But the chief reason is that most of this stuff collects in the region
of the milky way.
Let us now explain the origin, cause, and nature of the milky way.
And here too let us begin by discussing the statements of others on
(1) Of the so-called Pythagoreans some say that this is the path
of one of the stars that fell from heaven at the time of Phaethon's
downfall. Others say that the sun used once to move in this circle and
that this region was scorched or met with some other affection of this
kind, because of the sun and its motion.
But it is absurd not to see that if this were the reason the
circle of the Zodiac ought to be affected in the same way, and
indeed more so than that of the milky way, since not the sun only
but all the planets move in it. We can see the whole of this circle
(half of it being visible at any time of the night), but it shows no
signs of any such affection except where a part of it touches the
circle of the milky way.
(2) Anaxagoras, Democritus, and their schools say that the milky way
is the light of certain stars. For, they say, when the sun passes
below the earth some of the stars are hidden from it. Now the light of
those on which the sun shines is invisible, being obscured by the of
the sun. But the milky way is the peculiar light of those stars
which are shaded by the earth from the sun's rays.
This, too, is obviously impossible. The milky way is always
unchanged and among the same constellations (for it is clearly a
greatest circle), whereas, since the sun does not remain in the same
place, what is hidden from it differs at different times. Consequently
with the change of the sun's position the milky way ought to change
its position too: but we find that this does not happen. Besides, if