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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

another.

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.

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