COPERNICUS. 117 



nicus accepted. The precession of the equinoxes was discovered by 

 Hipparchus by comparing his own observations of stars with preceding 

 ones. He saw that the longitudes of the stars changed progressively 

 and fixed the annual change as 1° in seventy-five years. Later ob- 

 servers determined the amount of precession by comparing their own 

 observations with preceding ones. The motion of the origin of longi- 

 tudes — the equinox — is really uniform. An unlucky Jew — Tabit ben 

 Korra — in the ninth century, came to the conclusion that the motion 

 was not uniform, but variable, sometimes at one rate, sometimes at 

 another. The variable motion was the trepidation. Copernicus ad- 

 mitted the reality of this phenomenon and thereby introduced a fault. 

 Tycho Brahe, who had no important data on this point that was inac- 

 cessible to Copernicus, rejected the idea of trepidation and freed astron- 

 omy from a blemish that had endured for centuries. 



It is impossible and unnecessary to exhibit in this place the details 

 of the heliocentric theory of Copernicus. In Kepler's account of Coper- 

 nican astronomy there is a section on the explanation of the retrogra- 

 dations of the planets. "Here," he says, "is the triumph of the 

 Copernican astronomy. The old astronomy can only be silent and 

 admire; the new speaks and gives rational account of every appearance; 

 the old multiplies its epicycles; the new, far simpler, preserves every- 

 thing by the single motion of the earth around the sun. ' ' In describing 

 the stationary points of the planets he declared : ' ' Here the old astron- 

 omy has naught to say." 



We must try to put ourselves in the place of the students of those 

 days who heard the two explanations of the world — the geocentric and 

 the heliocentric — expounded by the same professor in the same lecture- 

 room as alternative hypotheses. Each hypothesis offered a possible 

 explanation. That of Copernicus was so simple that its intellectual 

 acceptance was immediate. It was possible; but was it true? If it 

 were accepted, what implications did it bring in its train? The real 

 difficulty was moral, not intellectual. "Was the whole edifice of 

 Ptolemy to be destroyed? No — some of it was indubitably true. If 

 some, why not all ? What was to become of the authority he had held 

 for a thousand years? Was all knowledge to be made over? Even 

 the idea that part of the 'Almagest' was true and part false was not 

 to be lightly accepted. 



The conception that every physical problem has one and only one 

 solution was also entirely new; until it was fully received students 

 balanced one explanation against another, and even held two at once, 

 strange as this may seem to us with our new standards in such matters. 

 The heliocentric theory eventually prevailed not because the logic of 

 Ptolemy was broken down, but because all mere authority was weak- 

 ened. The dicta of philosophers were looked at in a new light. It was 



