no POPULAR SCIENCE MONTHLY. 



nicus was received by him on the footing of a friend and helper, rather 

 than as a pupil ; and the association was, without doubt, of great benefit 

 to the younger man. All the systematized knowledge of the time was 

 opened to him; what was known was examined and discussed, not 

 received uncritically. Best of all, observation was practised as a test 

 of theory and as the only basis for its advancement. 



The first recorded observation of Copernicus is an occultation of 

 Aldebaran by the moon in 1497 at Bologna; in 1500 he observed a 

 conjunction of Saturn with the moon at the same place, and a lunar 

 eclipse at Eome. Other eclipses were observed in 1509, 1511, 1522 

 and 1523 ; and positions of Venus, Mars, Jupiter and Saturn in 1512, 

 1514, 1518, 1520, 1523, 1526, 1527, 1529, 1532, 1537. These recorded 

 observations extend over a period of forty years. Though they are few 

 in number, there is no reason to doubt that they are merely excerpts 

 from a more considerable collection. They were made with very 

 simple wooden instruments constructed by the observer's own hands. 

 One of them, a triquetum, was sent as a present to Tycho Brahe in 

 1584, forty-one years after the death of Copernicus. It was made of 

 pine wood, eight feet long, with two equal cross arms. They were 

 divided, in ink, into 1,000 equal parts, and the long arm into 1,414 

 parts. This precious relic, together with a portrait of Copernicus, 

 was long preserved in Tycho 's observatory at Uraniborg, and finally 

 removed to Bohemia, where it perished in the confusions incident to 

 the Thirty Years' War (1618-48). 



Eheticus once urged upon him the need of making astronomical 

 observations with all imaginable accuracy. Copernicus laughed at his 

 friend for being disturbed about so small an error as a minute of arc, 

 and declared that if he were sure of his observations to ten minutes, 

 he would be as pleased as was Pythagoras when he discovered the 

 properties of the right-angled triangle. Copernicus determined the 

 latitude of Frauenburg to be 54° 19%', which is 2' too small. This 

 seems to us a large error. Even with his instruments he could have 

 been more precise if he had repeated his observations many times. But 

 the determination was excellent for the times, as we may see by remem- 

 bering that the latitude of Paris was given by Tycho as 48° 10', by 

 Fernel as 48° 40', by Vieta as 48° 49', by Kepler as 48° 39'. His cal- 

 culated longitude of Spica Virginis, which he took as a standard star, 

 was 40' in error. He concluded that Krakau and Frauenburg were 

 on the same meridian — an error of 17%' of arc. The observations of 

 Albategnius, five centuries earlier, were far more precise, and this was 

 not entirely owing to the superiority of the Arab instruments. 



At the University of Bologna Copernicus mastered Greek. The 

 knowledge was subsequently utilized in a translation into Latin of the 

 epistles of Theophylactos Simokatta (630 A. D.), which he printed in 



