69] Regiomontnnus and Others 91 



to explain correctly the dim illumination seen over the 

 rest of the surface of the moon when the bright part is 

 only a thin crescent. He pointed out that when the 

 moon was nearly new the half of the earth which was 

 then illuminated by the sun was turned nearly directly 

 towards the moon, and that the moon was in consequence 

 illuminated slightly by this earthshine, just as we are by 

 moonshine. The v explanation is interesting in itself, and 

 was also of some value as shewing an analogy between 

 the earth and moon which tended to break down the 

 supposed barrier between terrestrial and celestial bodies 

 (chapter vi., 119). 



Jerome Fracastor (1483-1543) and Peter Apian (1495- 

 1552), two voluminous writers on astronomy, made obser- 

 vations of comets of some interest, both noticing that 

 a comet's tail continually points away from the sun, as 

 the* comet changes its position, a fact which has been 

 used in modern times to throw some light on the structure 

 of comets (chapter xin., 304). 



Peter Nonius (1492-1577) deserves mention on account 

 of the knowledge of twilight which he possessed ; several 

 problems as to the duration of twilight, its variation in 

 different latitudes, etc., were correctly solved by him ; but 

 otherwise his numerous books are of no great interest.* 



A new determination of the size of the earth, the first 

 since the time of the Caliph Al Mamun ( 57), was made 

 about 1528 by the French doctor John Feme I (1497-1558), 

 who arrived at a result the error in which (less than i per 

 cent.) was far less than could reasonably have been ex- 

 pected from the rough methods employed. 



The life of Regiomontanus overlapped that of Copper- 

 nicus by three years ; the four writers last named were 

 nearly his contemporaries ; and we may therefore be said to 

 have come to the end of the comparatively stationary period 

 dealt with in this chapter. 



* He did not invent the measuring instrument called the vernier, 

 often attributed to him, but something quite different and of very 

 inferior value. 



