Twilight - Length

Length

The length of twilight after sunset and before sunrise is heavily influenced by the latitude of the observer. In the Arctic and Antarctic regions, twilight (if there is any) can last for several hours. There is no civil twilight at the poles within a month on either side of the winter solstice. At the poles, civil twilight can be as long as two weeks, while at the equator, conditions can go from day to night in as little as twenty minutes. This is true because at low latitudes the sun's apparent movement is perpendicular to the observer's horizon. As one gets closer to the Arctic and Antarctic circles, the sun's disk moves toward the observer's horizon at a lower angle. The observer's earthly location will pass through the various twilight zones less directly, taking more time.

Within the polar circles, twenty-four hour daylight is encountered in summer, and in regions very close to the poles, twilight can last for weeks on the winter side of the equinoxes. Outside the polar circles, where the angular distance from the polar circle is less than the angle which defines twilight (see above), twilight can continue through local midnight near the summer solstice (June in the Northern Hemisphere, December in the Southern Hemisphere). The precise position of the polar circles—and thus of the regions where twilight can continue through local midnight—varies slightly from year to year with Earth's axial tilt. The lowest latitudes at which the various twilights can continue through local midnight are approximately 60.561° (60° 33’ 43”) for civil twilight, 54.561° (54° 33’ 43”) for nautical twilight and 48.561° (48° 33’ 43”) for astronomical twilight.

These are the largest cities, of their respective countries, where the various twilights can continue through local solar midnight:

• Civil twilight from sunset to sunrise: Arkhangelsk, Tampere, Umeå, Trondheim, Mid Yell, Tórshavn, Reykjavik, Nuuk, Whitehorse, Anchorage and Baltasound.
• Nautical twilight from civil dusk to civil dawn: Petropavl, Moscow, Vicebsk, Vilnius, Riga, Tallinn, Wejherowo, Flensburg, Helsinki, Stockholm, Copenhagen, Oslo, Newcastle upon Tyne, Glasgow, Belfast, Letterkenny, Edmonton, Grande Prairie, Juneau, Ushuaia and Puerto Williams.
• Astronomical twilight from nautical dusk to nautical dawn: Hulun Buir, Erdenet, Astana, Samara, Kiev, Minsk, Warsaw, Košice, Northernmost points of Hungary, Zwettl, Prague, Berlin, Paris, Luxembourg City, Brussels, Amsterdam, London, Cardiff, Dublin, Winnipeg, Calgary, Vancouver, International Falls, Bellingham, Rio Gallegos and Punta Arenas.

Although Helsinki, Oslo, Stockholm, Tallinn and Saint Petersburg do not actually receive civil twilight from sunset to sunrise, they do have noticeably lighter skies at night (known as white nights) around the summer solstice.

During the winter solstice there are various twilights that can go through solar noon. Here are the highest latitudes that twilight go through solar noon 73.439° ( 73° 26' 17" ) for Civil twlight,79.439°( 79° 26' 17" ) for Nautical twlight,85.439°( 85° 26' 17 ) for Astronomical twlight.

These are the cities where twilights can continue through solar noon:

• Astronomical twilight from dawn to dusk: Alert, Canada