* 74, 7s] Publication of the " De Revolutionibus " 99 



appears to have been much alarmed at the thought of the 

 disturbance which the heretical ideas of Coppernicus would 

 cause, and added a prefatory note of his own (which he 

 omitted to sign), praising the book in a vulgar way, and 

 declaring (what was quite contrary to the views of the 

 author) that the fundamental principles laid down in it 

 were merely abstract hypotheses convenient for purposes 

 of calculation; he also gave the book the title De 

 Revolutionibus Orbium Celestium (On the Revolutions of 

 the Celestial Spheres), the last two words of which were 

 probably his own addition. The printing was finished in 

 the winter 1542-3, and the author received a copy of his 

 book on the day of his death (May 24th, 1543), when his 

 memory and mental vigour had already gone. 



75. The central idea with which the name of Coppernicus 

 is associated, and which makes the De Revolutionibus one 

 of the most important books in all astronomical literature, by 

 the side of which perhaps only the Almagest and Newton's 

 Prindpia (chapter ix., 177 seqq.) can be placed, is that 

 the apparent motions of the celestial bodies are to a great 

 extent not real motions, but are due to the motion of the 

 earth carrying the observer with it. Coppernicus tells us 

 that he had long been struck by the unsatisfactory nature 

 of the current explanations of astronomical observations, 

 and that, while searching in philosophical writings for some 

 better explanation, he had found a reference of Cicero to 

 the opinion of Hicetas that the earth turned round on its 

 axis daily. He found similar views held by other Pytha- 

 goreans, while Philolaus and Aristarchus of Samos had 

 also held that the earth not only rotates, but moves 

 bodily round the sun or some other centre (cf. chapter n., 

 24). The opinion that the earth is not the sole centre 

 of motion, but that Venus and Mercury revolve round the 

 sun, he found to be an old Egyptian belief, supported 

 also by Martianus Capella, who wrote a compendium of 

 science and philosophy in the 5th or 6th century AD. 

 A more modern authority, Nicholas of Cusa (1401-1464), a 

 mystic writer who refers to a possible motion of the earth, 

 was ignored or not noticed by Coppernicus. None of 

 the writers here named, with the possible exception of 

 Aristarchus of Samos, to whom Coppernicus apparently 



