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Tropic of Cancer

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Tropic of Cancer

World map showing the Tropic of Cancer

The Tropic of Cancer, also referred to as the Northern Tropic, is the most northerly circle of latitude on the Earth at which the Sun may appear directly overhead at its culmination. This event occurs once per year, at the time of the Northern solstice, when the Northern Hemisphere is tilted toward the Sun to its maximum extent. As of 15 June 2016, it lies at 23°26′13.7″ (or 23.43715°) north of the Equator.[1]

Its Southern Hemisphere counterpart, marking the most southerly position at which the Sun may appear directly overhead, is the Tropic of Capricorn. These tropics are two of the five major degree measures or major circles of latitude that mark maps of the Earth, besides the Arctic and Antarctic Circles and the Equator. The positions of these circles of latitude (other than the Equator) are dictated by the tilt of the Earth's axis of rotation relative to the plane of its orbit.

Contents

  • Name 1
  • Geography 2
  • Climate 3
  • Circumnavigation 4
  • External links 5
  • See also 6
  • References 7

Name

Tropic of cancer passes through Gujarat, India
Road sign South of Dakhla, Western Sahara marking the Tropic of Cancer. The sign was placed by the Budapest-Bamako rally participants, thus the inscription is in English and Hungarian.
Tropic of Cancer passes through National Highway 34 in Nadia District, West Bengal, India

The imaginary line is called the Tropic of Cancer because when the Sun reaches the zenith at this latitude, it is entering the astrological sign of Cancer (summer solstice in the northern hemisphere). When it was named, the Sun was also in the direction of the constellation Cancer (Latin for crab). However, this is no longer true due to the precession of the equinoxes. According to International Astronomical Union boundaries, the Sun now is in Taurus at the June solstice. According to sidereal astrology, which divides the zodiac into 12 equal parts, the Sun is in Gemini at that time. However, according to tropical astrology, which divides the ecliptic in twelve 30° sectors, starting with the vernal equinox, the Sun is always entering Cancer at this time, as the Earth's axial tilt is most inclined towards the Sun. The word "tropic" itself comes from the Greek "trope (τροπή)", meaning turn (change of direction, or circumstances), inclination, referring to the fact that the sun appears to "turn back" at the solstices.

Tropic of cancer passes through Madhya Pradesh, India

Geography

Carretera 83 (Vía Corta) Zaragoza-Victoria, km 27+800. Of all crossings of the Tropic of Cancer with Mexican federal highways, this is the only place where the latitude is marked with precision and where the annual drift between the years 2005 and 2010 can be appreciated.
Tropic of cancer Mark in Rueisuei Township, Hualian County, Taiwan

The Tropic of Cancer position is not fixed, but varies in a complicated manner over time. It drifts south almost half an arcsecond (0.47″) of latitude per year (it was at exactly 23° 27′ in year 1917).[2] See axial tilt and circles of latitude for further information.

North of the tropic are the subtropics and the North Temperate Zone. The equivalent line of latitude south of the Equator is called the Tropic of Capricorn, and the region between the two, centered on the Equator, is the tropics.

Starting at the Prime Meridian and heading eastward, the Tropic of Cancer passes through:

Co-ordinates Country, territory or sea Notes
 Algeria
 Niger
 Libya The Tropic touches on the northernmost point of  Chad at
 Egypt The Tropic passes through Lake Nasser
Red Sea
 Saudi Arabia
 United Arab Emirates Abu Dhabi emirate only
 Oman
Indian Ocean Arabian Sea
 India States of Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand and West Bengal
 Bangladesh Khulna, Dhaka, and Chittagong Divisions
 India State of Tripura
 Bangladesh Chittagong Division
 India State of Mizoram
 Myanmar (Myanmar) Chin State, Sagaing Division, Mandalay Division, Shan State
 China Provinces of Yunnan (passing about 7 km north of the border with  Vietnam), Guangxi, and Guangdong
Taiwan Strait
 Taiwan Chiayi County, Hualien County
Pacific Ocean Passing just south of Necker Island, Hawaii,  United States
 Mexico State of Baja California Sur
Gulf of California
 Mexico States of Sinaloa, Durango, Zacatecas, San Luis Potosí, Nuevo León, and Tamaulipas
Gulf of Mexico
Atlantic Ocean Passing through the Straits of Florida and the Nicholas Channel
Passing just south of the Anguilla Cays ( Bahamas)
Passing through the Santaren Channel and into the open ocean
 Bahamas Exuma Islands and Long Island
Atlantic Ocean
Western Sahara Claimed by  Morocco and the  Sahrawi Arab Democratic Republic
 Mauritania
 Mali
 Algeria
Relationship between Earth's axial tilt (ε) to the tropical and polar circles

Climate

Excluding cooler highland regions in China, the climate at the Tropic of Cancer is generally hot and dry except for easterly coastal areas where orographic rainfall can be very heavy, in some places reaching 4 metres (160 in). Most regions on the Tropic of Cancer experience two distinct seasons: an extremely hot summer with temperatures often reaching 45 °C (113 °F) and a warm winter with maxima around 22 °C (72 °F). Much land on or near the Tropic of Cancer is part of the Sahara Desert, whilst to the east the climate is torrid monsoonal with a short wet season from June to September and very little rainfall for the rest of the year.

The highest mountain on or adjacent to the Tropic of Cancer is Yushan in Taiwan; though it had glaciers descending as low as 2,800 metres (9,190 ft) during the Last Glacial Maximum, none survive and at present no glaciers exist within 470 kilometres (290 mi) of the Tropic of Cancer; the nearest currently surviving are the Minyong and Baishui in the Himalayas to the north and on Iztaccíhuatl to the south.

Circumnavigation

According to the rules of the Fédération Aéronautique Internationale, for a flight to compete for a round-the-world speed record, it must cover a distance no less than the length of the Tropic of Cancer, cross all meridians, and end on the same airfield where it started.

To calculate the length of the Tropic:

  • At present, the radius of the Tropic of Cancer (or Capricorn) circle, computed from the radius of Earth's equatorial circle, is 6378 km x cos(23° 26′ 16″) which results in 5,851.77 kilometres (3,636.12 mi).
  • Then the length of either Tropic circle is \ell = \pi \cdot 2r = 3.1416 x 2 x 5851.77 which equals 36,768 kilometres (22,847 mi).
  • Therefore, assuming the Earth to be a perfect sphere, the length of the circumference at the parallel of the Tropic of Cancer or Capricorn is 36,768 kilometres (22,847 mi).

For an ordinary circumnavigation the rules are somewhat relaxed and the distance is set to a rounded value of at least 37,000 kilometers.

External links

  • Article on the Tropic of Cancer in Oman
  • Temporal Epoch Calculations
  • Useful constants See: Obliquity of the ecliptic

See also

References

  1. ^
  2. ^ Montana State University: Milankovitch Cycles & Glaciation Archived August 6, 2011 at the Wayback Machine
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