What are the definitions of dawn, dusk and twilight?

Dawn occurs at the time that the geometric center of the Sun is 18° below the horizon in the morning. Respectively, dusk occurs when the geometric center of the Sun is 18° below the horizon in the evening. Twilight refers to the periods between the dawn and sunrise and sunset and dusk, where sunrise and sunset are defined as the exact times when the upper edge of the disc of the Sun is at the horizon. The hazy light during this period is an effect caused by the scattering of the sunlight in the upper atmosphere and reflecting towards Earth. It is very subjective as far as the time of day that twilight occurs, because it depends on the location and elevation of the observer, the time of year and local weather conditions.

In addition, twilight is divided into three durations based on the angle of the Sun below the horizon. Astronomical twilight is the period when the Sun is between 18° and 12° below the horizon. Nautical twilight is the period when the Sun is between 12° and 6° below the horizon, and civil twilight is the period when the Sun is between 6° below the horizon until sunrise. The same designations are used for periods of evening twilight.