We see that motion is able to dissolve and inflame the air;
indeed, moving bodies are often actually found to melt. Now the
sun's motion alone is sufficient to account for the origin of
terrestrial warmth and heat. For a motion that is to have this
effect must be rapid and near, and that of the stars is rapid but
distant, while that of the moon is near but slow, whereas the sun's
motion combines both conditions in a sufficient degree. That most heat
should be generated where the sun is present is easy to understand
if we consider the analogy of terrestrial phenomena, for here, too, it
is the air that is nearest to a thing in rapid motion which is
heated most. This is just what we should expect, as it is the
nearest air that is most dissolved by the motion of a solid body.
This then is one reason why heat reaches our world. Another is
that the fire surrounding the air is often scattered by the motion
of the heavens and driven downwards in spite of itself.
Shooting-stars further suffix to prove that the celestial sphere
is not hot or fiery: for they do not occur in that upper region but
below: yet the more and the faster a thing moves, the more apt it is
to take fire. Besides, the sun, which most of all the stars is
considered to be hot, is really white and not fiery in colour.
Having determined these principles let us explain the cause of the
appearance in the sky of burning flames and of shooting-stars, and
of 'torches', and 'goats', as some people call them. All these
phenomena are one and the same thing, and are due to the same cause,
the difference between them being one of degree.
The explanation of these and many other phenomena is this. When
the sun warms the earth the evaporation which takes place is
necessarily of two kinds, not of one only as some think. One kind is
rather of the nature of vapour, the other of the nature of a windy