Twilight follows sunset and precedes sunrise. Each evening of astrophotography begins or ends when the top of the sun touches the horizon at sunset.
(Moonrise and moonset also begin when the top of the moon touches the local horizon.)
- Civil: sun is between 0 and 6 degrees below horizon.
- Nautical: sun is between 6 and 12 degrees below horizon.
- Astronomical: sun is between 12 and 18 degrees below horizon.
Dark night is defined as the period when the sun is 18 degrees or more below the horizon.
Note that the sun itself is about 0.5 degrees in diameter.