156 A Short History of Astronomy [Cn. vi 



apparent motion from A to B, as seen by the observer 

 at o, is measured by the angle A o B, and is obviously 

 much less than that from D to E, measured by the angle 

 DOE, and consequently an object attached to the sun 

 must appear to move more slowly from A to B, i.e. near 

 the sun's edge, than from D to E, near the centre. On the 

 other hand, if the spot be a body revolving round the sun 

 at some distance from it, e.g. along the do' ted circle c d e, 

 then if c, d, e be taken at equal distances from one another, 

 the apparent motion from c to d, measured again by the 

 angle c o d, is only very slightly less than that from d to , 

 measured by the angle d o e. Moreover, it required only 

 a simple calculation, performed by Galilei in several cases, 



FIG. 56. Galilei's proof that sun-spots are not planets. 



to express these results in a numerical shape, and so to 

 infer from the actual observations that the spots could not 

 be more than a very moderate distance from the sun. The 

 only escape from this conclusion was by the assumption 

 that the spots, if they were bodies revolving round the sun, 

 moved irregularly, in such a way as always to be moving 

 fastest when they happened to be between the centre of 

 the sun and the earth, whatever the earth's position might 

 be at the time, a procedure for which, on the on^ hand, 

 no sort of reason could be given, and which, on the other, 

 was entirely out of harmony with the uniformity to which 

 mediaeval astronomy clung so firmly. 



The rotation of the sun about an axis, thus established, 

 might evidently have been used as an argument in support 

 of the view that the earth also had such a motion, but, 

 as far as I am aware, neither Galilei nor any contemporary 

 noticed the analogy. Among other facts relating to the 



