(meteorobs) Astro versus Nautical Twilight

[wisetdf at Safe-mail.net]

If I remember rightly civil, nautical and astronomical twilight are defined as starting when the Sun is 6, 12 and 18 degrees below the horizon respectively.

In most northern midlatitues to high ones this means at least a coupla months without astronomical twilght at all, even at 50 degrees North.

The astronomical almanac should have the definitions.

If you've ever observed for long periods over several seasons you'll see that the sky is never as dark this time of year.

But as long as you are not on top of a mountain etc, it is as others have said, the other more regional and nightly factors probably dominate the seeing.

Cheers

John

-------- Original Message --------
From: "Robert Lunsford" <lunro.imo.usa at cox.net>
Apparently from: meteorobs-bounces at meteorobs.org
To: "Bill Godley" <wwgj180 at yahoo.com>, "Meteor science and meteor observing" <meteorobs at meteorobs.org>
Subject: Re: (meteorobs) Astro versus Nautical Twilight
Date: Thu, 4 Aug 2011 13:13:24 -0700

> 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!
> 
> Bob
> 
> -----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,
> Bill,
> Coweta, OK
> 
> 
> 
> 
> 
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