FreeLists / az-observing / [AZ-Observing] Re: MM date limits (again)

["Tom Polakis" <tpolakis@xxxxxxx>]

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



> Well here's some definitions which I stole from USNO. It's the 6-12-18
> degree definition of twilight: civil, nautical and astronomical, the 
details
> which I am always forgetting. So Tom, at 10 degrees, is somewhere 
between
> civil and nautical. Myself, I think he could be a little less nautical 
and a
> little more civil.  :-)
>
> Jack
>
> http://aa.usno.navy.mil/faq/docs/RST_defs.html
>
> Civil twilight is defined to begin in the morning, and to end in the 
evening
> when the center of the Sun is geometrically 6 degrees below the horizon.
> This is the limit at which twilight illumination is sufficient, under 
good
> weather conditions, for terrestrial objects to be clearly distinguished; 
at
> the beginning of morning civil twilight, or end of evening civil 
twilight,
> the horizon is clearly defined and the brightest stars are visible under
> good atmospheric conditions in the absence of moonlight or other
> illumination. In the morning before the beginning of civil twilight and 
in
> the evening after the end of civil twilight, artificial illumination is
> normally required to carry on ordinary outdoor activities. Complete
> darkness, however, ends sometime prior to the beginning of morning civil
> twilight and begins sometime after the end of evening civil twilight.
>
> Nautical twilight is defined to begin in the morning, and to end in the
> evening, when the center of the sun is geometrically 12 degrees below 
the
> horizon. At the beginning or end of nautical twilight, under good
> atmospheric conditions and in the absence of other illumination, general
> outlines of ground objects may be distinguishable, but detailed outdoor
> operations are not possible, and the horizon is indistinct.
>
> Astronomical twilight is defined to begin in the morning, and to end in 
the
> evening when the center of the Sun is geometrically 18 degrees below the
> horizon. Before the beginning of astronomical twilight in the morning 
and
> after the end of astronomical twilight in the evening the Sun does not
> contribute to sky illumination; for a considerable interval after the
> beginning of morning twilight and before the end of evening twilight, 
sky
> illumination is so faint that it is practically imperceptible.
>
>
> > Why did you pick the sun at -10 deg?  Is this from experience
> > or a gut
> > feeling?  Could it be from some actual calculations that
> > balance M30's
> > magnitude and sky brightness.  One would also have to factor
> > in not only
> > the mountains, but the elevation of the site plus sky glow
> > from Tucson
> > lights.
> >
> >
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