Sometimes on a fine night we see a variety of appearances that
form in the sky: 'chasms' for instance and 'trenches' and blood-red
colours. These, too, have the same cause. For we have seen that the
upper air condenses into an inflammable condition and that the
combustion sometimes takes on the appearance of a burning flame,
sometimes that of moving torches and stars. So it is not surprising
that this same air when condensing should assume a variety of colours.
For a weak light shining through a dense air, and the air when it acts
as a mirror, will cause all kinds of colours to appear, but especially
crimson and purple. For these colours generally appear when
fire-colour and white are combined by superposition. Thus on a hot
day, or through a smoky, medium, the stars when they rise and set look
crimson. The light will also create colours by reflection when the
mirror is such as to reflect colour only and not shape.
These appearances do not persist long, because the condensation of
the air is transient.
'Chasms' get their appearance of depth from light breaking out of
a dark blue or black mass of air. When the process of condensation
goes further in such a case we often find 'torches' ejected. When
the 'chasm' contracts it presents the appearance of a 'trench'.
In general, white in contrast with black creates a variety of
colours; like flame, for instance, through a medium of smoke. But by
day the sun obscures them, and, with the exception of crimson, the
colours are not seen at night because they are dark.
These then must be taken to be the causes of 'shooting-stars' and
the phenomena of combustion and also of the other transient
appearances of this kind.