perfect movement around the earth in the centre.

It is somewhat strange that Ptolemy did not advance one step further,
as by so doing he would have given great simplicity to his system. He
might, for instance, have represented the movements of Venus equally
well by putting the centre of the moving circle at the sun itself,
and correspondingly enlarging the circle in which Venus revolved. He
might, too, have arranged that the several circles which the outer
planets traversed should also have had their centres at the sun. The
planetary system would then have consisted of an earth fixed at the
centre, of a sun revolving uniformly around it, and of a system of
planets each describing its own circle around a moving centre placed
in the sun. Perhaps Ptolemy had not thought of this, or perhaps he
may have seen arguments against it. This important step was,
however, taken by Tycho. He considered that all the planets revolved
around the sun in circles, and that the sun itself, bearing all these
orbits, described a mighty circle around the earth. This point
having been reached, only one more step would have been necessary to
reach the glorious truths that revealed the structure of the solar
system. That last step was taken by Copernicus.



COPERNICUS


[PLATE: THORN, FROM AN OLD PRINT.]

The quaint town of Thorn, on the Vistula, was more than two centuries
old when Copernicus was born there on the 19th of February, 1473. The