All About Space Astronomer Book - 2014 UK

Moon was high in the sky and they rotated the image with a mirror

while Richard noted the angles and measured the Moon’s apparent

size. They found that the Moon appeared much larger when low

down, confirming the presence of the so-called “Moon illusion”.

As an aside, the Moon’s actual size is slightly smaller when it’s

closer to the horizon because it’s an Earth radius further away and

slightly squashed in height due to atmospheric refraction.

Gegenschein

The gegenschein is an elusive phenomenon that may be seen

from extremely-dark-sky sites. The name in German means

“counterglow” and refers to a faint patch of light that occurs opposite

the Sun’s position in the sky. This is due to interplanetary dust in

the plane of the Solar System appearing fully illuminated in this

direction. PM had seen it from Britain only once, in 1942 when the

whole country was blacked out as a precaution against German air

raids. However it is often seen in countries far from artificial light

where the sky is completely clear.

Moonbow

A moonbow is precisely the same phenomenon as a rainbow but

is caused by the light of the Moon rather than that of the Sun as in

a conventional rainbow. Of course it is much fainter, because the

Moon is fainter than the Sun. How many full Moons would be needed

to send as much light as the Sun? Many would guess five or six. The

answer is close to half a million!

A moonbow is always seen in the part of the sky opposite the

Moon and the colours are so subdued that they cannot be seen with

the naked eye. A moonbow is most easily viewed when the Moon is

with trees and houses, while when it’s high up, there’s nothing to
compare it with. This, according to Ptolemy, is why the low-down
Moon looks much larger.
In modern times there are many explanations for the effect.
One of the most plausible is that our brains interpret the sky as
a flattened bowl above our heads rather than a sphere. When the
Moon’s on the horizon, our brains compensate for its perceived
greater distance and we think it’s larger than it actually is.
PM once tried an experiment on the beach at Selsey Bill, where
he was joined by an old friend, Professor Richard Gregory. The full

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Earth and sky