66 SIXTEENTH CENTURY. pt. hi. 



He now 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 obser- 

 vations on the different positions of the sun during the 

 year, all proving how naturally the movements of the dif- 

 ferent 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 why Ptolemy was mistaken in believing the 

 earth to be the centre of the universe, and added a dia- 

 gram 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 publish 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 the 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. 



Work of Vesalius on Anatomy, 1542. — While Coper- 

 nicus was proving to himself that Ptolemy's theory of the 

 heavens was not a true one, a Belgian, named Vesalius, was 

 beginning to suspect that Galen, though a good physician, 



