[AZ-Observing] Re: MM date limits (again)

  • From: "Tom Polakis" <tpolakis@xxxxxxx>
  • To: "Reply-To:az-observing"@freelists.org
  • Date: Thu, 18 Mar 2004 14:29:22 -0700

I have found that astronomical twilight (sun 18 degrees below horizon) 
corresponds pretty well with the time that no glow from the sun is visible 
even in the direction where it just set.  At about -15 degrees, the 
western horizon is bright, but mostly colorless, and the zenith is not any 
brighter that at midnight, to my eyes.  I think you could say that the 
definition of nautical twilight is "sea-oriented" rather than "ground 
oriented".

Tom


> Although I don't like the definitions much since they are 
"ground-oriented",
> I like the references to the visibility of the horizon - civil: horizon 
is
> clearly defined - nautical: horizon is indistinct. It's a continuum of
> course, but I will try to memorize what the horizon looks like when a 
target
> object goes invisible as a reference to warn how much time I have to 
play
> with. (I'm also going to be running Starry Night on the notebook in real
> time.) But usually I freak at the end of astron. twilight when first 
light
> shows and I have 20 left.
>
> Jack
>
> >
> >
> > The 10 degrees was a sort of arbitrary choice, chosen as a
> > time when M30
> > would be several degrees above the horizon.  I do remember
> > looking at M30
> > in 2001 in very bright twilight.  I could see scopes and cars
> > clearly at
> > the other end of the field.  I would guess it was well past nautical
> > twilight when we saw it.
> >
> > In any event, I don't recall the distant mountains presenting
> > nearly as
> > much of a problem for seeing M30 as the horizon extinction and sky
> > brightness.
> >
> > Tom
> >
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