Darkness, Light and What Lies Between
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.
and Writing Connections >>
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
- 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.
- 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.
- 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!
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: