XXVI
1
What then is the reason, you may ask, for the apparent retrogression of some heavenly bodies? The appearance of slowness in their motion is caused by their encountering the sun, as well as by the character of their paths and the position of their orbits, which are at certain periods calculated to deceive the eye. Ships in the same way moving under full sail seem withal to be stationary. Men will some day be able to demonstrate in what regions comets have their paths, why their course is so far removed from the other stars, what is their size and constitution. Let us be satisfied with what we have discovered, and leave a little truth for our descendants to find out.
2
We cannot, Apollonius says, see through the stars what is beyond, but sight passes easily through the comets. Well, in the first place, if that is the case, it is not so in the part of the body which consists of dense solid fire, but only where the dispersed glow extends as it breaks up into the appearance of hair. One can see through the gaps in the fire and not through the fire itself. Stars again, it is said, are all round, comets extended; whence it is plain that they are not true stars. But who, pray, will allow that comets are long? Their tendency like that of other stars is to a globe shape, only the light from them is prolonged. The sun shoots out his rays far and wide, but has himself a shape different from that of the light that streams from him. So in comets, the body is rounded, but the glow from them presents the appearance of being longer than that of the other stars.