After his time astronomy, though it was not neglected, appeared to droop, and it is at a comparatively late period that we again open the records—viz., in 1543, the year in which Copernicus died. This philosopher, who was born in 1473, promulgated the true theory of the solar system. He placed the sun in the centre of the planets, and by this he explained their motion around the sun, though they appeared to be carried round the earth. The book in which he explained his theory, “De Revolutionibus Orbium Celestium,” was not finished till a day or two before he died.
The justly celebrated Tycho Brahé was the most important of the successors of Copernicus, but he opposed the Copernican theory, while other able philosophers agreed with it. Brahé was a Dane; he died in 1601. He adopted the theory that the sun and moon revolved around the earth, while the (other) planets moved around the sun. This theory did not gain much credence, but he, again, though he could not defeat Copernicus, and though he was wrong in his assumption, made many important investigations. After him came Kepler, whose observations upon the planet Mars cleared away many complications, and he laid down three laws, which are as follows:—
1. Every planet describes an elliptic orbit about the sun, which occupies one focus of each such ellipse.
Fig. 495.—Copernican System.
2. If a line be drawn from the sun, continually, to any planet, this line will sweep over equal areas in equal times.
3. The squares of the periodic times of the planets are proportional to the cubes of their mean distances from the sun.
Kepler also remarked that gravity was a power existing between all bodies, and reasoned upon the tides being caused by the attraction of the moon for the waters.
Fig. 496.—Ellipse.