64 SIXTEENTH CENTURY. PT. in. 



called the ' Theory of Epicycles,' Copernicus, after twenty 

 years of labour, turned back to the simple explanation 

 which Aristarchus had given (p. 21), and which was called 

 the Pythagorean System, namely, that the sun stands still 

 in the centre of our system, and that the earth and other 

 planets revolve round it. 



He had made a large quadrant, that is, an instrument 

 for measuring the angular height of the sun and stars, 

 and with this he made an immense number of observations 

 on the different positions of the sun during the year, all 

 proving how naturally the movements of the different planets 

 are explained by supposing the sun to stand still in the 

 middle. This he wrote down in his great work called ' The 

 Revolutions of the Heavenly Bodies,' in which he taught that 

 the earth must be round, and must make a journey every 

 year round the sun. He gave his reasons for believing that 

 Ptolemy was mistaken in believing the earth to be the 

 centre of the universe, and added a diagram of the orbits 

 of our earth and of the planets round the sun. He then, 

 went on to found upon this a whole system of Astronomy, 

 too complicated for us to follow here ; but he did not pub- 

 lish it, because he was afraid of public opinion ; for people 

 did not like to believe that our world is not the centre of 

 the whole universe. At last his friends persuaded him to 

 let his book be printed, and a perfect copy reached him 

 only a few days before his death, which occurred in 1543, 

 when he was seventy years of age. 



This work was the foundation of modern astronomy, 

 and the theory that the earth and planets move round the 

 sun has ever since been called the Copernican Theory ; but 

 at the time it was published very few persons believed in it, 

 and it was not till more than sixty years after the death of 

 Copernicus that Galileo's discoveries brought it into general 

 notice. 



