Page:A short history of astronomy(1898).djvu/208

156 apparent motion from to, as seen by the observer at , is measured by the angle , and is obviously much less than that from  to , measured by the angle , and consequently an object attached to the sun must appear to move more slowly from  to , i.e. near the sun's edge, than from  to , 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 dotted 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 e, measured by the angle d o e. Moreover, it required only a simple calculation, performed by Galilei in several cases,

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 one hand, no sort of reason could be given, and which, on the other, was entirely out of harmony with the uniformity to which mediæval 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