Moonlit colour 1
Question: Recently, on a very crisp, bright autumn night in the Appalachian
Mountains in Pennsylvania I noticed that the full Moon was producing an inverse
rainbow in the 12 o鈥檆lock position directly above the Moon. There were also very
bright spots of light in the 3 and 9 o鈥檆lock positions on either side of the
Moon that also showed patches of rainbow. It wasn鈥檛 actually raining at the
time, but there was a high, thin layer of misty cloud. Both my friends and I
have no idea how this effect was created because we were not standing between
the light source and the rainbow image. Can anybody explain it to us?
Answer: Occasionally a faint, thin cloud of high-altitude ice crystals or
water droplets of just the right uniform size and shape lie between you and the
Moon or the Sun. They refract the light to give a circular rainbow effect鈥攁
moonbow or a sunbow as they are sometimes called.
Jamie Fryer
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Grantham, Lincolnshire
Answer: Long and prism-like ice crystals in the atmosphere, which have a
hexagonal cross section, refract light twice: once when the light enters the
crystal and once when it exits through the second facet from the point of entry.
The two facets are inclined to each other at 60掳 and act as a prism, bending
the light through about 22掳, and bending the blue end of the spectrum more
than the red to produce a circle of radius 22掳 about the Moon with the red
on the inner side and blue on the outer.
This contrasts with a normal rainbow, where the process includes internal
reflection within liquid drops of water. In a normal rainbow, the bow is more
than 90掳 from the Sun (it鈥檚 138掳 from the Sun, therefore 42掳 in
radius about the antisolar point) and has red on the outer edge. So yes, the
22掳 halo, which can form around either the Sun or the Moon, is indeed
鈥渋nverse鈥� in colour and in positioning with respect to its illuminating
body.
The bright spots at the 3, 9 and 12 o鈥檆lock positions are called mock moons
or moondogs. It is quite possible that the crystals would be preferentially
oriented both horizontally and vertically rather than obliquely. So the
intensifications at the 3 and 9 o鈥檆lock positions are caused by a majority of
the prism-like ice crystals being aligned vertically, producing horizontal
deflections of the light. At the 12 o鈥檆lock position, the intensification comes
from horizontally oriented crystals. Excellent descriptions of the phenomena can
be found in Marcel Minnaert鈥檚 The Nature of Light and Color in the Outdoors.
Charles Kluepfel
Bloomfield, New Jersey
Answer: Many features of the spectrum of colour seen in a rainbow can be
reproduced by rapidly rotating a transparent hexagonal ballpoint pen tube about
its long axis (many real ice crystals also have holes down the middle, so it
does not affect the result).
Richard White
Woodford Bridge, Essex
Answer: Halo phenomena caused by thin, high cloud (cirrus) are very common by
day, but rarely noticed because few people look up high enough. Aircraft
condensation trails in the right place can produce spectacularly bright effects.
Because the Moon is much fainter than the Sun, its haloes are fainter and less
likely to be seen as coloured. In towns, street lighting makes the weaker lunar
haloes hard to see.
Rodney Blackall
Retired senior editor
The Meteorological Office, London
Moonlit colour 2
Question: One early morning in November, about an hour before sunrise, I was
driving in the rain. Briefly the clouds cleared behind me to reveal a full Moon.
In front of me there suddenly appeared what I can only describe as a lunar
rainbow. It was much dimmer than a normal rainbow and looked as though it had
been washed through in sepia. Is this rare? Has anybody else seen one? Why did
it appear as it did?
Answer: Although the full Moon is sufficiently bright to give rise to visible
rainbows, Moon rainbows are only rarely seen because the conditions need to be
just right. They can only be seen within a day or two of a full Moon, otherwise
the Moon and its rainbow are less bright.
The Moon should be fairly low in the sky. If it is above 42掳, the rainbow
will be below the horizon. The timing of the sighting was just about perfect:
the Moon must have been about 10掳 above the horizon, close to setting (and
behind the driver, who must have been looking backwards). Dark sky is essential,
because rainbows are often only visible against dark clouds, not against nearby
blue sky. Close to sunrise, Moon rainbows will occur in the east, in the
direction of the rising Sun. But an hour before sunrise, the November sky in
Britain is still dark. Obviously, it should also be showery.
A Moon rainbow looks greyish for two reasons. At low light levels, our eyes
only provide black-and-white vision鈥攏ote how only the brightest stars show
a hint of colour. But as important is the fact that moonlight is just not
colourful. The Moon is very dark (only reflecting 7 per cent of the incoming
sunlight) but also quite brown, and a rainbow can only show colours present in
incoming light. During sunset, a rainbow sometimes turns red, reflecting the
changing colour of the Sun. If a rainbow made of 鈥渆arthlight鈥� could be seen on
the Moon, it would be incredibly beautiful, as the Earth is colourful. But,
sadly, there are no rainbows on the Moon because there is no water vapour in its
atmosphere.
Albert Zijlstra
Department of Physics
University of Manchester Institute of Science and Technology
This week鈥檚 questions
The hole answer: How is the hole in fine surgical needles produced?
G. Ertsch
By e-mail, no address supplied
Chorus line: Ever since finding a millipede in my bath, I鈥檝e wondered why
this creature has so many legs. What advantage do they provide and how did it
get them?
Sarah Crew
Ongar, Essex
A low heat: I鈥檝e always understood that hot air rises and cold air sinks. But
on a recent visit to Death Valley in the US, I learned that it is the hottest
and driest area in North America despite being the lowest point in the western
hemisphere. Why is this?
Katie Megg
Toronto, Canada