• Position of an object on a sphere east or west of some arbitrary meridian line – in the case of Earth, one running through Greenwich in London, England. On the celestial sphere longitude is called Right Ascension. Other objects in the solar system have also had longitude systems defined in recent years – Mercury’s answer to Greenwich is a crater called Hun Kal, and for Venus a point called Alpha Regio.
  • Imaginary half-circles that are drawn parallel to the prime meridian, or another reference meridian, and which serve to measure angular displacement from such a meridian. The prime meridian has an angle of 0°, while those to the east work their way towards 180 °E, and those to the west work their way towards 180 °W. These half-circles cross the parallels of latitude at right angles, and together serve to define locations on the planet. Its abbreviation is long.

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