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(meteorobs) What allows darker skies? (was Re: Florida Geminids very iffy+vision)
>I guess lack of Oxygen affects unacclimated observers differently. My best
>limiting magnitude is always from mountain skies, where I have seen to 7.5
>magnitude, but can't do better than 6.5 at sea level. I wonder why this is?
George, I'm living proof that people react to altitude differently: when Malcolm
Currie kindly took us on a tour of the UKIRT on Mauna Kea's summit this summer,
faint paint fumes were enough to overcome me for a while! And I was surprised
how LITTLE improved (maybe 0.5 magnitude?) skies at the Visitor Center (9000')
were vs. my sea-level site in the FL Keys. Part of it was surely my littoral
lungs, struggling to gasp in enough of the Olympian air up there.
But part of it may also be WHERE on the Coast you are observing from: as I said
before, I've never found a site anywhere on the main Eastern Seaboard where
skies are really comparable to the Keys - let alone Mauna Kea! Part of this may
be development - but I know the Keys and Florida in general in recent years have
seen a sad EXPLOSION of new development. So perhaps there is another factor to
consider: I know for example that on RARE nights in my back yard in Medford, I
HAVE achieved LMs approaching 6.0. Why? I don't think that light pollution was
any less on those nights: no, I suspect it was because the reflective (and maybe
scattering and reemiting?) CONTENT of the atmosphere was reduced.
Light pollution is only a problem when it is reflected, scattered or reemited so
that it decreases contrast in your sky: the quantity of particulates, aerosols
and other compounds in the column of air above you critically affects how much
light pollution affects your sky. It's conceivable that this, as a SECONDARY
effect of the reduced "optical depth" of your air column, is a major reason
mountain sites are often darker than sea-level sites: in other words, contrast
reduction should probably be considered orthogonally from extinction!
And in areas where prevailing wind patterns will tend to bring in (or hold on
to) less contrast-reducing "junk", it stands to reason skies will be better. Of
course, the fact that the Keys and parts of the Everglades are surrounded by
less total AREA of developed land (being islands on the ocean on one hand, and
islands in the sea of grass on the other) may also help!
Clear (and really, really, really dark) skies to all,
Lew
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