HOW MUCH DOES TWILIGHT VARY AROUND THE WORLD?
A common phrase of Northerners who visit the tropics is
"It gets dark almost immediately after sunset." While this
is not quite true (there's generally around 50 minutes of
nautical twilight at the equator), there is considerably
more twilight closer to the poles than the equator, since
the "angular momentum" of the earth is less near the poles.
Twilight lasts longer during summer than winter, with the
difference becoming greater as one traverses away from the
equator. On the first day of summer (and usually other
nearby days), north of about 54 degrees North (or south of
about 54 deg. S.), twilight will last all night, except
north of the Arctic Circle (or south of the Antarctic Circle)
where there is no twilight because the sun is up all night!
Nautical twilight most closely corresponds to the period
of time after sunset (or before sunrise) when some solar
light can still be seen in the sky. Technically, nautical
twilight ends when the sun is 12 degrees (angle) below the
horizon. Of course, other factors, such as cloudiness,
haze or mountains will also determine how long light can
be seen in the sky; usually the actual twilight will be a
bit shorter. Other types of twilight include
"civil twilight" (the period of time when objects can easily
be seen outdoors and occurs until the sun is 6 degrees below
the horizon) and "astronomical twilight" (when the sun
contributes at least a very minor amount to the sky's
illumination, whether visible by humans or not, and occurs
until the sun is 18 degrees below the horizon).
Here are some summer and winter solstice (nautical) twilight
lengths for cities around the world, in minutes:
Fairbanks, Alaska (USA)
Anchorage, Alaska (USA)
London, United Kingdom
Vancouver, B.C. (Canada)
Montreal, Quebec (Canada)
Toronto, Ontario (Canada)
Chicago, Illinois (USA)
New York, New York (USA)
Washington, D.C. (USA)
San Francisco, California (USA)
Los Angeles, Californa (USA)
Dallas, Texas (USA)
Miami, Florida (USA)
Hong Kong, China
Honolulu, Hawaii (USA)
Mexico City, Mexico
* Nautical twilight near the equator is longest during the
winter and summer solstices (50 min.) and shortest during
the spring and fall equinoxes (47 min.).
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