[AZ-Observing] Re: MM date limits (again)
- From: "Jones, Jack (AZ75)" <jack.jones@xxxxxxxxxxxxx>
- To: "'az-observing@xxxxxxxxxxxxx'" <az-observing@xxxxxxxxxxxxx>
- Date: Thu, 18 Mar 2004 09:40:54 -0700
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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