1 64 <A Short History of Astronomy [CH. VI. 



enough if the stars are regarded as rigidly attached to a 

 material sphere, is shewn in a quite different aspect if, 

 as even Simplicio admits, no such sphere exists, and each 

 star moves in some sense independently. A star near the 

 pole must then be supposed to move far more slowly than 

 one near the equator, since it describes a much smaller 

 circle in the same time ; and further an argument very 

 characteristic of Galilei's ingenuity in drawing conclusions 

 from known facts owing to the precession of the equinoxes 

 (chapter n., 43, and iv., 84) and the consequent change 

 of the position of the pole among the stars, some of those 

 stars which in Ptolemy's time were describing very small 

 circles, and therefore moving slowly, must now be describing 

 large ones at a greater speed, and vice versa. An extremely 

 complicated adjustment of motions becomes therefore 

 necessary to account for observations which Coppernicus 

 explained adequately by the rotation of the earth and a 

 simple displacement of its axis of rotation. 



Salviati deals also with the standing difficulty that the 

 annual motion of the earth ought to cause a corresponding 

 apparent motion of the stars, and that if the stars be 

 assumed so far off that this motion is imperceptible, then 

 some of the stars themselves must be at least as large as 

 the earth's orbit round the sun. Salviati points out that 

 the apparent or angular magnitudes of the fixed stars, 

 avowedly difficult to determine, are in reality almost entirely 

 illusory, being due in great part to an optical effect known 

 as irradiation, in virtue of which a bright object always 

 tends to appear enlarged ; * and that there is in consequence 

 no reason to suppose the stars nearly as large as they might 

 otherwise be thought to be. It is suggested also that the 

 most promising way of detecting the annual motion of stars 

 resulting from the motion of the earth would be by 

 observing the relative displacement of two stars close 

 together in the sky (and therefore nearly in the same direc- 

 tion), of which one might be presumed from its greater 



* This is illustrated by the well-known optical illusion whereby a 

 white circle on a black background appears larger than an equal 

 black one on a white background. The apparent size of the hot 

 filament in a modern incandescent electric lamp is another good 

 illustration. 



