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Pages of Meteorology

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However, we must accept the account we have given of these things in

the theory of sensation, and take some things for granted while we

explain others.


Let us begin by explaining the shape of the halo; why it is a circle

and why it appears round the sun or the moon or one of the other

stars: the explanation being in all these cases the same.

Sight is reflected in this way when air and vapour are condensed

into a cloud and the condensed matter is uniform and consists of small

parts. Hence in itself it is a sign of rain, but if it fades away,

of fine weather, if it is broken up, of wind. For if it does not

fade away and is not broken up but is allowed to attain its normal

state, it is naturally a sign of rain since it shows that a process of

condensation is proceeding which must, when it is carried to an end,

result in rain. For the same reason these haloes are the darkest. It

is a sign of wind when it is broken up because its breaking up is

due to a wind which exists there but has not reached us. This view

finds support in the fact that the wind blows from the quarter in

which the main division appears in the halo. Its fading away is a sign

of fine weather because if the air is not yet in a state to get the

better of the heat it contains and proceed to condense into water,

this shows that the moist vapour has not yet separated from the dry

and firelike exhalation: and this is the cause of fine weather.

So much for the atmospheric conditions under which the reflection

takes place. The reflection is from the mist that forms round the

sun or the moon, and that is why the halo is not seen opposite the sun

like the rainbow.

Since the reflection takes place in the same way from every point

the result is necessarily a circle or a segment of a circle: for if

the lines start from the same point and end at the same point and

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