[GeoStL] Re: OT? Resource for determining when it gets "Dark"?
- From: "tnands" <sydstyr@xxxxxxxxxx>
- To: <geocaching@xxxxxxxxxxxxx>
- Date: Sun, 24 Aug 2008 18:43:30 -0500
Based on all of this, it's easy to see why I couldn't just find the answer to
the question, "When does it get dark?"
Although 72 minutes after sunset would seem to make sense. Assuming that whole
3.999 minutes/degree thing. And hey, who's to argue?
And the center of the sun? Now we have to factor in the size of the sun?
I think an outdoor movie would best be shown at dark dark don't you you? LOL
-L.
Thanks for you help!
Nancy
----- Original Message -----
From: GC-RGS
To: geocaching@xxxxxxxxxxxxx
Sent: Sunday, August 24, 2008 6:31 PM
Subject: [GeoStL] Re: OT? Resource for determining when it gets "Dark"?
Nothing to do with thinking I are one, just proves I can read:-) And I have
NO idea what all this means!
And no one posted the answer???????
Well, since the sun rotates 1 degree just under 4 minutes (3.999 roughly), it
s/b easy to multiply 4 by 18 and come up with 72 minutes. Of course that's if
you want dark dark. Light dark s/b about 1 minute after sunset and medium dark
somewhere in between the two.
I'd think the plan would be to go out at sunset tonight since it's a clear
day and see when you think it's dark enough for a movie. Sunset is at 7: PM
but for north county there's no telling.
The following information is provided for St. Louis, Missouri (longitude
W90.2, latitude N38.6):
Sunday
24 August 2008 Central Daylight Time
SUN
Begin civil twilight 5:56 a.m.
Sunrise 6:23 a.m.
Sun transit 1:03 p.m.
Sunset 7:42 p.m.
End civil twilight 8:10 p.m.
So now another term has entered, "civil twilight". So here is that
explanation:
Civil twilight
This begins in the morning when the center of the Sun is less than 6° below
the horizon (the point of civil dawn), and ends at sunrise. Evening civil
twilight begins at sunset and ends when the center of the Sun is more than 6°
below the horizon (the point of civil dusk).
The brightest stars appear during civil twilight, as well as planets, such as
Venus which is known as the 'morning star' and/or 'evening star'. During this
period there is enough light from the Sun that artificial sources of light may
not be needed to carry on outdoor activities. This concept is sometimes
enshrined in laws, like when drivers of automobiles must turn on their
headlights, or if the crime of burglary is to be treated as night-time
burglary, which carries stiffer penalties in some jurisdictions. A fixed period
of time (most commonly 30 minutes after sunset or before sunrise) is typically
used in such statutes, rather than how many degrees the Sun is below the
horizon. Civil twilight can also be described as 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
----- Original Message -----
From: Mike Griffin
To: geocaching@xxxxxxxxxxxxx
Sent: Sunday, August 24, 2008 5:29 PM
Subject: [GeoStL] Re: OT? Resource for determining when it gets "Dark"?
It should have read... "You think you are one..."
LOL
----- Original Message -----
From: Libby Morehead
To: geocaching@xxxxxxxxxxxxx
Sent: Sunday, August 24, 2008 4:19 PM
Subject: [GeoStL] Re: OT? Resource for determining when it gets "Dark"?
No wonder your caches demonstrate a stroke of genius. You are one I think.
L
----- Original Message -----
From: GC-RGS
To: geocaching@xxxxxxxxxxxxx
Sent: Sunday, August 24, 2008 3:50 PM
Subject: [GeoStL] Re: OT? Resource for determining when it gets "Dark"?
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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