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HM Nautical Almanac Office: Miscellanea

Twilights

Twilight is the time preceding sunrise and following sunset when the sky is partially illuminated. In polar regions twilight can persist for long periods when the Sun is below the horizon for extended periods of time. A definition of sunrise and sunset is a helpful starting point in this description of twilight.

Sunrise and sunset are taken to be the times at which the apparent upper limb of the Sun is on the (astronomical) horizon. For our purposes, they are computed by calculating when the zenith distance of the centre of the Sun's disk is 90° 50′, adopting 34′ for horizontal refraction and 16′ for the semi-diameter of the Sun.

The term depression can also be used in the context of an object's position relative to the horizon. The depression of an object is its angular distance below the horizon i.e. the zenith distance of the object minus 90°. Hence, the depression of the centre of the Sun's disk at sunset is 50′.

For the purposes of this discussion, a reference to the summer solstice means the northern hemisphere summer solstice taking place on or about June 21st.

The Astronomical Almanac tabulates the following times of twilight in addition to the times of sunrise and sunset.

For a typical location in non-polar regions, the normal daily sequence of events and the corresponding zenith distance (Z.D.) and depression (Dep.) of the Sun is as follows:

Daily sequence of eventsZ.D.Dep.Illumination conditions
(Ignoring the effects of moonlight)
Beginning Morning Astronomical Twilight 108°18°Sixth magnitude stars are no longer visible to the naked eye under good conditions
Beginning Morning Nautical Twilight102°12°It may now be possible to discern the sea horizon and it is no longer dark for normal practical purposes.
Beginning Morning Civil Twilight96°Large terrestrial objects can be now be distinguished. The sea horizon is clearly defined and the brightest stars and planets are still visible.
End of the Hours of DarknessN/AN/AEnds 30 minutes before sunrise as defined by the United Kingdom Road Vehicle Lighting Regulations (1989) and always occurs during civil twilight in the UK.
Sunrise90° 50′50′Daylight
Sunset90° 50′50′
Beginning of the Hours of DarknessN/AN/ABegins 30 minutes after sunset as defined by the United Kingdom Road Vehicle Lighting Regulations (1989) and always occurs during civil twilight in the UK.
End Evening Civil Twilight96°Large terrestrial objects can be seen but no detail can be distinguished. The sea horizon is clearly defined and the brightest stars and planets are visible.
End Evening Nautical Twilight102°12°The sea horizon is no longer visible and it can be considered to be dark for normal practical purposes.
End Evening Astronomical Twilight108°18°Sixth magnitude stars are now visible to the naked eye under good conditions.

By way of a historical footnote, sunrise, sunset, moonrise and moonset times were first introduced into The Nautical Almanac and Astronomical Ephemeris in 1925. Astronomical twilight followed in 1928 and nautical and civil twilight were first tabulated in 1937.

Further information on twilights can be found in HMNAO Astronomical Information Sheet No. 7, A note on sunrise, sunset, and twilight times and on the illumination conditions during twilight.

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Last modified: Friday, 31 May 2013 at 11:41:23 BST