256 Astronomy. [Lecture 17. 



high tower fell at its foot, argued that the Earth 

 must be without motion, never reflecting that 

 the Earth, in that case, is like a vessel in full 

 sail, when if a stone is thrown from the mast, 

 it would fall at the foot of that mast, provided 

 the motion of the vessel was neither accelerated 

 nor retarded during the fall. Tycho-Brahe, 

 therefore, invented a system between that of 

 Ptolemy and that of Copernicus. He supposed 

 that the Earth was at rest, and that the other 

 planets revolving round the Sun, turned also 

 with him round the Earth in twenty-four hours. 

 It was towards the end of the sixteenth century 

 that he proposed his system. He placed the 

 Earth (fig. 108) immoveable, as the centre, and 

 made the Moon turn round her, as well as the 

 Sun S, and the fixed stars : the other planets, viz. 

 Mercury, Venus, Mars, Jupiter, and Saturn, 

 turning round the Sun, in orbits which are carried 

 with him in his revolution round the Earth. As 

 the system of Tycho-Brahe requires the same 

 rapidity of motion as that of Ptolemy and of the 

 Egyptians, it is at once annihilated by the same 

 arguments. 



Leaving, however, for the present the history 

 of astronomical discoveries, I shall request your 

 attention to the celestial phenomena. 



There are evidently two sorts of stars ; the one 

 luminous of themselves, and throwing light on 

 every object which surrounds them to a certain 

 distance ; such as our Sun, and those which we 



