Civil twilight: Morning civil twilight starts when the geometric centre of the sun is 6° below the horizon. Evening civil twilight begins at sunset and ends when the centre of the sun reaches 6° below the horizon. The evening civil twilight is otherwise known as dusk. This is when the brightest stars will be visible under normal atmospheric conditions
Nautical Twilight: Ahoy, mates! Nautical twilight is the time when the centre of the sun is between 6° and 12° below the horizon. During nautical twilight the illumination is such that the horizon is still visible even on a Moonless night, allowing mariners to take reliable star sights for navigational purposes, hence the name. At the beginning or end of nautical twilight, under good atmospheric conditions, vague outlines of ground objects may be distinguishable, but details are not likely.
Astronomical Twilight: Darker than civil or nautical, astronomical twilight occurs when the center of the sun is a full 18 degrees below the horizon. As its name suggests, this twilight period is of most interest to astronomers. There is no color in the sky during astronomical twilight.