This article needs additional citations for verification .(April 2012)
|Lines of longitude appear vertical with varying curvature in this projection, but are actually halves of great ellipses, with identical radii at a given latitude.|
|Lines of latitude appear horizontal with varying curvature in this projection; but are actually circular with different radii. All locations with a given latitude are collectively referred to as a circle of latitude.|
|The equator divides the planet into a Northern Hemisphere and a Southern Hemisphere, and has a latitude of 0°.|
A circle of latitude or line of latitude on Earth is an abstract east–west small circle connecting all locations around Earth (ignoring elevation) at a given latitude coordinate line.
Circles of latitude are often called parallels because they are parallel to each other; that is, planes that contain any of these circles never intersect each other. A location's position along a circle of latitude is given by its longitude. Circles of latitude are unlike circles of longitude, which are all great circles with the centre of Earth in the middle, as the circles of latitude get smaller as the distance from the Equator increases. Their length can be calculated by a common sine or cosine function. For example, the 60th parallel north or south is half as long as the Equator (disregarding Earth's minor flattening by 0.335%), stemming from . On the Mercator projection or on the Gall-Peters projection, a circle of latitude is perpendicular to all meridians.  On the ellipsoid or on spherical projection, all circles of latitude are rhumb lines, except the Equator.
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 Pole and South Pole are at 90° north and 90° south, respectively. The Equator is the longest circle of latitude and is the only circle of latitude which also is a great circle. As such, it is perpendicular to all meridians.
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).
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 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 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 is at 41° N while the southern border is at 37° N. Roughly half the length of border between the United States and Canada follows 49° N.
There are five major circles of latitude, listed below from north to south. The position of the 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 21 February 2023: 
These circles of latitude, excluding the Equator, mark the divisions between the five principal geographical zones.
The equator is the circle that is equidistant from the North Pole and South Pole. It divides the 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.
The 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 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).
The latitude of the polar circles is equal to 90° minus the Earth's axial tilt.
The Tropic of Cancer and 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.
By definition, the positions of the Tropic of Cancer, Tropic of Capricorn, Arctic Circle and 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 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. At noon of January 1st 2000 AD, the mean value of the tilt was 23° 26′ 21.406″ (according to IAU 2006, theory P03).
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 0.468″ 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 m per year, and the area of the Tropics is decreasing by 1,100 km2 (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 m 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 m) 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.
These circles of latitude can be defined on other planets with axial inclinations relative to their orbital planes. Objects such as Pluto with tilt angles greater than 45 degrees will have the tropic circles closer to the poles and the polar circles closer to the equator.
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 metres) by sighting the North Star.
|81°N||In Svalbard, Norway, the northern and southern limits of the area comprised by the Svalbard Treaty of 9 February 1920.|
|70°N||On Victoria Island, 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 and Saskatchewan; and the southern border of mainland Nunavut with the northern border of 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 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.|
|51°N||The southern limit of Russian America from 1799 to 1821.|
|49°N||Much of the border between 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 and New Brunswick.|
|46°N||In the United States, part of the border between Washington and Oregon.|
|45°N||Approximates the portion of the Canada–United States border between Quebec (Canada) and New York and Vermont (US). Also approximates most of the border between Montana and Wyoming.|
|43°30'N||In the US, the border between Minnesota and Iowa.|
|43°N||In the US, much of the border between South Dakota and Nebraska.|
|42°30'N||In the US, the border between Wisconsin and Illinois.|
|42°N||Originally the northward limit of New Spain. In the US, the southern borders of Oregon and Idaho where they meet the northern borders of California, Nevada and Utah. The parallel also defines much of the border between Pennsylvania and New York.|
|41°N||In the US, part of the border between Wyoming and Utah, the border between Wyoming and Colorado, and part of the border between Nebraska and Colorado.|
|40°N||In the US, the border between Nebraska and Kansas. The parallel was originally chosen for the 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 and South Korea, from 1945 until the Korean War (1950–1953).|
|37°N||In the US, the southern border of Utah with the northern border of Arizona. The southern border of Colorado with the northern borders of New Mexico and Oklahoma. The southern border of Kansas with the northern border of Oklahoma.|
|36°30'N||The historic Missouri Compromise line. In the US, defines part of the border between Oklahoma and Texas, most of the border between Missouri and Arkansas. Geographically it is a Westward extension of the border between Virginia and North Carolina and part of the border between Kentucky and Tennessee.|
|36°N||In the US, a short section of the border between the Missouri Bootheel and Arkansas.|
|35°N||In the US, the southern border of Tennessee, which meets Mississippi, Alabama and Georgia. Also, part of the border between North Carolina and Georgia.|
|33°N||In the US, the southern border of Arkansas, which meets the northern border of Louisiana, is approximated by the parallel. Historically, it defined the southern border of the Louisiana Territory.|
|32°N||In the US, part of the border between New Mexico and Texas.|
|31°20'N||Part of the border between the US and Mexico (Sonora and Chihuahua); the southern border of Arizona and the New Mexico Bootheel.|
|31°N||Part of the border between Iran and Iraq. In the US, part of the border between Mississippi and Louisiana, and part of the border between Alabama and Florida.|
|28°N||In Mexico, the border between Baja California and Baja California Sur.|
|26°N||Part of the border between Western Sahara (claimed by Morocco) and Mauritania.|
|25°N||Part of the border between Mauritania and Mali.|
|22°N||Much of the border between Egypt and Sudan, partly disputed (see also Hala'ib Triangle).|
|20°N||A short section of the border between 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 and Nicaragua. |
|13°05'N||Part of the border between Chad and Cameroon, over a stretch of 41.6 km, partly in Lake Chad|
|10°N||Part of the border between Guinea and Sierra Leone.|
|8°N||Part of the border between Somalia and Ethiopia.|
|1°N||Part of the border between Equatorial Guinea and Gabon.|
|1°S||Most of the border between Uganda and Tanzania, and a very short section of the border between Kenya and Tanzania in Lake Victoria.|
|7°S||A short section of the border between Democratic Republic of the Congo and 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 and Peru.|
|13°S||Part of the border between Angola and Zambia.|
|16°S||Part of the border between Mozambique and Zimbabwe.|
|22°S||A short section of the border between Namibia and Botswana, and parts of the border between Bolivia and Argentina.|
|26°S||In Australia, the border between South Australia and the Northern Territory, and part of the border between South Australia and Queensland.|
|28°S||In Argentina, the border between Chaco Province and Santa Fe Province.|
|29°S||In Australia, much of the border between 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 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 and Santa Cruz Province.|
|52°S||Part of the border between Argentina and Chile.|
|60°S||The northern boundary of Antarctica for the purposes of the Antarctic Treaty System (see map). The northern boundary of the Southern Ocean.|
Normally the circles of latitude are defined at zero elevation. Elevation has an effect on a location with respect to the plane formed by a circle of latitude. Since (in the geodetic system) altitude and depth are determined by the normal to the Earth's surface, locations sharing the same latitude—but having different elevations (i.e., lying along this normal)—no longer lie within this plane. Rather, all points sharing the same latitude—but of varying elevation and longitude—occupy the surface of a truncated cone formed by the rotation of this normal around the Earth's axis of rotation.
In geography, latitude is a coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pole, with 0° at the Equator. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude and longitude are used together as a coordinate pair to specify a location on the surface of the Earth.
The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's North Pole.
A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In many countries, the seasons of the year are determined by the solstices and the equinoxes.
The Tropic of Cancer, which is also referred to as the Northern Tropic, is the most northerly circle of latitude on Earth at which the Sun can be directly overhead. This occurs on the June solstice, when the Northern Hemisphere is tilted toward the Sun to its maximum extent. It also reaches 90 degrees below the horizon at solar midnight on the December Solstice. Using a continuously updated formula, the circle is currently 23°26′10.6″ (or 23.43628°) north of the Equator.
The Tropic of Capricorn is the circle of latitude that contains the subsolar point at the December solstice. It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reaches 90 degrees below the horizon at solar midnight on the June Solstice. Its northern equivalent is the Tropic of Cancer.
A polar circle is a geographic term for a conditional circular line (arc) referring either to the Arctic Circle or the Antarctic Circle. These are two of the keynote circles of latitude (parallels). On Earth, the Arctic Circle is currently drifting northwards at a speed of about 14.5 m per year and is now at a mean latitude of 66°33′49.3″ N; the Antarctic Circle is currently drifting southwards at a speed of about 14.5 m per year and is now at a mean latitude of 66°33′49.3″ S. Polar circles are often equated with polar regions of Earth. Due to their inherent climate environment, the bulk of the Arctic Circle, much of which is sea, is sparsely settled whereas this applies to all of Antarctica which is mainly land and sheltered ice shelves.
The ascendant is the astrological sign on the eastern horizon when the person was born.
Midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight. When midnight sun is seen in the Arctic, the Sun appears to move from left to right, but in Antarctica the equivalent apparent motion is from right to left. This occurs at latitudes from 65°44' to 90° north or south, and does not stop exactly at the Arctic Circle or the Antarctic Circle, due to refraction.
The polar regions, also called the frigid zones or polar zones, of Earth are the regions of the planet that surround its geographical poles, lying within the polar circles. These high latitudes are dominated by floating sea ice covering much of the Arctic Ocean in the north, and by the Antarctic ice sheet on the continent of Antarctica and the Southern Ocean in the south.
Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this illumination occurs.
The middle latitudes are a spatial region on Earth located between the Tropic of Cancer to the Arctic Circle (66°33'39"), and Tropic of Capricorn (-23°26'22") to the Antarctic Circle (-66°33'39"). They include Earth's subtropical and temperate zones, which lie between the two tropics and the polar circles. Weather fronts and extratropical cyclones are usually found in this area, as well as occasional tropical cyclones or subtropical cyclones, which have traveled from their areas of formation closer to the Equator.
An orbital pole is either point at the ends of an imaginary line segment that runs through the center of an orbit and is perpendicular to the orbital plane. Projected onto the celestial sphere, orbital poles are similar in concept to celestial poles, but are based on the body's orbit instead of its equator.
The 30th parallel north is a circle of latitude that is 30 degrees north of the Earth's equatorial plane. It stands one-third of the way between the equator and the North Pole and crosses Africa, Asia, the Pacific Ocean, North America and the Atlantic Ocean. The parallel is used in some contexts to delineate Europe or what is associated with the continent of Europe as a southernmost limit, e.g. to qualify for membership of the European Broadcasting Union.
The five main latitude regions of Earth's surface comprise geographical zones, divided by the major circles of latitude. The differences between them relate to climate. They are as follows:
Daytime as observed on Earth is the period of the day during which a given location experiences natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemisphere facing the Sun. In direct sunlight the movement of the sun can be recorded and observed using a sundial that casts a shadow that slowly moves during the day. Other planets and natural satellites that rotate relative to a luminous primary body, such as a local star, also experience daytime, but this article primarily discusses daytime on Earth.
The summer solstice, also called the estival solstice or midsummer, occurs when one of Earth's poles has its maximum tilt toward the Sun. It happens twice yearly, once in each hemisphere. For that hemisphere, the summer solstice is the day with the longest period of daylight and shortest night of the year, when the Sun is at its highest position in the sky. Within the Arctic circle or Antarctic circle, there is continuous daylight around the summer solstice. The opposite event is the winter solstice.
Sun path, sometimes also called day arc, refers to the daily and seasonal arc-like path that the Sun appears to follow across the sky as the Earth rotates and orbits the Sun. The Sun's path affects the length of daytime experienced and amount of daylight received along a certain latitude during a given season.
The equator is a circle of latitude that divides a spheroid, such as Earth, into the northern and southern hemispheres. On Earth, it is an imaginary line located at 0 degrees latitude, about 40,075 km (24,901 mi) in circumference, halfway between the North and South poles. The term can also be used for any other celestial body that is roughly spherical.
The Antarctic Circle is the most southerly of the five major circles of latitude that mark maps of Earth. The region south of this circle is known as the Antarctic, and the zone immediately to the north is called the Southern Temperate Zone. South of the Antarctic Circle, the Sun is above the horizon for 24 continuous hours at least once per year and the centre of the Sun is below the horizon for 24 continuous hours at least once per year ; this is also true within the equivalent polar circle in the Northern Hemisphere, the Arctic Circle.
The Arctic Circle is one of the two polar circles, and the most northerly of the five major circles of latitude as shown on maps of Earth. Its southern equivalent is the Antarctic Circle.