44 CLASSICS OF MODERN SCIENCE 



at a tremendous rate in immense circles, others slowly in smaller 

 circles, according as they are placed at greater or smaller distances 

 from the pole. And not only the size of the different circles and 

 so the velocity of movement varies greatly in different fixed stars, 

 but also the same stars change their courses and their velocity ; herein 

 is the fifth difficulty. That is, those stars which 2,000 years ago stood 

 on the equator of the stellar sphere and thereafter moved in the 

 greatest circles, must now, since to-day they have moved several 

 degrees from it, move more slowly and in smaller circles. Within a 

 conceivable time it will happen that one of those which have been 

 continually moving will eventually reach the pole and cease to re- 

 volve, then later, after a period of rest, begin to move again. The 

 other stars, however, which undoubtedly move, all have, as has been 

 said, as orbit an immense circle and move in it without change. 



The improbability is increased (and this may be called a sixth 

 difficulty) for him who investigates basic principles, by the fact that 

 one cannot imagine the firmness which that immense sphere must 

 possess, in whose depths so many stars are so solidly fixed that in 

 spite of such varieties of motions they are held together in the 

 revolution without in any way changing their relative positions. But 

 if according to the most probable view the heavens are fluid, so that 

 each star may describe its own orbit, by what law and according to 

 what principles are their orbits governed, so that seen from the earth 

 they appear as if held in one sphere? To accomplish this it seems to 

 me it would be easier and more convenient to make them stationary 

 instead of movable, just as the paving stones in the market place are 

 kept in order more easily than the troops of children who race over 

 them. 



Finally the seventh objection ; if we ascribe the daily revolution to 

 the highest heavens we must suppose this to be of such power and 

 force that it bears along the innumerable crowd of fixed stars, every 

 one a body of immense mass and much larger than the earth, further, 

 all the planets, although these by their nature move in an opposite 

 direction. Moreover, we must suppose that the element of fire and 

 the greater portion of the air is also borne along ; therefore, singly and 

 alone the little earth ball withstands stubbornly and independently this 

 mighty force : a supposition that seems to me to have much against it. 

 I cannot explain how the earth, a body freely suspended and balanced 



