Astronomy, the oldest of the sciences, had made much progress in the tabulation of material. The apparent orbits of the sun, moon, planets, and stars had been correctly observed, so that eclipses might be predicted, conjunction of planets calculated, and that {617} gradual movement of the sun through the signs of the zodiac known as the precession of the equinoxes, taken account of. To explain these movements the ancients started on the theory that each heavenly body moved in a perfect circle around the earth; the fixed stars were assigned to one of a group of revolving spheres, the sun, moon and five planets each to one, making eight in all. But it was soon observed that the movements of the planets were too complicated to fall into this system; the number of moving spheres was raised to 27 before Aristotle and to 56 by him. To these concentric spheres later astronomers added eccentric spheres, moving within others, called epicycles, and to them epicycles of the second order; in fact astronomers were compelled:
To build, unbuild, contrive,
To save appearances, to gird the sphere
With centric and eccentric scribbled o'er
Cycle and epicycle, orb in orb.
The complexity of this system, which moved the mirth of Voltaire and, according to Milton, of the Almighty, was such as to make it doubted by some thinkers even in antiquity. Several men thought the earth revolved on its axis, but the hypothesis was rejected by Aristotle and Ptolemy. Heracleides, in the fourth century B. C., said that Mercury and Venus circled around the sun, and in the third century Aristarchus of Samos actually anticipated, though it was a mere guess, the heliocentric theory.
Just before Copernicus various authors seemed to hint at the truth, but in so mystical or brief a way that little can be made of their statements. Thus, Nicholas of Cusa [Sidenote: Nicholas of Cusa, 1400-64] argued that "as the earth cannot be the center of the universe it cannot lack all motion." Leonardo believed that the earth revolved on its axis, and stated that it was a star and would look, to a man on {618} the moon, as the moon does to us. In one place he wrote, "the sun does not move,"—only that enigmatical sentence and nothing more.
[Sidenote: Copernicus, 1473-1543]
Nicholas Copernicus was a native of Thorn in Poland, himself of mixed Polish and Teutonic blood. At the age of eighteen he went to the university of Cracow, where he spent three years. In 1496 he was enabled by an ecclesiastical appointment to go to Italy, where he spent most of the next ten years in study. He worked at the universities of Bologna, Padua and Ferrara, and lectured—though not as a member of the university—at Rome. His studies were comprehensive, including civil law, canon law, medicine, mathematics, and the classics. At Padua, on May 31, 1503, he was made doctor of canon law. He also studied astronomy in Italy, talked with the most famous professors of that science and made observations of the heavens.
Copernicus's uncle was bishop of Ermeland, a spiritual domain and fief of the Teutonic Order, under the supreme suzerainty, at least after 1525, of the king of Poland. Here Copernicus spent the rest of his life; the years 1506-1512 in the bishop's palace at Heilsberg, after 1512, except for two not long stays at Allenstein, as a canon at Frauenburg.
This little town, near but not quite on the Baltic coast, is ornamented by a beautiful cathedral. On the wall surrounding the close is a small tower which the astronomer made his observatory. Here, in the long frosty nights of winter and in the few short hours of summer darkness, he often lay on his back examining the stars. He had no telescope, and his other instruments were such crude things as he put together himself. The most important was what he calls the Instrumentum parallacticum, a wooden isosceles triangle with legs eight feet long divided into 1000 {619} divisions by ink marks, and a hypotenuse divided into 1414 divisions. With this he determined the height of the sun, moon and stars, and their deviation from the vernal point. To this he added a square (quadrum) which told the height of the sun by the shadow thrown by a peg in the middle of the square. A third instrument, also to measure the height of a celestial body, was called the Jacob's staff. His difficulties were increased by the lack of any astronomical tables save those poor ones made by Greeks and Arabs. The faults of these were so great that the fundamental star, i.e., the one he took by which to measure the rest, Spica, was given a longitude nearly 40 degrees out of the true one.
[Sidenote: Copernican hypothesis]
Nevertheless with these poor helps Copernicus arrived, and that very early, at his momentous conclusion. His observations, depending as they did on the weather, were not numerous. His time was spent largely in reading the classic astronomers and in working out the mathematical proofs of his hypothesis. He found hints in quotations from ancient astronomers in Cicero and Plutarch that the earth moved, but he, for the first time, placed the planets in their true position around the sun, and the moon as a satellite of the earth. He retained the old conception of the primum mobile or sphere of fixed stars though he placed it at an infinitely greater distance than did the ancients, to account for the absence of any observed alteration (parallax) in the position of the stars during the year. He also retained the old conception of circular orbits for the planets, though at one time he considered the possibility of their being elliptical, as they are. Unfortunately for his immediate followers the section on this subject found in his own manuscript was cut out of his printed book.