Astronomy. ^Lecture 17, 



were turned about the Earth, as they supposed 

 the Sun himself turned, they would sometimes 

 appear opposite to the Sun, or more distant from 

 him than one hundred and eighty degrees; 

 which never happens. This is the reason why 

 the Egyptians regarded these two planets as 

 satellites of the Sun, and thought that they 

 turned about him, their orbits being carried with 

 him in his revolutions about the Earth. They 

 therefore supposed the Earth T (fig. 106) im- 

 mov cable, as the centre of the system ; and they 

 supposed the other celestial bodies to turn round 

 her : first, the Moon D ; secondly, the Sun ; 

 about which they made Mercury $ and Venus 

 to revolve, till they came to Mars $ , Jupiter 

 i;, and to Saturn T?; and lastly to the fixed 

 stars. 



At the present day, however, when we know the 

 immense distance at which the stars are placed, 

 both these systems become insupportable. They 

 require that all the heavenly bodies should go 

 through the whole course of their orbits in about 

 24 hours, which would give to the fixed stars a 

 rapidity of motion that exceeds all belief: nay, 

 the Sun himself would in a single second have to 

 describe a space of more than two thousand five 

 hundred miles. 



Copernicus, with a view of obviating the 

 inconveniences of the imaginary systems that 

 preceded him, commenced at first by admitting 

 the diurnal motion of the Earth, or her motion 



