Tropic of Cancer
Tropic of Cancer is also the name of a novel by Henry Miller, first published in 1934. |
World map showing the Tropic of Cancer in red |
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.
:The Tropic of Cancer passes through the following countries (going east):
*
United States of America (
Hawaii) - misses every island, passing between
Nihoa and
Necker Island*
Mexico*
Bahamas*
Western Sahara (claimed by
Morocco)
*
Mauritania*
Mali*
Algeria*
Niger*
Libya*
Chad - North-most point is defined by Tropic of Cancer
*
Egypt*
Saudi Arabia*
United Arab Emirates*
Oman*
India*
Bangladesh*
Myanmar*
People's Republic of China*
Republic of ChinaThe 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
Taurus.
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 36 787 559 metres. (A number implying an accuracy which does not exist, considering the variations of the tropic as described above).
For an ordinary
circumnavigation the rules are somewhat relaxed and the distance is set to more sensible rounded value of 37000 kilometres.
*
Tropic of Capricorn
