150 A Short History of Astronomy [Cn. vi. 



three times greater (in breadth and height), and he was 

 soon able to make telescopes which in the same way 

 magnified thirty-fold.- 



That the new instrument might be applied to celestial 

 as well as to terrestrial objects was a fairly obvious idea, 

 which was acted on almost at once by the English mathe- 

 matician Thomas Harriot (1560-1621), by Simon Marius 

 (1570-1624) in Germany, and by Galilei. That the credit 

 of first using the telescope for astronomical purposes is 

 almost invariably attributed to Galilei, though his first 

 observations were in all probability slightly later in date 

 than those of Harriot and Marius, is to a great extent 

 justified by the persistent way in which he examined object 

 after object, whenever there seemed any reasonable prospect 

 of results following, by the energy and acuteness with which 

 he followed up each clue, by the independence of mind 

 with which he interpreted his observations, and above all 

 by the insight with which he realised their astronomical 

 importance. 



119. His first series of telescopic discoveries were pub- 

 lished early in 1610 in a little book called Sidereus Nuncius, 

 or The Sidereal Messenger. His first observations at 

 once threw a flood of light on the nature of our nearest 

 celestial neighbour, the moon. It was commonly believed 

 that the moon, like the other celestial bodies, was perfectly 

 smooth and spherical, and the cause of the familiar dark 

 markings on the surface was quite unknown.* 



Galilei discovered at once a number of smaller markings, 

 both bright and dark (fig. 53), and recognised many of 

 the latter as shadows of lunar mountains cast by the 

 sun ; and further identified bright spots seen near the 

 boundary of the illuminated and dark portions of the moon 

 as mountain-tops just catching the light of the rising or 

 setting sun, while the surrounding lunar area was still in 

 darkness. Moreover, with characteristic ingenuity and love 

 of precisiori, he calculated from observations of this nature 

 the height of some of the more conspicuous lunar moun- 



* A fair idea of mediaeval views on the subject may be derived from 

 one of the most tedious Cantos in Dante's great poem (Paradiso, II.;, 

 i i which the poet and Beatrice expound two different " explanations " 

 cf the spots on the moon. 



