Lighting-up time, sunrise/sunset and twilights

View of the Royal Observatory Greenwich at night taken from the roof of Flamsteed House.View over the Royal Observatory, Greenwich at twilight, with the dome open housing the 28-inch telescope. Taken from the roof of Flamsteed House. Repro ID: D3155. ©National Maritime Museum, Greenwich, London The Road Vehicles Lighting Regulations, 1989, make the use of front and rear position lamps compulsory on vehicles during the period between sunset and sunrise.

Headlamps are required on vehicles during the hours of darkness which are defined by these regulations as being the interval between one half-hour after sunset to one half-hour before sunrise.

Lights are also required at other times when visibility is restricted.

Sunrise and sunset are defined by these regulations as local sunrise and sunset.

Sunrise and sunset

The times of sunrise and sunset refer to the times when the Sun's upper limb, as affected by refraction, is on the true horizon of an observer at sea-level. This occurs when the Sun's centre is 50 arcminutes below the true horizon, the upper limb then being 34 arcminutes (just more than the Sun's apparent diameter) below the true horizon.

Twilights

There are three different definitions of twilight:

  • Civil twilight – when the Sun's centre is 6° below the horizon, is roughly equivalent to Lighting-up Time. In the UK, it is between 30 and 60 minutes after sunset. The brightest stars are visible and at sea the horizon is clearly defined.
  • Nautical twilight – when the Sun's centre is 12° below the horizon, is to all intents and purposes the time when it is dark. For nautical purposes it is that time when the horizon ceases to be clearly visible and it is impossible to determine altitudes with reference to the horizon.
  • Astronomical twilight – when the Sun's centre is 18° below the horizon, is when it is truly dark and no remnant of the Sun's afterglow can be seen. It is possible to see the Zodiacal light which comes from light from the Sun reflected by small particles between the Earth and the Sun; this can be mistaken for the Sun's afterglow.