THE PREDECESSORS OF COPERNICUS. 341 



through the deepest layer of atmosphere there) and must he taken 

 account of, even with rude observing apparatus. Kefraction had been 

 studied by Ptolemy and more deeply by Alhazen and Eoger Bacon. 

 Twilight, the scattering of the rays of the sun from the particles of 

 dust and the like in the upper atmosphere, was investigated by Peter 

 Nonius (1492-1577) a voluminous writer on astronomical matters. 



All that was known in astronomy was familiar to Eegiomontanus, 

 and during his seven years' residence in Italy his relations were with 

 the best instructed savants of Eome, who were then concerned with 

 projects for improving and correcting the calendar. When Copernicus 

 went to Italy in 1496 the best traditions of all Europe had spread 

 throughout its universities and he was, therefore, familiar with all that 

 his predecessors had accomplished. 



A passage from the 'Principles of Astronomy' of Gemma Frisius 

 (died 1558) is worth translation, since it fixes an important date and 

 describes methods of determining longitudes and latitudes which are 

 used to-day. He says: "People are beginning to make use of little 

 clocks that are called watches. They are not too heavy to be carried 

 about; they will run nearly twenty-four hours, and even longer if you 

 aid them a bit; they afford a very easy method of determining longi- 

 tude. Before starting on a journey, set your watch carefully to the 

 local time of the country you are leaving; take pains that the watch 

 doesn't stop on the road; when you have gone twenty leagues, for 

 instance, determine the local time of the place where you are, with an 

 astrolabe ; compare this with the time by your watch, and you will have 

 the difference of longitude." The latitude of the place can be had 

 by measuring the altitude of the pole-star. Watches, which were 

 invented about 1525, varied several minutes a day, and the portable 

 astrolabes of the time could hardly give the altitude so close as 10'; 

 but the methods were correct, and are those to-day employed in using 

 the chronometer and the sextant. 



Mention must be made of Peter Bienewitz, otherwise Peter Apianus 

 (born 1495, died 1552), who expounded the Ptolemaic system in a 

 great volume — Astronomicum Ccesareum (1540). Apianus was the 

 first to observe the sun through colored glasses. The astronomers of 

 Bagdad had observed an eclipse, when the sun was low, by its reflec- 

 tion in water, and Eeinhold had proposed to project the solar image 

 on a card in a camera obscura, a method which was used by the 

 astronomers of Galileo's time. His best contribution to astronomy 

 was the discovery that the tails of comets are generally directed away 

 from the sun, a remark independently made by Fracastor. 



Comets in his day were usually supposed to be atmospheric phe- 

 nomena. Why this connection between them and the sun? Wliy 

 should the sun, and not the earth, control their forms? The comet 

 of 1472 had been studied by Eegiomontanus and its course among the 



