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A circle of latitude on the Earth is an imaginary east-west circle connecting all locations (ignoring elevation) with a given latitude. A location's position along a circle of latitude is given by its longitude. 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. Unlike ''circles of longitude'', which all are great circles with the centre of Earth in the middle, 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. The 60th circle of latitude is half as long as the equator (disregarding Earth’s minor ovality, which is less than 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 the Earth's centre. The equator is at 0°, and the North 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. 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 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 1888 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. ==Major circles of latitude== There are five major circles of latitude, listed below from north to south. The position of the Equator is fixed (90 degrees from the Earth's axis of rotation) but the latitudes of the other circles depend on the tilt of this axis relative to the plane of the Earth's orbit, and so are not perfectly fixed. The values below are for : *Arctic Circle ( N) *Tropic of Cancer ( N) *Equator (0° latitude) *Tropic of Capricorn ( S) *Antarctic Circle ( S) These circles of latitude, excluding the Equator, mark the divisions between the five principal geographical zones. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Circle of latitude」の詳細全文を読む スポンサード リンク
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