circles round the sun; for besides the fact that this hypothesis agrees very well with observation, there is a simplicity in the circle which leads philosophers to believe that nature would adopt it in preference to any more complicated curve. Has it not been found that all apparent deviation from simplicity was in reality due to the fact that our point of view is a movable one, and does not this lead us to believe that the truth will be found in a circular orbit? 70. While such speculations are indulged in, Tycho Brahe is busy with his instruments. He is a thoroughly accurate man of science, and makes most excellent observations of the various planets. These are ultimately discussed by Kepler, who finds that the planets do not move round the sun in circles, but in ellipses, having the sun in one focus. He finds too that any one planet describes areas which are proportional to the times of description; while the squares of the periodic times of the various planets are proportional to the cubes of their mean distances from the sun. These are Kepler’s laws; they are yet, however, only empirical. We know them to be true, but we cannot tell why they should be as they are and not otherwise.