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.
The beginning of morning civil twilight and the end of evening civil twilight occur when the zenith distance of the centre of the Sun's disk is 96°.
For latitudes north of approximately 60.5° north, civil twilight will not occur around the summer solstice. Civil twilight occurs all year round for nearly all of the UK except the northernmost parts of the Shetland Islands e.g. Yell, Fetlar, Unst and the northern part of Mainland. For example, civil twilight does not occur at Norwick on Unst between June 13 and June 29.
The beginning of morning nautical twilight and the end of evening nautical twilight occur when the zenith distance of the centre of the Sun's disk is 102°.
For latitudes north of approximately 54.5° north, nautical twilight will not occur around the summer solstice. The table below shows the period for which nautical twilight does not occur for a sample of UK locations.
|Location||No nautical twilight|
|Edinburgh||from||June 2||to||July 11|
|Inverness||from||May 23||to||July 20|
|Kirkwall||from||May 16||to||July 27|
|Lerwick||from||May 11||to||August 1|
The beginning of morning astronomical twilight and the end of evening astronomical twilight occur when the zenith distance of the centre of the Sun's disk is 108°.
For latitudes north of approximately 48.5° north, astronomical twilight will not occur around the summer solstice. The table below shows the period for which astronomical twilight does not occur for a sample of UK locations.
|Location||No astronomical twilight|
|St. Helier||from||June 8||to||July 4|
|Southampton||from||May 26||to||July 17|
|Birmingham||from||May 18||to||July 25|
|Leeds||from||May 13||to||July 31|
|Edinburgh||from||May 5||to||August 8|
|Inverness||from||April 30||to||August 13|
|Kirkwall||from||April 25||to||August 18|
|Lerwick||from||April 22||to||August 21|
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 events||Z.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 Twilight||102°||12°||It may now be possible to discern the sea horizon and it is no longer dark for normal practical purposes.|
|Beginning Morning Civil Twilight||96°||6°||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 Darkness||N/A||N/A||Ends 30 minutes before sunrise as defined by the United Kingdom Road Vehicle Lighting Regulations (1989) and always occurs during civil twilight in the UK.|
|Beginning of the Hours of Darkness||N/A||N/A||Begins 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 Twilight||96°||6°||Large terrestrial objects but no detail can be distinguished. The sea horizon is clearly defined and the brightest stars and planets are visible.|
|End Evening Nautical Twilight||102°||12°||The sea horizon is no longer visible and it can be considered to be dark for normal practical purposes.|
|End Evening Astronomical Twilight||108°||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.