As for the halo we shall explain its cause later.

  The fact that comets when frequent foreshadow wind and drought
must be taken as an indication of their fiery constitution. For
their origin is plainly due to the plentiful supply of that secretion.
Hence the air is necessarily drier and the moist evaporation is so
dissolved and dissipated by the quantity of the hot exhalation as
not readily to condense into water.-But this phenomenon too shall be
explained more clearly later when the time comes to speak of the
winds.-So when there are many comets and they are dense, it is as we
say, and the years are clearly dry and windy. When they are fewer
and fainter this effect does not appear in the same degree, though
as a rule the is found to be excessive either in duration or strength.
For instance when the stone at Aegospotami fell out of the air-it
had been carried up by a wind and fell down in the daytime-then too
a comet happened to have appeared in the west. And at the time of
the great comet the winter was dry and north winds prevailed, and
the wave was due to an opposition of winds. For in the gulf a north
wind blew and outside it a violent south wind. Again in the archonship
of Nicomachus a comet appeared for a few days about the equinoctial
circle (this one had not risen in the west), and simultaneously with
it there happened the storm at Corinth.

  That there are few comets and that they appear rarely and outside
the tropic circles more than within them is due to the motion of the
sun and the stars. For this motion does not only cause the hot
principle to be secreted but also dissolves it when it is gathering.
But the chief reason is that most of this stuff collects in the region
of the milky way.

                                 8

  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
the subject.

  (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
astronomical demonstrations are correct and the size of the sun is
greater than that of the earth and the distance of the stars from
the earth many times greater than that of the sun (just as the sun
is further from the earth than the moon), then the cone made by the
rays of the sun would terminate at no great distance from the earth,
and the shadow of the earth (what we call night) would not reach the
stars. On the contrary, the sun shines on all the stars and the
earth screens none of them.

  (3) There is a third theory about the milky way. Some say that it is
a reflection of our sight to the sun, just as they say that the
comet is.

  But this too is impossible. For if the eye and the mirror and the
whole of the object were severally at rest, then the same part of
the image would appear at the same point in the mirror. But if the
mirror and the object move, keeping the same distance from the eye
which is at rest, but at different rates of speed and so not always at
the same interval from one another, then it is impossible for the same
image always to appear in the same part of the mirror. Now the
constellations included in the circle of the milky way move; and so
does the sun, the object to which our sight is reflected; but we stand
still. And the distance of those two from us is constant and
uniform, but their distance from one another varies. For the Dolphin
sometimes rises at midnight, sometimes in the morning. But in each
case the same parts of the milky way are found near it. But if it were
a reflection and not a genuine affection of these this ought not to be
the case.

  Again, we can see the milky way reflected at night in water and
similar mirrors. But under these circumstances it is impossible for
our sight to be reflected to the sun.

  These considerations show that the milky way is not the path of
one of the planets, nor the light of imperceptible stars, nor a
reflection. And those are the chief theories handed down by others