There are three established and widely accepted subcategories of twilight: civil twilight (nearest the horizon), nautical twilight, and astronomical twilight (farthest from the horizon).
In the United States' military, the initialisms BMCT (begin morning civil twilight, i.e.
Civil dawn is preceded by morning nautical twilight and civil dusk is followed by evening nautical twilight.
Evening nautical twilight begins when the geometric center of the sun is 6° below the horizon in the evening and ends (nautical dusk) when the geometric center of the sun is 12° below the horizon in the evening.
Nautical dawn is the moment when the geometric center of the Sun is 12 degrees below the horizon in the morning.
When the Sun reaches 18 degrees below the horizon, the twilight's brightness is nearly zero, and evening twilight becomes nighttime.
When the Sun again reaches 18° below the horizon, nighttime becomes morning twilight.
civil dawn) and EECT (end evening civil twilight, i.e.