Tropic of Cancer
From Wikipedia, the free encyclopedia
- Tropic of Cancer is also the name of a novel by Henry Miller, first published in 1934.
The Tropic of Cancer (cancer is Latin for crab), or Northern tropic is one of the five major circles of latitude that mark maps of the Earth. It is the parallel of latitude that lies currently 23° 26′ 22″ north of the Equator.
It is the farthest northern latitude at which the sun can appear directly overhead, which occurs on the June solstice. North of this line is the subtropics and Northern Temperate Zone. The Tropic of Capricorn is at the opposite latitude south of the Equator. South of the Tropic of Cancer and north of the Tropic of Capricorn are the Tropics.
It is difficult to specify an exact location for both tropic circles because the obliquity of the ecliptic varies between 21.5° and 24.5° with a 41,000 year periodicity. Currently the mean value of the obliquity diminishes with 0.5" per year, resulting in a southwards shift of the Tropic of Cancer over the Earth surface with 15 metres per year. Together with a likewise northwards shift of the Tropic of Capricorn, this means that the mean area of the tropics reduces worldwide by 1100 square kilometres per year. In addition the apparent or real value of the obliquity is subject to extra variations due to the nutation, of which the mainterm with a period of 18.6 year has an amplitude of 9"21 (corresponding to almost 300 m north and south). And then there are still many smaller terms, resulting in daily varying shifts of some meters in any direction.
Contents |
[edit] Geography
- The Tropic of Cancer passes through the following countries (going east):
- United States of America (Hawaii) - sea area only, misses every island, passing between Nihoa and Necker Island
- Mexico, just north of Mazatlan on the Pacific
- Bahamas
- Western Sahara (claimed by Morocco)
- Mauritania
- Mali
- Algeria
- Niger
- Libya
- Chad – Its northernmost point is defined by the Tropic of Cancer.
- Egypt
- Saudi Arabia
- United Arab Emirates
- Oman capital city Muscat
- India
- Bangladesh
- Myanmar
- China, just north of Guangzhou, also known as Canton
- Taiwan
[edit] History
The line is called the Tropic of Cancer because when it was named, the sun was in the constellation of Cancer when it appeared directly overhead at this latitude during the June solstice. However, due to precession of the equinoxes, the June solstice is now in the constellation Gemini.
[edit] Trivia
According to the Fédération Aéronautique Internationale's rules, for a flight to compete for a round-the-world speed record, it must cover a distance no less than the length of Tropic of Cancer as well as cross all meridians and end on the same airfield where it started. This length is set to be 36787.559 kilometres - a number implying an accuracy which does not exist, considering the variations of the tropic described above.
For an ordinary circumnavigation the rules are somewhat relaxed and the distance is set to a rounded value of 37000 kilometres.
