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Twilight occurs according to the solar elevation angle θ s, which is the position of the geometric center of the Sun relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (nearest the horizon), nautical twilight, and astronomical twilight (farthest from the horizon).
Twilight (both dusk and dawn) is divided into three 6° segments that mark the Sun's position below the horizon. At civil twilight, the center of the Sun's disk appears to be between 1/4° and 6° below the horizon. At nautical twilight, the Sun's altitude is between –6° and –12°. At astronomical twilight, the Sun is between –12° and ...
Nautical twilight begins when there is enough light for sailors to distinguish the horizon at sea, but the sky is still too dark to perform outdoor activities. It begins when the center of the Sun is 12 degrees below the horizon in the morning. Nautical dawn marks the start of nautical twilight, which lasts until civil dawn. [6] [5]
Civil twilight happens when the Sun is between 0 and 6° below the horizon, so this phenomenon can also be referred to as civil polar night. Nowhere on mainland Europe is this definition met. The Norwegian town of Longyearbyen, Svalbard, experiences nautical polar twilight from about 11 November until 30 January. Dikson, in Russia, experiences
Blue hour at the Old Cathedral of the Holy Name of Jesus, Bragança in Portugal. The blue hour (from French l'heure bleue; [1] [a] pronounced [lœʁ blø]) is the period of twilight (in the morning or evening, around the nautical stage) when the Sun is at a significant depth below the horizon.
The first one is civil twilight, which begins once the Sun has disappeared below the horizon, and continues until it descends to 6 degrees below the horizon. The early to intermediate stages of twilight coincide with predusk. The second phase is nautical twilight, between 6 and 12 degrees below the horizon
Above 60° latitude, the Sun would be even closer to the horizon, only 6.56° away from it. Then civil twilight continues almost all night, only a little bit of nautical twilight around the local midnight. Above 66.56° latitude, there is no sunset at all, a phenomenon referred to as the midnight sun.
After sunset, the civil twilight sets in; it ends when the Sun drops more than 6° below the horizon. This is followed by the nautical twilight, when the Sun is between 6° and 12° below the horizon (depth between −6° and −12°), after which comes the astronomical twilight, defined as the period between −12° and −18°. When the Sun ...