[GeoStL] Re: OT? Resource for determining when it gets "Dark"?
- From: "GC-RGS" <gc-rgs@xxxxxxxxxx>
- To: <geocaching@xxxxxxxxxxxxx>
- Date: Sun, 24 Aug 2008 15:50:19 -0500
Here's something that Google picked out. From this info, it becomes "truly
dark" (Astronomical Twilight) when the sun's center is 18 degrees below the
horizon. Now all you need to do is determine how many minutes it takes the sun
to drop 18 degrees and you'll have the exact time :-)
SUNRISE/SUNSET AND TWILIGHTS
At the last public open house I was asked to explain why it took so long to see
any stars. I started to go into the light-pollution issues and how all of the
unshielded lights were robbing us of our night skies and the persons who had
asked me the question stopped me and said, "No, that's not what we meant. We
see sunrise and sunset times in the newspaper but its still light out long
after sunset, how come?"
Well, this prompted me to try and explain the different twilights, and I
thought I might share the Royal Greenwich Observatory's official definitions.
Sunrise and Sunset - The times of Sunrise and Sunset refer to the times when
the Sun's upper limb, as affected by refraction, is on the true horizon of an
observer at sea-level. This occurs when the Sun's center is 50 arcminutes below
the true horizon, the upper limb then being 34 arcminutes just more than the
Sun's apparent diameter) below the true horizon.
Twilights - There are three different definitions of twilight.
Civil Twilight, when the Sun's center is 6 degrees below the horizon, is
roughly equivalent to between 30 and 60 minutes after sunset. The brightest
stars are visible and at sea the horizon is clearly defined.
Nautical Twilight, when the Sun's center is 12 degrees below the horizon, is
to all intents and purposes the time when it is dark. For nautical purposes it
is that time when the horizon ceases to be clearly visible and it is impossible
to determine altitudes with reference to the horizon.
Astronomical Twilight, when the Sun's center is 18 degrees below the horizon,
is when it is truly dark and no remnant of the Sun's afterglow can be seen. It
is possible to see the Zodiacal light which comes from light from the Sun
reflected by small particles between the Earth and the Sun; this can be
mistaken for the Sun's afterglow.
----- Original Message -----
From: tnands
To: Geocaching Freelist
Sent: Sunday, August 24, 2008 2:41 PM
Subject: [GeoStL] OT? Resource for determining when it gets "Dark"?
Is there a rule of thumb, or better yet a chart, for determining when "Dark"
sets in after sunset? We're planning an outdoor movie night for our city on
September 19 and no one is sure when it will actually be dark if sunset is
7:02. Does "Dark" differ depending on the time of year?
This also fits in with the idea of the night time cache being discussed
earlier.
Chicken wings and brown rice - last night's dinner -- which no longer makes
this post completely off topic.
Nancy
BTW -- Andrew -- we'll expect to see you all there!
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