Dusk is the darkest stage of twilight in the evening. Pre-dusk, during early to intermediate stages of twilight, there may be enough light in the sky under clear-sky conditions to read outdoors without artificial illumination, but at the end of civil twilight, when the earth rotates to a point at which the center of the sun is at 6° below the local horizon, artificial illumination is required to read outside.
The time of dusk can be thought of relative to the time of twilight, which has several alternative technical definitions:
- Dusk as the last part of civil twilight ends when the sun is 6 degrees below the horizon in the evening. At this time objects are distinguishable and some stars and planets are visible to the naked eye.
- Dusk as the last part of nautical twilight ends when the sun is 12 degrees below the horizon in the evening. At this time, objects are no longer distinguishable, and the horizon is no longer visible to the naked eye.
- Dusk as the last part of astronomical twilight ends when the sun is 18 degrees below the horizon in the evening. At this time the sun no longer illuminates the sky, and thus no longer interferes with astronomical observations.