Circle of latitude
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|Lines of longitude appear |
|Lines of latitude appear |
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
The latitude of the circle is approximately the
There are 89
On a map, the circles of latitude may or may not be parallel, and their spacing may vary, depending on which
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
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 24 September 2018:
These circles of latitude, excluding the Equator, mark the divisions between the five principal
The equator is the circle that is equidistant from the
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
The latitude of the polar circles is equal to 90° less the Earth's
The latitude of the tropical circles is equal to the Earth's axial tilt.
By definition, the positions of the
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
The Earth's axial tilt has additional shorter-term variations due to
Finally, the Earth's rotational axis is not exactly fixed in the Earth, but undergoes small fluctuations (on the order of 15 meters) called
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