A circle of latitude is an imaginary east-west circle on the Earth, that connects all locations with a given latitude. It is perpendicular to all meridians. The position on the circle of latitude is given by the longitude.
Circle of latitude are based on the rotation of the Earth. Four special ones are based on the relationship with the Earth's orbit around the Sun.
The five major circles of latitude are:
The Arctic Circle and Antarctic circle represent the southernmost and northernmost locations where it is possible to have a day without a sunrise.
The Tropic of Cancer and Tropic of Capricorn represent the northernmost and southernmost locations where the sun may be seen directly overhead (at midsummer and midwinter respectively)
The circles of latitude are loxodromes, but, apart from the equator, they are not great circles, hence not the shortest distance between points, as opposed to what is suggested by maps that show them as straight lines. It is for this reason that an aeroplane travelling between a European and North American city on the same latitude will fly further north, over Greenland for example. Thus they are not really "lines" in the geometry of the sphere. See also great circle distance.
A circle of latitude is often called a "parallel", because circles of latitude are a fixed distance apart and on some map projections, including the Mercator projection, they are parallel.
For a low latitude a circle of latitude can be said to be a line around the Earth, while at a high latitude it is a circle around a pole.
Circles of latitude are often used as boundaries between countries or regions. Notable parallels include: