The Definitions of Twilight Content from the guide to life, the universe and everything

The Definitions of Twilight

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Many people, when they think of twilight, will think of the period shortly before sunrise when the birds begin to sing, there is a warm glow in the eastern sky and the air has a stillness and serenity about it that cannot be experienced at any other time of day. Likewise in the evening just after sunset, there is a red afterglow in the west, birds begin to roost for the night, lovers meet to whisper sweet nothings to each other and we can imagine vampires might be stirring from their sleep.

However, twilight has a specific scientific definition which varies depending on whether you are a civilian, a mariner or an astronomer. There are three types of twilight; Civil Twilight, Nautical Twilight and Astronomical Twilight.

Civil Twilight

In the morning civil twilight begins when the sun is six degrees below the horizon before sunrise; in the evening it lasts after sunset until the sun is six degrees past the horizon. Between these times it is considered light enough to work outside without the aid of artificial lighting. It is used today for work that needs to be done at quieter times of the day when there are not many people around.

North of latitude 60 degrees, civil twilight lasts all night during midsummer as the sun will never be more than six degrees below the horizon. It is the same phenomenon that stops the termination of daylight for months at a time in Polar Regions. 60 degrees passes through the northern part of the Shetland Isles and forms the border between the Northwest Territories and the rest of Canada.

The civil twilight zone ends at the Arctic Circle which is at 66.56 degrees north. North of the Arctic Circle there is at least one 24-hour period when the sun never sets. The closer you get to the North Pole, the longer the periods of daylight.

Nautical Twilight

In the morning nautical twilight begins when the sun is 12 degrees below the horizon at sunrise; in the evening it lasts after sunset until the sun is 12 degrees below the horizon. Between these times the horizon is distinguishable at sea. It is important to mariners because at this time they can use the horizon as a reference for taking bearings using a sextant, yet it is still dark enough to see most of the major star groupings.

North of latitude 55 degrees, nautical twilight lasts all night during midsummer. Newcastle-upon-Tyne and Grand Prairie in Alberta are at a latitude of 55 degrees north.

Astronomical Twilight

In the morning astronomical twilight begins when the sun is 18 degrees below the horizon at sunrise; in the evening it lasts after sunset until the sun is 18 degrees below the horizon. The time between the end and the start of astronomical twilight is important to astronomers as this is the time when the sky is completely dark which is required for observation of faint stellar objects.

North of latitude 50 degrees, astronomical twilight lasts all night during midsummer. Abbeville in northern France and Medicine Hat in southern Alberta are about 50 degrees north. Therefore it can be seen that astronomical observations during midsummer in the UK can be rather difficult.

Astronomical twilight differs slightly from the other two as it is the time of darkness between the end and the start of twilight when there is no light that interests astronomers. With civil and nautical twilight, it is the time between the start and the end when the presence of sunlight is the important factor.

The time between twilight and sunset or sunrise varies depending on your latitude. At UK latitudes civil twilight lasts about an hour. The further north you go, the longer the period, until you get to the Shetland Isles where civil twilight last as long as two hours. At equatorial latitudes twilight lasts only a few minutes - consequently it can go from pitch dark to broad daylight very quickly.

Everything said above relating to the Northern Hemisphere also applies to the Southern Hemisphere. Just change north for south and Arctic for Antarctic. In the Southern Hemisphere 50 degrees south is south of Australia and New Zealand. The Falkland Islands are about 51 degrees south. After that there is just ocean and Antarctica.

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