Twilight

The lower the Sun is beneath the horizon, the dimmer the sky (other factors such as atmospheric conditions being equal).

When the Sun reaches 18° below the horizon, the illumination emanating from the sky is nearly zero, and evening twilight becomes nighttime.

Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the lit sky, twilight has long been popular with photographers and painters, who often refer to it as the blue hour, after the French expression l'heure bleue.

Twilight occurs according to the solar elevation angle θs, which is the position of the geometric center of the Sun relative to the horizon.

Under good atmospheric conditions with the absence of other illumination, during nautical twilight, the human eye may distinguish general outlines of ground objects but cannot participate in detailed outdoor operations.

A military unit may treat BMNT and EENT with heightened security, e.g. by "standing to", for which everyone assumes a defensive position.

Observers within 63°26' of the Equator can view twilight twice each day on every date between the month of the autumnal equinox and the month of vernal equinox between astronomical dawn, nautical dawn, or civil dawn, and sunrise as well as between sunset and civil dusk, nautical dusk, or astronomical dusk, i.e., from September 1 to March 31 of the following year in the Northern Hemisphere and from March 1 to September 30 in the Southern Hemisphere.

The nighttime/twilight boundary solar midnight's latitude varies depending on the month: At latitudes greater than about 48°34' North or South, on dates near the summer solstice (June 21 in the Northern Hemisphere or December 21 in the Southern Hemisphere), twilight can last from sunset to sunrise, since the Sun does not sink more than 18 degrees below the horizon, so complete darkness does not occur even at solar midnight.

In Arctic and Antarctic latitudes in wintertime, the polar night only rarely produces complete darkness for 24 hours each day.

At all other latitudes and dates, the polar night includes a daily period of twilight, when the Sun is not far below the horizon.

At latitudes greater than 81°25' North or South, as the Sun's angular elevation difference is less than 18 degrees, twilight can last for the entire 24 hours.

The only permanent settlement to experience this condition is Alert, Nunavut, Canada, where it occurs from February 22–26, and again from October 15–19.

As one gets closer to the Arctic and Antarctic circles, the Sun's disk moves toward the observer's horizon at a lower angle.

Within the polar circles, twenty-four-hour daylight is encountered in summer, and in regions very close to the poles, twilight can last for weeks on the winter side of the equinoxes.

[18] In Christian practice, "vigil" observances often occur during twilight on the evening before major feast days or holidays.

[citation needed] Hinduism prescribes the observance of certain practices during twilight, a period generally called sandhya.

According to Hindu scriptures, an asura king, Hiranyakashipu, performed penance and obtained a boon from Brahma that he could not be killed during day or night, neither by human nor animal, neither inside his house nor outside.

Vishnu appeared in a half-man half-lion form (neither human nor animal), and ended Hiranyakashipu's life at twilight (neither day nor night) while he was placed in the threshold of his house (neither inside nor outside).

[citation needed] In Judaism, twilight is considered neither day nor night; consequently it is treated as a safeguard against encroachment upon either.