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smallest when the sun is on the meridian; for the higher H is the

lower the pole and the centre of the circle will be.

In the shorter days after the autumn equinox there may be a

rainbow at any time of the day, but in the longer days from the spring

to the autumn equinox there cannot be a rainbow about midday. The

reason for this is that when the sun is north of the equator the

visible arcs of its course are all greater than a semicircle, and go

on increasing, while the invisible arc is small, but when the sun is

south of the equator the visible arc is small and the invisible arc

great, and the farther the sun moves south of the equator the

greater is the invisible arc. Consequently, in the days near the

summer solstice, the size of the visible arc is such that before the

point H reaches the middle of that arc, that is its point of

culmination, the point is well below the horizon; the reason for

this being the great size of the visible arc, and the consequent

distance of the point of culmination from the earth. But in the days

near the winter solstice the visible arcs are small, and the

contrary is necessarily the case: for the sun is on the meridian

before the point H has risen far.



6



Mock suns, and rods too, are due to the causes we have described.

A mock sun is caused by the reflection of sight to the sun. Rods are

seen when sight reaches the sun under circumstances like those which

we described, when there are clouds near the sun and sight is

reflected from some liquid surface to the cloud. Here the clouds

themselves are colourless when you look at them directly, but in the

water they are full of rods. The only difference is that in this

latter case the colour of the cloud seems to reside in the water,

but in the case of rods on the cloud itself. Rods appear when the

composition of the cloud is uneven, dense in part and in part rare,

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