It's caused by Rayleigh scattering, which is by definition strongly
wavelength-dependent and is brought about by particles of the same order of
size as the wavelength of light, usually from fine dust or smoke. The blue is
scattered most so you see an orange/red moon (or sun, or planet, even, if it's
bright enough). Absolutely nothing whatsoever to do with refraction. Nothing
whatsoever to do with Saharan dust, which is too large to cause Rayleigh
scattering. Nothing whatsoever to do with ozone, whose scattering
characteristics are the same as nitrogen and oxygen. Very common indeed.
Happens all the time. Seen it thousands of times. Why the fuss?

Tudor Hughes, Warlingham, Surrey.

It's certainly been very red/orange for a while now, spectacular blood
red full moon very low on the horizon about two weeks ago. I was out
taking photos of it.. alas, none captured it with any justice. Isn't
this what is referred to as a 'harvest' moon? which I presumed was to
do with the dust in the low atmosphere. I wonder if it is helping to
give Mars a much more pronounced red glow than usual, it sure is easy
to spot in the night sky.

regards,

Jim
Banbury


(TudorHgh) wrote in message ...
It's caused by Rayleigh scattering, which is by definition strongly
wavelength-dependent and is brought about by particles of the same order of
size as the wavelength of light, usually from fine dust or smoke. The blue is
scattered most so you see an orange/red moon (or sun, or planet, even, if it's
bright enough). Absolutely nothing whatsoever to do with refraction. Nothing
whatsoever to do with Saharan dust, which is too large to cause Rayleigh
scattering. Nothing whatsoever to do with ozone, whose scattering
characteristics are the same as nitrogen and oxygen. Very common indeed.
Happens all the time. Seen it thousands of times. Why the fuss?

Tudor Hughes, Warlingham, Surrey.

On 07 Aug 2003 00:07:22 GMT, (TudorHgh) wrote:

It's caused by Rayleigh scattering,
snip
Nothing whatsoever to do with Saharan dust, which is too large
to cause Rayleigh scattering.

Are you saying that none of the Saharan particles are small enough to
cause this effect?

I would have thought that at the end of a Saharan dust spell, such as
occurred yesterday, the final particles to settle out of suspension
would be the finest particles which, if small enough, could have
contributed to the observed discolouration. Even today,
http://forecast.uoa.gr/MODEL_OUTPUT/dload/000.gif
there are dusty areas quite near to the UK and I'd expect some fine
dust still to be up there. But are the particles small enough?

--
Dave

Are you saying that none of the Saharan particles are small enough to
cause this effect?

I would have thought that at the end of a Saharan dust spell, such as
occurred yesterday, the final particles to settle out of suspension
would be the finest particles which, if small enough, could have
contributed to the observed discolouration.

What you say is quite plausible, and may well have contributed to the
reddening in the way you describe. Most Saharan dust seems ( to my eyes) to
cause a milkiness to the sky, which indicates an equal scattering of all
wavelengths similar to Cs or fog or very thin low St, but since there must be a
distribution of particle sizes there could well be some small stuff as well.
But the phenomenon of the reddish moon is so common I can't quite
understand why it has caused so much comment and so many exotic explanations
invoked. It affects the sun, too. I remember an occasion in Dec 2000 when,
with a cloudless sky, the sun disappeared from view as a dim red globe, several
degrees above the horizon. It was nice to watch but not exactly noteworthy. I
would say it is less commonly observed than, say, 40 yrs ago, because the
atmosphere is less smoky these days.

Tudor Hughes, Warlingham, Surrey.