But, as a matter of fact, no planet has been observed besides the
five. And all of them are often visible above the horizon together
at the same time. Further, comets are often found to appear, as well
when all the planets are visible as when some are not, but are
obscured by the neighbourhood of the sun. Moreover the statement
that a comet only appears in the north, with the sun at the summer
solstice, is not true either. The great comet which appeared at the
time of the earthquake in Achaea and the tidal wave rose due west; and
many have been known to appear in the south. Again in the archonship
of Euclees, son of Molon, at Athens there appeared a comet in the
north in the month Gamelion, the sun being about the winter
solstice. Yet they themselves admit that reflection over so great a
space is an impossibility.

  An objection that tells equally against those who hold this theory
and those who say that comets are a coalescence of the planets is,
first, the fact that some of the fixed stars too get a tail. For
this we must not only accept the authority of the Egyptians who assert
it, but we have ourselves observed the fact. For a star in the thigh
of the Dog had a tail, though a faint one. If you fixed your sight
on it its light was dim, but if you just glanced at it, it appeared
brighter. Besides, all the comets that have been seen in our day
have vanished without setting, gradually fading away above the
horizon; and they have not left behind them either one or more
stars. For instance the great comet we mentioned before appeared to
the west in winter in frosty weather when the sky was clear, in the
archonship of Asteius. On the first day it set before the sun and
was then not seen. On the next day it was seen, being ever so little
behind the sun and immediately setting. But its light extended over
a third part of the sky like a leap, so that people called it a
'path'. This comet receded as far as Orion's belt and there dissolved.
Democritus however, insists upon the truth of his view and affirms
that certain stars have been seen when comets dissolve. But on his
theory this ought not to occur occasionally but always. Besides, the
Egyptians affirm that conjunctions of the planets with one another,
and with the fixed stars, take place, and we have ourselves observed
Jupiter coinciding with one of the stars in the Twins and hiding it,
and yet no comet was formed. Further, we can also give a rational
proof of our point. It is true that some stars seem to be bigger
than others, yet each one by itself looks indivisible. Consequently,
just as, if they really had been indivisible, their conjunction
could not have created any greater magnitude, so now that they are not
in fact indivisible but look as if they were, their conjunction will
not make them look any bigger.

  Enough has been said, without further argument, to show that the
causes brought forward to explain comets are false.

                                 7

  We consider a satisfactory explanation of phenomena inaccessible
to observation to have been given when our account of them is free
from impossibilities. The observations before us suggest the following
account of the phenomena we are now considering. We know that the
dry and warm exhalation is the outermost part of the terrestrial world
which falls below the circular motion. It, and a great part of the air
that is continuous with it below, is carried round the earth by the
motion of the circular revolution. In the course of this motion it
often ignites wherever it may happen to be of the right consistency,
and this we maintain to be the cause of the 'shooting' of scattered
'stars'. We may say, then, that a comet is formed when the upper
motion introduces into a gathering of this kind a fiery principle
not of such excessive strength as to burn up much of the material
quickly, nor so weak as soon to be extinguished, but stronger and
capable of burning up much material, and when exhalation of the
right consistency rises from below and meets it. The kind of comet
varies according to the shape which the exhalation happens to take. If
it is diffused equally on every side the star is said to be fringed,
if it stretches out in one direction it is called bearded. We have
seen that when a fiery principle of this kind moves we seem to have
a shooting-star: similarly when it stands still we seem to have a star
standing still. We may compare these phenomena to a heap or mass of
chaff into which a torch is thrust, or a spark thrown. That is what
a shooting-star is like. The fuel is so inflammable that the fire runs
through it quickly in a line. Now if this fire were to persist instead
of running through the fuel and perishing away, its course through the
fuel would stop at the point where the latter was densest, and then
the whole might begin to move. Such is a comet-like a shooting-star
that contains its beginning and end in itself.

  When the matter begins to gather in the lower region independently
the comet appears by itself. But when the exhalation is constituted by
one of the fixed stars or the planets, owing to their motion, one of
them becomes a comet. The fringe is not close to the stars themselves.
Just as haloes appear to follow the sun and the moon as they move, and
encircle them, when the air is dense enough for them to form along
under the sun's course, so too the fringe. It stands in the relation
of a halo to the stars, except that the colour of the halo is due to
reflection, whereas in the case of comets the colour is something that
appears actually on them.

  Now when this matter gathers in relation to a star the comet
necessarily appears to follow the same course as the star. But when
the comet is formed independently it falls behind the motion of the
universe, like the rest of the terrestrial world. It is this fact,
that a comet often forms independently, indeed oftener than round
one of the regular stars, that makes it impossible to maintain that
a comet is a sort of reflection, not indeed, as Hippocrates and his
school say, to the sun, but to the very star it is alleged to
accompany-in fact, a kind of halo in the pure fuel of fire.