(meteorobs) Astro versus Nautical Twilight

Robert Lunsford lunro.imo.usa at cox.net
Thu Aug 4 16:13:24 EDT 2011

Bill and All,

I would suggest that you start observing at start of astronomical twilight, when the sun lies 12 degrees below the horizon. The 
change in LM between the start and end of astronomical twilight is minimal. The LM at different stages of twilight depends on the 
clarity of the air. The sky is still much too bright to begin observing at the start of nautical twilight, when the sun lies 6 
degrees below the horizon. The sky rapidly darkens during nautical twilight when the sun lies between 6 and 12 degrees below the 
horizon. Observations are possible mid-way through nautical twilight, when the sun lies 9 degrees below the horizon. I would still 
wait until the start of astronomical twilight, when the sky is nearly fully dark.

I hope this helps!


-----Original Message----- 
From: Bill Godley
Sent: Thursday, August 04, 2011 8:45 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 week.

Clear skies and cool temperatures,
Coweta, OK

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