In order to understand the difference between dusk and twilight, it is important to understand a few important terms with regard to the observed movement of the sun in relation to the earth. There are four terms that are interrelated and help clarify the meaning of “twilight.” Dawn is the moment at which the sun’s light begins to illuminate an area though the sun is not visible, and sunrise is the point at which the sun becomes visible; sunset is the moment when the sun is no longer visible, and dusk is the point at which an area becomes dark. Twilight is the time between dawn and sunrise and between sunset and dusk, during which light is scattered indirectly from the sun to illuminate an area.
Dusk and twilight are different concepts, though they are closely related with regard to how the sun appears to move throughout the sky. Even though twilight is commonly associated with sunset and dusk, it actually occurs twice each day. In the morning, dawn is the moment in which the sun begins to illuminate the sky and twilight is the period when sunlight is scattered by the atmosphere to provide indirect lighting. This is followed by sunrise, when the sun becomes visible over the horizon in the morning.
At the end of the day is a similar process, played out in reverse, in which both dusk and twilight occur. Sunset is the moment when the sun is no longer visible over the horizon, which begins a period of twilight in the evening when indirect lighting is created again by sunlight scattered by the atmosphere. This twilight technically ends at dusk, when this light is replaced by darkness. The basic difference between the two is, therefore, that twilight is a period in which evening light is still provided indirectly by the sun, while dusk is when this light has faded for the night.
Timing for both dusk and twilight can be affected by a number of factors. The position of an observer on the earth, with regard to his or her latitude, can have a direct impact on when dusk and twilight occur for him or her. Seasonal changes based on the tilt of the earth can alter the duration of twilight in the morning and evening. There are also three different types of twilight, based on the position of the sun: civil twilight occurs when the sun is not visible but its center is below the horizon less than 6°; nautical twilight is observed when sun's center is 6° to 12° below the horizon; and astronomical twilight happens as the sun is 12° to 18° below the horizon.