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Parallel A line of constant latitude, special ones are the equator, which separates north from south latitudes, the tropics (at 23.45° North and South) that mark the limits of the sun's north and south excursions, and the polar circles (at 90° - 23.45° North and South) that mark the latitudes above which it is possible for the sun to stay above or below the horizon for more than 24 hours. In this usage, a "parallel" of latitude is analogous to a "meridian" of longitude. Parallel can, of course, also refer geometrically to two lines in a plane that never intersect, or two planes in space that never touch.

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