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of the planets； but that it appears at great intervals of time and
only rises a little above the horizon。 This is the case with Mercury
too； because it only rises a little above the horizon it often fails
to be seen and consequently appears at great intervals of time。
A view like theirs was also expressed by Hippocrates of Chios and
his pupil Aeschylus。 Only they say that the tail does not belong to
the comet iself； but is occasionally assumed by it on its course in
certain situations； when our sight is reflected to the sun from the
moisture attracted by the comet。 It appears at greater intervals
than the other stars because it is slowest to get clear of the sun and
has been left behind by the sun to the extent of the whole of its
circle before it reappears at the same point。 It gets clear of the sun
both towards the north and towards the south。 In the space between the
tropics it does not draw water to itself because that region is
dried up by the sun on its course。 When it moves towards the south
it has no lack of the necessary moisture； but because the segment of
its circle which is above the horizon is small； and that below it many
times as large； it is impossible for the sun to be reflected to our
sight； either when it approaches the southern tropic； or at the summer
solstice。 Hence in these regions it does not develop a tail at all。
But when it is visible in the north it assumes a tail because the
arc above the horizon is large and that below it small。 For under
these circumstances there is nothing to prevent our vision from
being reflected to the sun。
These views involve impossibilities； some of which are common to all
of them； while others are peculiar to some only。
This is the case； first； with those who say that the comet is one of
the planets。 For all the planets appear in the circle of the zodiac；
whereas many comets have been seen outside that circle。 Again more
comets than one have often appeared simultaneously。 Besides； if
their tail is due to reflection； as Aeschylus and Hippocrates say；
this planet ought sometimes to be visible without a tail since； as
they it does not possess a tail in every place in which it appears。
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