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The Twilight Zone
Darkness, Light and What Lies Between

 
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How would you like to experience day after day of three-hour sunsets complete with some of the most beautiful golds, reds and violets found in nature. Our friends living in the high latitudes of the Arctic regions treasure an astronomical phenomenon most of us in the warmer regions are unaware of, a winter full of twilight.

Sunset
If it is not dark, and not light, twilight is the term used to describe the period of incomplete darkness that occurs after sunset and before sunrise. This word is derived from Saxon or Middle English terms, which implies light that occurs twice daily. With the sun below the horizon, the multiple scattering of light produced by particles in the upper atmosphere may commonly produce a purple, red or yellow glow. This is possible because refraction (a phenomenon where the light is bent when it passes from one medium to another) apparently lifts the sun a little more than its diameter when it is lying on the horizon.

Variations in the Twilight Zone
Historically, three subdivisions of twilight have been used to define outdoor visibility:
  1. Civil twilight refers to the interval of incomplete darkness that occurs when the sun's center is approximately 6 degrees below the horizon. The amount of light is still sufficient to carry on outdoor work without the aid of artificial light.
  2. Nautical twilight refers to the interval of incomplete darkness that occurs when the sun's center is approximately 12 degrees below the horizon. The amount of light is still sufficient to navigate using visible features on the surface of water or land. This subdivision ends when it becomes too hard for a sailor to pick out the line between sea and sky.
  3. Astronomical twilight refers to the interval of incomplete darkness that occurs when the sun's center is approximately 18 degrees below the horizon. There is no discernible horizon glow left over the sun's azimuth. Stars (sixth magnitude) directly overhead.

In winter, someone would have to go beyond 67 degrees by a bit not to see the sun at all on the winter solstice. A place has to lie above 66 degrees 33 minutes north (or south) latitude before the sun would be either above or below the horizon for a full 24 hours at some time during the year. The Arctic or Antarctic Circle is at that 66 degree 33 minute line of latitude. To find true night at noon on winter solstice in the high latitudes, a traveler would have to stand within about 5´ degrees of the pole.

One of our newest Journey North classrooms, Hopson Middle School, is located in Barrow, Alaska. This special place is the northernmost community in North America, and is located on the Chukchi Sea coast. Barrow's latitude is 71 N. In Barrow, the disc of the sun doesn't climb above the horizon for roughly a month on each side of the winter solstice. However, at noon on the winter solstice the sun's upper rim is only about 4´ degrees below the horizon.
Imagine winter's twilight skies in Barrow!


Try This!
Find out when the sun sets in your area then go outside and watch it set. Does it get dark immediately? Why not? How many minutes pass from the time the sun sets until you can't read the print in a book without extra light? Do you think this time interval changes depending on the time of the year?

Refracted light is a fun concept to experiment with. Try your hand at some experiments to help you understand how light appears to bend:

 
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