(IAAC) Re: Limiting Magnitude and altitude (was Re: [amastro] M81 with the unaided eye)

>(Before I wade into the oft-debated and -conflated topic of Limiting Mags,
>I wanted to report that I've sighted M81 naked-eye on just a few occasions.
>For me, the problem is usually not dark skies - but that my dark-sky sites
>are specifically chosen to give the darkest and clearest possible SOUTHERN
>horizons! After all, no site (I've ever seen) is truly uniformly dark, all
>around the horizon. So maybe, just maybe, that makes my relative paucity of
>naked-eye M81 sightings an aberration? Anyway, on to LMs... :>)
Dear all,
I have seen with surprise recent threads or articles stating that a
"transparent" sky is a "dark" sky (even in the excellent article John
Bortle wrote recently in S&T). It surprises me because I have another way
of seeing this matter of transparency and would like to share it with you,
just to know if it seems logical or not.
I would, on the opposite, state that a transparent sky should show a little
background brightness because of ionization of high atmospheric molecules
by high energy outerspace radiations (airglow as it is mentioned even in
Bortle's article). If this background exist, a mountain top transparent sky
(ie one that let the maximum electromagnetic radiations go through) should
be brighter than say a rural sky with heavy moisture or atmospheric dust
which could look dark enough but not transparent.
Is is me just reasoning wrond or just a common saying stating a slightly
wrong thing for another ?
Carpe noctem, Yann.
Yann Pothier
11 impasse Canart, 75012 PARIS, FRANCE
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