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(meteorobs) Zodiacal Light (etc) and the Bortle Scale
Hi Everyone,
I have enjoyed, and learned, from everyone's responses to my question
about the ZL on Jan 3.
When the discussion migrated to zodiacal bands and gegenschein (sp?), I
realized you knowlegeable folks have darker skies than I do. Here in the DC
suburbs, even 40 miles to the south of the District, a dark night is about
+5.5 Lm,6.0 tops. After reading an article in Sky & Tel I was able to spot
the gegenschein on one dark occasion.
In the Feb 2001 S&T John Bortle shares his 9 point sky darkness scale
which makes use of the Lm measure in addition to other darkness indicators
like the ZD, G, and how clouds, and your observing site look under the
various skies. "Class 1" is the perfection of Dark and #9 is a meteor
observer's worst nightmare, the brightest sky.
In Bortle's scale I recognize my sky varies between #4 ("Rural/subuurban
transition") at best and #6 at worst ("Bright suburban sky"). These ratings
are at my best site in Newburg, MD about 40 miles south of the White House.
On Jan 3, I had a very clear but only moderately dark sky, I concluded.
Those of you who can reach Lms of 7.3, see ZDs and Gs, probably are
luxuriating under #2 ("Typical true dark site"). John tells us that under
Class 2 conditions we should see weak shadows cast by the ZL!!! Have any of
you noticed that? Bortle's article is provocative reading.
Clear Dark Skies!
Rich
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