This is followed by the nautical twilight, when the sun reaches heights of-6?and-12? after which comes the astronomical twilight defined as the period from-12?to-18?
12.
There is no astronomical twilight at the poles near the winter solstice ( for about 74 days at the North Pole and about 80 days at the South Pole ).
13.
In the evening, even when astronomical twilight has yet to end and in the morning when astronomical twilight has already begun, most casual observers would consider the entire sky fully dark.
14.
In the evening, even when astronomical twilight has yet to end and in the morning when astronomical twilight has already begun, most casual observers would consider the entire sky fully dark.
15.
This is followed by nautical twilight, when the sun reaches heights of 6?and 12? after which comes astronomical twilight, defined as the period from 12?to 18?
16.
Twilight patterns are produced during the time period between the beginning of astronomical twilight ( when the sun is 18?below the horizon ) and sunrise, or sunset and the end of astronomical twilight.
17.
Twilight patterns are produced during the time period between the beginning of astronomical twilight ( when the sun is 18?below the horizon ) and sunrise, or sunset and the end of astronomical twilight.
18.
The lowest latitudes at which the various twilights can continue through local midnight are approximately 60.561?( 60?32 433 ) for civil twilight, 54.561?( 54?32 433 ) for nautical twilight and 48.561?( 48?32 433 ) for astronomical twilight.
19.
At the winter solstice within the polar circle, twilight can extend through solar noon at latitudes below 72.561?( 72?32 433 ) for civil twilight, 78.561?( 78?32 433 ) for nautical twilight, and 84.561?( 84?32 433 ) for astronomical twilight.
20.
A civil twilight period of about two weeks occurs before sunrise and after sunset, a nautical twilight period of about five weeks occurs before sunrise and after sunset and an astronomical twilight period of about seven weeks occurs before sunrise and after sunset.