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! -------------------------------------------------------------------------- No virus found in this incoming message. Checked by AVG. Version: 7.5.524 / Virus Database: 270.6.6 - Release Date: 8/19/2008 12:00 AM