Let us go on to explain the nature of comets and the 'milky way',
after a preliminary discussion of the views of others.
Anaxagoras and Democritus declare that comets are a conjunction of
the planets approaching one another and so appearing to touch one
Some of the Italians called Pythagoreans say that the comet is one
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.