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| Twilight at our camp |
Twilight was a popular book series and movie franchise about 10 years ago, which I know simply because i have a daughter who loved the books and also watched the movies. I guess the title was based on that time of day that may have been conducive to the activity of the series characters who happened to be vampires. The books and movies were not of much interest to me but like several of the posts I did on atmospheric phenomena "Parhelia, And Other Strange Sky Stuff" or "Northern Lights Southern Lights", twilight is one of those things I didn't quite understand in technical terms. Sure, lots of people know about and refer to, but don't actually understand what it is and how it is scientifically determined.
So, what is twilight exactly? We know it's that time between day and night, when there is light outside but the sun isn't visible, but how exactly is that defined? Twilight occurs when the earth's upper atmosphere scatters and reflects light, illuminating the lower atmosphere. The actual measurement of this phenomena is based on the geometric centre of the sun and its' elevation compared to the earth, as measured in degrees. And here's something interesting, there are actually three different twilights, those being Civil, Nautical and Astronomical.
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| photo courtesy of timeanddate.com |
Regardless of whether it is dusk or dawn, the various stages of twilight occur at the angles described in the graphic above, just in reverse order as we enter one or the other. That is to say, civil twilight occurs first as day becomes night, followed by nautical and then astronomical and when night becomes day, astronomical twilight occurs first, followed by nautical and then civil.
Civil twilight is obviously the brightest form of twilight and artificial light is not required outdoors, hence the use of this period as a reference point for laws around hunting, aviation and automobiles' use of headlights.
Nautical twilight came about from the era of sailors using the stars for navigation, since it is dark enough to see many of the important stars and constellations during this time. The U.S.military uses this time for tactical purposes, BMNT (Begin Morning Nautical Twilight) and EENT ( End Evening Nautical Twilight)
Astronomical twilight means the sky is no longer illuminated and has two phases, Astronomical Dawn and Astronomical Dusk.
Twilight is of a shorter duration at the equator. There may be no distinction between Astronomical dawn or dusk twilight if the Solar Elevation Angle is less than 18 ° at midnight. Twilight at the poles can be skewed during the Equinox. That is to say during the March Equinox the sun rises and stays up all day at the north pole and there is no twilight until the September Equinox. Conversely, this occurs at the south pole between the September Equinox and the March Equinox.
Civil twilight is obviously the brightest form of twilight and artificial light is not required outdoors, hence the use of this period as a reference point for laws around hunting, aviation and automobiles' use of headlights.
Nautical twilight came about from the era of sailors using the stars for navigation, since it is dark enough to see many of the important stars and constellations during this time. The U.S.military uses this time for tactical purposes, BMNT (Begin Morning Nautical Twilight) and EENT ( End Evening Nautical Twilight)
Astronomical twilight means the sky is no longer illuminated and has two phases, Astronomical Dawn and Astronomical Dusk.
Twilight is of a shorter duration at the equator. There may be no distinction between Astronomical dawn or dusk twilight if the Solar Elevation Angle is less than 18 ° at midnight. Twilight at the poles can be skewed during the Equinox. That is to say during the March Equinox the sun rises and stays up all day at the north pole and there is no twilight until the September Equinox. Conversely, this occurs at the south pole between the September Equinox and the March Equinox.
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