(meteorobs) Astro versus Nautical Twilight
meteoreye at comcast.net
Thu Aug 4 13:22:59 EDT 2011
Bill, it's very hard to say, and to be honest, it changes minute by minute.
I typically observer until dawn, and generally stop at or just before
nautical twilight. The first 15 minutes of astro twilight can barely be
noticed. After that, things begin to deteriorate rapidly, which is why I
stop by nautical twilight, LM drops below 5.2. My typical LM during the
night is 5.6-5.8. Of course, the northern Europeans have to deal with these
conditions all summer long, so perhaps they are better adapted. The only
time I have ever continued beyond Nautical twilight was during the Leonid
storm years, in 2001. By civil twilight, my LM was about 2, and we were
still seeing dozens of Leonids a minute. Wow!
It also depends on how clear your air is. Any haze or particulates in the
air light up more rapidly.
Pretty much any observations after nautical twilight are at LMs such that
scientific visual data is pretty much worthless. I'd suggest you attempt to
do a LM reading just before you stop recording data. Of course, that's not
easy, since the IMO polygons don't work well at LM +5.0 . When I observe
during full moon conditions (As it will be for the Perseids this year) I
always note the faintest stars I can see in my field of view, and then look
the up later to try and obtain an accurate estimate. It's really WAG
From: meteorobs-bounces at meteorobs.org
[mailto:meteorobs-bounces at meteorobs.org] On Behalf Of Bill Godley
Sent: Thursday, August 04, 2011 11:45 AM
To: meteorobs at meteorobs.org
Subject: (meteorobs) Astro versus Nautical Twilight
Does anyone know off-hand the relationship between the two with respect to
LM? In other words, all other conditions being equal, if you have a LM of,
say, 6.0 at zenith at astronomical twilight, what would the LM be at
nautical twilight? At civil twilight?
As you might guess, I am planning for Perseid watching. There will be a few
hours of dark skies on 8/9, 9/10 and 10/11 and I am going to take a crack at
seeing some meteors.
The weather has been brutal this last month in Oklahoma. It has been over
100F (38C) for well over a month straight now and lately the highs have been
over 110F. It is likely to go down as the worst summer in recorded history
in these parts (after a pretty cold and snowy winter). Not only is it
extremely uncomfortable (at night 85F or so) but hazy skies and wispy clouds
have kept me inside. Hoping for some improvement and some meteors next
Clear skies and cool temperatures,
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