Twilight occurs just before sunrise and just after sunset (Figure 2). Three periods of twilight are generally recognized: civil, nautical, and astronomical twilights. Civil twilight occurs while the center of the Sun is between 50' and 6° below the horizon. In the evening, darkness forces us to stop normal outdoor activities towards the end of civil twilight, but even then it is not pitch black. In southern Canada, street lights are not usually turned on until half an hour after sunset, at the end of civil twilight. Twilight lasts considerably longer in the arctic winter, because the Sun rises and sets at a much gentler angle, and so takes longer to drop 6° below the horizon. Nautical twilight begins in the morning and ends in the evening when the center of the Sun is 12° below the horizon. At this point it is usually impossible to make out the horizon.

Astronomical twilight occurs while the center of the Sun is up to 18° below the horizon. In the Arctic there is generally no trace of the twilight glow in the southern sky when the Sun is this far below the horizon. In the evening, once the astronomical period is over, light from starlight and airglow, from chemical reactions 60 to 200 km above the Earth's surface, may be about twice that from scattered sunlight.