82 HISTORY OF SCIENCE. 



Kepler. NICHOLAS KOPERNIK (1473 Z 543)> or COPERNICUS as he 

 is more commonly called by English writers, was a native of Thorn, 

 in Polish Russia. From early youth he manifested a taste for science, 

 especially astronomy. Sent to the university at Cracow, he profited 

 so well by the studies there that he obtained the doctor's degree, and 

 soon afterwards was appointed professor of mathematics at Rome, a 

 position which he retained with great credit for several years. About 

 the beginning of the sixteenth century he left Rome; for having 

 entered the Church, he was appointed by a relative to an ecclesias- 

 tical office at Frauenberg in Prussia, where he had leisure' to pursue 

 his daily study in peace and comfort. After passing many years ob- 

 serving the motions of the celestial bodies and meditating upon the 

 systems of Ptolemy and the Pythagoreans, he drew up his* celebrated 

 treatise entitled " JDe Revolutionibus Orbium Cehstium" which com- 

 pletely changed for ever the science of astronomy. The great com- 

 plexity of the Ptolemaian system, its want of symmetry and order, 

 and the difficulty of conceiving so vast a machine as the sphere of 

 the fixed stars to revolve about the earth with the rapidity which the- 

 diurnal motion required, caused Copernicus to carefully compare his 

 own observations with the counter or helio-centric theory which had 

 been advocated by the Pythagoreans. The results convinced him of 

 the falsity of the then accepted theory, and led him to propose that 

 system the truth of which is now so abundantly demonstrated. But for 

 twenty-six years he continued by observation to test this now familiar 

 theory before announcing it to the world, and in fact his work was 

 published only a few days before its author's death in 1543. The 

 Copernican System places the sun motionless in the centre, and makes 

 the earth and the planets revolve about it in the following order : 

 Nearest the sun is Mercury, and in succession come Venus, the Earth, 

 Mars, Jupiter, and Saturn. The moon only revolves about the earth, 

 with which it is carried round in the annual revolution. Copernicus 

 supposed that the earth revolved in a circle, in the plane of which 

 circle the sun was situated, and that the earth also turned on its axis 

 every twenty-four hours, as if it rolled on the convex circumference of 

 its orbit, which would cause the sun to appear to move round the 

 heavens in a year, in a direction contrary to that of its diurnal mo- 

 tion. That the evidence of the senses which attribute movement to 

 the sun and stars is an illusion, Copernicus illustrates by a familiar ex- 

 perience and an apt quotation. "Why then do we hesitate to give 

 to the earth the mobility suitable to its form, rather than admit that 

 the universe, whose bounds we do not and cannot know, should re- 

 volve ? Why should we not confess that the diurnal revolution is ap- 

 parent only in the heavens, and real in the earth ? Thus ^Eneas in 

 Virgil exclaims, 



Provehimur portu terroeque urbesque recedunt. 



While the ship glides tranquilly along all external objects appear to the 



