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Re: (meteorobs) CALL FOR OBSERVATIONS: Gegenschein



At 07:04 AM 2/13/98 PST, John (Polard@usadot net) asked:
>Hi, this may be a stupid question but what is the gegenschein?

I'm glad you asked that, John! I should have thought to make that message I
forwarded a bit more explanatory! :(

"Gegenschein" is a German word - it means literally "counter-glow". If you
ever have the pleasure of observing from a DARK observing site, you will
begin to notice that, even without light pollution, the sky is NOT
uniformly bright all over. First of all, of course, there's the Milky Way!

But even away from the Milky Way, a sharp eye may pick up subtle variations
and "brightish spots" which clearly are not clouds or man-made lights.
(These faint, broad glows actually move with the stars as they circle the
sky!)

The most common cause of these faint glows is "zodiacal dust": billions of
tons of interplanetary dust, which are (very faintly) reflecting back the
light of the Sun into our night sky! There are three well known phenomena
associated with zodiacal dust: the brightest is generally seen just before
astronomical dawn or just after astronomical dust. This "false dawn" or
"zodiacal light" is tough to distinguish from ordinary dawn or dusk, but
has a distinctive shape, and is of course visible at times when dawn
shouldn't be!

But on REALLY dark nights, zodiacal light can actually be seen to stretch
across the entire sky, following the ecliptic (the line of the Sun across
the sky) from east to west, and being visible potentially all night. This
is rare.

More frequently though, you can see a PATCH of the zodiacal band, visible
for a few degrees around JUST the point on the ecliptic which is exactly
OPPOSITE to the Sun in the sky... And this is the Gegenschein!

Clear skies,
Lew