Without being precisely a great genius, this quiet and thoughtful monk seems to have been wise far beyond the age in which he lived, and remarkable for his independence of mind. He had a “profound sagacity,” and a wide general grasp of scientific subjects. The extremely complicated and cumbrous nature of the Ptolemaic system appeared to his judgment hardly compatible with the harmony and simplicity elsewhere characteristic of nature. Moreover, he was impressed and perplexed by the very marked changes in the brilliancy of the planets at different seasons. These changes _now_ are no difficulty at all. Venus, Mars, Jupiter, when on the same side of the sun as ourselves, are comparatively near to us, and naturally look much more bright in consequence of that nearness than when they are on the opposite side of the sun from ourselves. But under the Ptolemaic system, each planet was supposed to revolve round our earth, and to be always at about the same distance from us; therefore, why such variations in their brilliancy? [Illustration: NICOLAUS COPERNICUS.] During thirty-six long years he patiently worked out this theory, and during part of those thirty-six years he wrote the one great book of his lifetime, explaining the newer view of the Solar System which had taken hold of his reason and imagination. This book, named _De Revolutionibus Orbium Cœlestium_, or, “Concerning the Revolutions of the Celestial Spheres,” which came out only a few hours before his death, was dedicated to the Bishop of Rome--a little touch of worldly wisdom which doubtless staved off for a while the opposition of the Vatican. Copernicus was not the first who thought of interpreting the celestial motions by the theory of the earth’s motion. That immortal astronomer has taken care to give, with rare sincerity, the passages in the ancient writers from which he derived the first idea of the probability of this motion--especially Cicero, who attributed this opinion to Nicetas of Syracuse; Plutarch, who puts forward the names of Philolaus, Heraclides of Pontus, and Ecphantus the Pythagorean; Martianus Capella, who adopted, with the Egyptians, the motion of Mercury and Venus around the sun, etc. Even a hundred years before the publication of the work of Copernicus, Cardinal Nicolas of Cusa, in 1444, in his great theological and scientific encyclopedia, had also spoken in favor of the idea of the earth’s motion and the plurality of worlds. From ancient times to the age of Copernicus, the system of the earth’s immobility had been doubted by clear-sighted minds, and that of the earth’s motion was proposed under different forms. But all these attempts still leave to Copernicus the glory of establishing it definitely.