Three different twilight calculations are offered. Astronomical twilight is defined as the
time when the sun reaches 18 degrees below the horizon. This is as dark as it gets.
Nautical twilight is computed with the sun 12 degrees below the horizon. Civil twilight is
computed with the sun 6 degrees below.
Times can be displayed in Standard, Daylight, or Universal Time. This can be set
automatically if you wish. They can also be in 12 or 24 hour format.