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A CIRCLE OF LATITUDE on the Earth
Earth
is an abstract east –west circle connecting all locations around the Earth
Earth
(ignoring elevation) at a given latitude .

Circles of latitude are often called PARALLELS because they are parallel to each other – that is, any two circles are always the same distance apart. A location's position along a circle of latitude is given by its longitude . Circles of latitude are unlike circles of longitude, which all are great circles with the centre of Earth
Earth
in the middle, as the circles of latitude get smaller as the distance from the Equator
Equator
increases. Their length can be calculated by a common sine or cosine function. The 60th circle of latitude is half as long as the equator (disregarding Earth's minor flattening by 0.3%). A circle of latitude is perpendicular to all meridians .

The latitude of the circle is approximately the angle between the equator and the circle, with the angle's vertex at Earth's centre. The equator is at 0°, and the North and South poles are at 90° north and 90° south respectively. The Equator
Equator
is the longest circle of latitude and is the only circle of latitude which also is a great circle.

There are 89 integral (whole degree ) circles of latitude between the equator and the Poles in each hemisphere , but these can be divided into more precise measurements of latitude, and are often represented as a decimal degree (e.g. 34.637°N) or with minutes and seconds (e.g. 22°14'26"S). There is no limit to how precisely latitude can be measured, and so there are an infinite number of circles of latitude on Earth.

On a map, the circles of latitude may or may not be parallel, and their spacing may vary, depending on which projection is used to map the surface of the Earth
Earth
onto a plane. On an equirectangular projection , centered on the equator, the circles of latitude are horizontal, parallel, and equally spaced. On other cylindrical and pseudocylindrical projections, the circles of latitude are horizontal and parallel, but may be spaced unevenly to give the map useful characteristics. For instance, on a Mercator projection
Mercator projection
the circles of latitude are more widely spaced near the poles to preserve local scales and shapes, while on a Gall–Peters projection the circles of latitude are spaced more closely near the poles so that comparisons of area will be accurate. On most non-cylindrical and non-pseudocylindrical projections, the circles of latitude are neither straight nor parallel.

Arcs of circles of latitude are sometimes used as boundaries between countries or regions where distinctive natural borders are lacking (such as in deserts), or when an artificial border is drawn as a "line on a map", which was made in massive scale during the 1884 Berlin Conference , regarding huge parts of the African continent. North American nations and states have also mostly been created by straight lines, which are often parts of circles of latitudes. For instance, the northern border of Colorado
Colorado
is at 41°N while the southern border is at 37°N . Roughly half the length of border between the United States and Canada
Canada
follows 49°N .

CONTENTS

* 1 Major circles of latitude

* 1.1 Equator
Equator
* 1.2 Polar Circles * 1.3 Tropical Circles * 1.4 Movement of the Tropical and Polar circles

* 2 Other notable parallels * 3 Altitude * 4 See also * 5 References * 6 External links

MAJOR CIRCLES OF LATITUDE

Diagram showing the locations of the five major circles of latitude on an equirectangular projection of the Earth
Earth
. Relationship between Earth's axial tilt (ε) to the tropical and polar circles

There are five major circles of latitude, listed below from north to south. The position of the Equator
Equator
is fixed (90 degrees from Earth's axis of rotation) but the latitudes of the other circles depend on the tilt of this axis relative to the plane of Earth's orbit, and so are not perfectly fixed. The values below are for 28 July 2017:

* Arctic Circle
Arctic Circle
(66°33′46.8″ N) * Tropic of Cancer (23°26′13.2″ N) * Equator
Equator
(0° latitude) * Tropic of Capricorn
Tropic of Capricorn
(23°26′13.2″ S) * Antarctic Circle
Antarctic Circle
(66°33′46.8″ S)

These circles of latitude, excluding the Equator, mark the divisions between the five principal geographical zones .

EQUATOR

The equator is the circle that is equidistant from the North Pole and South Pole
South Pole
. It divides the Earth
Earth
into the Northern Hemisphere and the Southern Hemisphere . Of the parallels or circles of latitude, it is the longest, and the only 'great circle ' (a circle on the surface of the Earth, centered on Earth's center). All the other parallels are smaller and centered only on Earth's axis.

Equator
Equator

POLAR CIRCLES

The Arctic Circle
Arctic Circle
is the southernmost latitude in the Northern Hemisphere at which the sun can remain continuously above or below the horizon for 24 hours (at the June and December solstices respectively). Similarly, the Antarctic Circle
Antarctic Circle
marks the northernmost latitude in the Southern Hemisphere at which the sun can remain continuously above or below the horizon for 24 hours (at the December and June Solstices respectively).

Arctic Circle
Arctic Circle

Antarctic Circle
Antarctic Circle

TROPICAL CIRCLES

The Tropic of Cancer and Tropic of Capricorn
Tropic of Capricorn
mark the northernmost and southernmost latitudes at which the sun may be seen directly overhead (at the June solstice and December solstice respectively).

The latitude of the tropical circles is equal to the Earth's axial tilt , about 23°.

Tropic of Cancer

Tropic of Capricorn
Tropic of Capricorn

MOVEMENT OF THE TROPICAL AND POLAR CIRCLES

See also: Axial tilt

By definition, the positions of the Tropic of Cancer , Tropic of Capricorn , Arctic Circle
Arctic Circle
and Antarctic Circle
Antarctic Circle
all depend on the tilt of the Earth\'s axis relative to the plane of its orbit around the sun (the "obliquity of the ecliptic"). If the Earth
Earth
were "upright" (its axis at right angles to the orbital plane) there would be no Arctic, Antarctic, or Tropical circles: at the poles the sun would always circle along the horizon, and at the equator the sun would always rise due east, pass directly overhead, and set due west.

The positions of the Tropical and Polar circles are not fixed because the axial tilt changes slowly – a complex motion determined by the superimposition of many different cycles (some of which are described below) with short to very long periods. In 2000 the mean value of the tilt was about 23° 26′ 21″.

The main long-term cycle causes the axial tilt to fluctuate between about 22.1° and 24.5° with a period of 41,000 years. Currently, the average value of the tilt is decreasing by about 0.47″ per year. As a result, (approximately, and on average) the Tropical Circles are drifting towards the equator (and the Polar Circles towards the poles) by 15 metres per year, and the area of the Tropics is decreasing by 1,100 square kilometres (420 sq mi) per year.

The Earth's axial tilt has additional shorter-term variations due to nutation , of which the main term, with a period of 18.6 years, has an amplitude of 9.2" (corresponding to almost 300 metres north and south). There are many smaller terms, resulting in varying daily shifts of some metres in any direction.

Finally, the Earth's rotational axis is not exactly fixed in the Earth, but undergoes small fluctuations (on the order of 15 meters) called polar motion , which have a small effect on the Tropics and Polar circles and also on the Equator.

Short-term fluctuations over a matter of days do not directly affect the location of the extreme latitudes at which the sun may appear directly overhead, or at which 24-hour day or night is possible, except when they actually occur at the time of the solstices. Rather, they cause a theoretical shifting of the parallels, that would occur if the given axis tilt were maintained throughout the year.

OTHER NOTABLE PARALLELS

See also: Baseline (surveying)

A number of sub-national and international borders were intended to be defined by, or are approximated by, parallels. Parallels make convenient borders in the northern hemisphere because astronomic latitude can be roughly measured (to within a few tens of meters) by sighting the North Star .

PARALLEL DESCRIPTION

70°N On Victoria Island , Canada
Canada
, two sections of the border between Northwest Territories and Nunavut .

60°N In Canada, the southern border of Yukon with the northern border of British Columbia ; the southern border of Northwest Territories with the northern borders of British Columbia, Alberta
Alberta
and Saskatchewan
Saskatchewan
; and the southern border of mainland Nunavut with the northern border of Manitoba
Manitoba
, leading to the expression "north of sixty" for the territories.

54°40\'N The border between 19th century Russian territories to the north and conflicting American and British land claims in western North America. The conflicting claims led to the Oregon
Oregon
boundary dispute between Britain and the United States, giving rise to the slogan "Fifty-four forty or fight."

52°N In Canada, part of the border between Newfoundland and Labrador and Quebec
Quebec
.

51°N The southern limit of Russian America from 1799 to 1821.

49°N Much of the border between Canada
Canada
and the United States, from British Columbia to Manitoba; "49th parallel" is a common expression for the border, though the majority of Canada's population actually lives south of the parallel.

48°N In Canada, part of the border between Quebec
Quebec
and New Brunswick .

46°N In the United States, part of the border between Washington and Oregon
Oregon
.

45°N Part of the border between Canada
Canada
(Quebec) and the USA (New York and Vermont
Vermont
) is approximated by the parallel. Also, in the USA, it approximates most of the border between Montana
Montana
and Wyoming .

43°30\'N In the USA, the border between Minnesota and Iowa
Iowa
.

43°N In the USA, much of the border between South Dakota and Nebraska
Nebraska
.

42°30\'N In the USA, the border between Wisconsin
Wisconsin
and Illinois
Illinois
.

42°N Originally the northward limit of New Spain . In the USA, the southern borders of Oregon
Oregon
and Idaho
Idaho
where they meet the northern borders of California
California
, Nevada
Nevada
and Utah
Utah
. The parallel also defines much of the border between Pennsylvania
Pennsylvania
and New York .

41°N In the USA, part of the border between Wyoming and Utah, the border between Wyoming and Colorado
Colorado
, and part of the border between Nebraska and Colorado.

40°N In the USA, the border between Nebraska
Nebraska
and Kansas
Kansas
. The parallel was originally chosen for the Mason–Dixon line
Mason–Dixon line
, but the line was moved several miles south to avoid bisecting the city of Philadelphia .

38°N The boundary between the Soviet and American occupation zones in Korea, and later between North Korea
North Korea
and South Korea
South Korea
, from 1945 until the Korean War (1950–1953).

37°N In the USA, the southern border of Utah
Utah
with the northern border of Arizona
Arizona
. The southern border of Colorado
Colorado
with the northern borders of New Mexico and Oklahoma
Oklahoma
. The southern border of Kansas
Kansas
with the northern border of Oklahoma.

36°30\'N The historic Missouri Compromise
Missouri Compromise
line. In the USA, defines part of the border between Oklahoma
Oklahoma
and Texas
Texas
, most of the border between Missouri
Missouri
and Arkansas
Arkansas
. Geographically it is a Westward extension of the border between Virginia
Virginia
and North Carolina and part of the border between Kentucky
Kentucky
and Tennessee
Tennessee
.

36°N In the USA, a short section of the border between the Missouri Bootheel and Arkansas.

35°N In the USA, the southern border of Tennessee, which meets Mississippi
Mississippi
, Alabama
Alabama
and Georgia . Also, part of the border between North Carolina and Georgia.

33°N In the USA, the southern border of Arkansas, which meets the northern border of Louisiana
Louisiana
, is approximated by the parallel. Historically, it defined the southern border of the Louisiana Territory .

32°N In the USA, part of the border between New Mexico and Texas.

31°20\'N Part of the border between the USA and Mexico
Mexico
( Sonora
Sonora
and Chihuahua ); the southern border of Arizona
Arizona
and the New Mexico Bootheel .

31°N Part of the border between Iran
Iran
and Iraq
Iraq
. In the USA, part of the border between Mississippi
Mississippi
and Louisiana
Louisiana
, and part of the border between Alabama
Alabama
and Florida
Florida
.

28°N In Mexico, the border between Baja California
California
and Baja California Sur .

26°N Part of the border between Western Sahara
Western Sahara
(claimed by Morocco
Morocco
) and Mauritania
Mauritania
.

25°N Part of the border between Mauritania
Mauritania
and Mali
Mali
.

22°N Much of the border between Egypt
Egypt
and Sudan
Sudan
, partly disputed (see also Hala\'ib Triangle ).

20°N A short section of the border between Libya
Libya
and Sudan, and within Sudan, the northern border of the Darfur region.

17°N The division between Republic of Vietnam ( South Vietnam ) and Democratic Republic of Vietnam ( North Vietnam ) during the Vietnam War .

15°N de facto maritime border between Honduras
Honduras
and Nicaragua .

13°05\'N Part of the border between Chad
Chad
and Cameroon
Cameroon
, over a stretch of 41.6 km, partly in Lake Chad
Chad

10°N Part of the border between Guinea
Guinea
and Sierra Leone
Sierra Leone
.

8°N Part of the border between Somalia
Somalia
and Ethiopia
Ethiopia
.

1°N Part of the border between Equatorial Guinea
Guinea
and Gabon .

1°S Most of the border between Uganda
Uganda
and Tanzania
Tanzania
, and a very short section of the border between Kenya
Kenya
and Tanzania
Tanzania
in Lake Victoria .

7°S A short section of the border between Democratic Republic of the Congo and Angola
Angola
.

8°S Two short sections of the border between Democratic Republic of the Congo and Angola.

10°S A short section of the border between Brazil
Brazil
and Peru
Peru
.

13°S Part of the border between Angola
Angola
and Zambia .

16°S Part of the border between Mozambique
Mozambique
and Zimbabwe
Zimbabwe
.

22°S A short section of the border between Namibia
Namibia
and Botswana
Botswana
, and parts of the border between Bolivia
Bolivia
and Argentina
Argentina
.

26°S In Australia
Australia
, the border between South Australia
Australia
and the Northern Territory , and part of the border between South Australia
Australia
and Queensland
Queensland
.

28°S In Argentina, the border between Chaco Province
Chaco Province
and Santa Fe Province .

29°S In Australia, much of the border between Queensland
Queensland
and New South Wales .

35°S In Argentina, part of the border between Córdoba Province and La Pampa Province .

36°S In Argentina, part of the border between Mendoza Province and La Pampa Province, and part of the border between San Luis Province
San Luis Province
and La Pampa Province.

42°S In Argentina, the border between Río Negro Province and Chubut Province .

46°S In Argentina, the border between Chubut Province
Chubut Province
and Santa Cruz Province .

52°S Part of the border between Argentina
Argentina
and Chile
Chile
.

60°S The northern boundary of Antarctica
Antarctica
for the purposes of the Antarctic Treaty System (see map). The northern boundary of the Southern Ocean .

ALTITUDE

Note that the features of the spheroid cross-section (orange) in this image are exaggerated with respect to the Earth.

Altitude has an effect on a location's position relative to the plane formed by a circle of latitude. Since altitude is determined by the normal to the Earth's surface, locations sharing the same latitude—but having different elevations (e.g., lying along this normal)—no longer lie within this plane. Rather, all points sharing the same latitude and of varying elevation occupy a cone formed by the rotation of this normal around the Earth's axis.

SEE ALSO

* List of circles of latitude

REFERENCES

* ^ "\'\'Trópico en movimiento\'\' (in Spanish)". Groups.google.com. Retrieved 2014-05-13. * ^ "Basics of Space Flight, Chapter 2". Jet Propulsion Laboratory. Jet Propulsion Laboratory/NASA. 2013-10-29. Retrieved 2015-03-26. * ^ "Maritime Delimitation between Nicaragua and Honduras
Honduras
in the Caribbean Sea ( Nicaragua v. Honduras)" (PDF). Retrieved 2014-05-13.

EXTERNAL LINKS

Wikimedia Commons has media related to LATITUDES .

* U.S. Naval Observatory - mean