702 COSMOS. 



he first directed towards the mountainous parts of the moon, 

 and showed how their summits might be measured, whilst he, 

 like Leonardo da Vinci and Mostlin, ascribed the ash- 

 coloured light of the moon to the reflection of solar light from 

 the earth to the moon. He observed with low magnifying 

 powers the group of the Pleiades, the starry cluster in Can- 

 cer, the Milky Way, and the group of stars in the head of 

 Orion. Then followed in quick succession the great discove- 

 ries of the four satellites of Jupiter, the two handles of 

 Saturn (his indistinctly-seen rings, the form of which was not 

 recognised), the solar spots, and crescent shape of Venus. 



The moons of Jupiter, the first of all the secondary planets 

 discovered by the telescope, were first seen, almost simul- 

 taneously and wholly independently, on the 29th of Decem- 

 ber, 1609, by Simon Marius at Ansbach, and on the 7th of 

 January, 1610, by Galileo at Padua. In the publication 

 of this discovery Galileo, by the Nuncius Siderius (1610), 

 preceded the Mundus Jovialis (1614), of Simon Marius,* 



cally, with some degree of exactness, byFeuille'e, in 1724, Galileo, like 

 all other observers up to the close of the eighteenth century, believed 

 in the existence of many seas and of a lunar atmosphere. 



* I here again find occasion (Cosmos, p. 179) to refer to the proposition 

 laid down by Arago : "The only rational and just method of writing 

 the history of science is to base it exclusively on works, the date of 

 whose publication is certain. All beyond this must be confused and ob- 

 scure." The singularly-delayed publication of the Frankische Kalender 

 or Practica (1612), and of the astronomically important memoir entitled 

 "Mundus Jovialis anno 1609 detectus ope perspicilli Belgici (Feb. 

 1614)," may indeed have given occasion to the suspicion that Marius 

 had drawn his materials from the Nuncius Sidereus of Galileo, the 

 dedication of which is dated March, 1610, or even from earlier manu- 

 script communications. Galileo, irritated by the still remembered law- 

 suit against Balthasar Capra, a pupil of Marius, calls him the usurper 

 of the system of Jupiter, "Usurpatore del sistema di Giove," and he 

 even accuses the heretical Protestant astronomer of Gunzenhausen, of 

 having founded his apparently earlier observation on a confusion be- 

 tween the calendars. " Tace il Mario di far cauto il lettore, come 

 essendo egli separate della chiesa nostra, ne avendo accettato 1'emenda- 

 tione gregoriana, il giorno 7 di gennaio del 1610, di noi cattolici (the 

 day on which Galileo discovered the satellites) e 1'istesso, che il di 28 di 

 Decembre del 1609, di loro eretici, e questa e tutta la precedeuza delle 

 sue finte osservationi" (Venturi, Memoire e Letter e di G. Galilei, 1818, 

 P. i. p. 279 ; and Delambre, Hist. dcVAstr. mod. t. i. p. 696). According 

 to a letter written by Galileo in 1614, to the Accademia di Lincei, it would 



