twi- + light

Twilight comes from Middle English, combining the prefix twi- (two, half) with 'light.' The twi- prefix is the same as in 'twin,' 'twine,' 'twist,' and 'between' — all words about doubleness or division. Twilight is two-light: the half-light, the divided light, the moment when the sky holds both day and night. German has Zwielicht (two-light) with the same construction.

Astronomers define three types of twilight. Civil twilight: the sun is up to 6 degrees below the horizon, and you can still read a newspaper outdoors. Nautical twilight: 6 to 12 degrees below, and sailors can still see the horizon. Astronomical twilight: 12 to 18 degrees below, and the sky is dark enough for telescopes. Below 18 degrees, it is full night. These definitions matter because twilight is not a moment — it is a gradual process that can last from minutes to hours depending on latitude.

The figurative meaning appeared early. The twilight of a career, the twilight of an era, the twilight of the gods — all describe endings that are gradual rather than sudden. Richard Wagner's Götterdämmerung (1876) translates as 'Twilight of the Gods,' and the Old Norse Ragnarök has the same sense of a slow, inevitable darkening. Twilight is not a cutoff. It is a fade.

At high latitudes, twilight can last all night. In Tromsø, Norway (69°N), the sun stays just below the horizon for weeks in winter, producing continuous twilight without ever reaching full darkness. This polar twilight has psychological effects — seasonal affective disorder is linked to extended low-light periods. The word 'twilight' covers a condition that, near the poles, can last for months.