A HISTORY OF SCIENCE 



about the planet, precisely as the moon circles about 

 the earth. Here, obviously, was a miniature solar 

 system a tangible object-lesson in the Copernican 

 theory. In honor of the ruling Florentine house of the 

 period, Galileo named these moons of Jupiter, Medicean 

 stars. 



Turning attention to the sun itself, Galileo ob- 

 served on the surface of that luminary a spot or 

 blemish which gradually changed its shape, suggesting 

 that changes were taking place in the substance of 

 the sim changes obviously incompatible with the 

 perfect condition demanded by the metaphysical 

 theorists. But however disquieting for the conser- 

 vative, the sun's spots served a most useful purpose 

 in enabling Galileo to demonstrate that the sun itself 

 revolves on its axis, since a given spot was seen to pass 

 across the disk and after disappearing to reappear in 

 due course. The period of rotation was found to be 

 about twenty-four days. 



It must be added that various observers disputed 

 priority of discovery of the sun's spots with Galileo. 

 Unquestionably a sun-spot had been seen by earlier 

 observers, and by them mistaken for the transit of an 

 inferior planet. Kepler himself had made this mis- 

 take. Before the day of the telescope, he had viewed 

 the image of the sun as thrown on a screen in a camera- 

 obscura, and had observed a spot on the disk which 

 he interpreted as representing the planet Mercury, but 

 which, as is now known, must have been a sun-spot, 

 since the planetary disk is too small to have been 

 revealed by this method. Such observations as these, 

 however interesting, cannot be claimed as discoveries 



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