Prime meridian

"Prime Meridian" redirects here. For Earth's prime meridian, often called the Prime Meridian, see IERS Reference Meridian. For the historical meridian it is based on, see Prime meridian (Greenwich).

Line across the Earth
0°
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Modern IERS Reference Meridian on Earth

A prime meridian is an arbitrarily chosen meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great circle. This great circle divides a spheroid, like Earth, into two hemispheres: the Eastern Hemisphere and the Western Hemisphere (for an east-west notational system). For Earth's prime meridian, various conventions have been used or advocated in different regions throughout history.[1] Earth's current international standard prime meridian is the IERS Reference Meridian. It is derived, but differs slightly, from the Greenwich Meridian, the previous standard.[2]

Gerardus Mercator in his Atlas Cosmographicae (1595) used a prime meridian somewhere close to 25°W, passing just to the west of Santa Maria Island in the Azores in the Atlantic Ocean. His 180th meridian runs along the Strait of Anián (Bering Strait)

A prime meridian for a planetary body not tidally locked (or at least not in synchronous rotation) is entirely arbitrary, unlike an equator, which is determined by the axis of rotation. However, for celestial objects that are tidally locked (more specifically, synchronous), their prime meridians are determined by the face always inward of the orbit (a planet facing its star, or a moon facing its planet), just as equators are determined by rotation.

Longitudes for the Earth and Moon are measured from their prime meridian (at 0°) to 180° east and west. For all other Solar System bodies, longitude is measured from 0° (their prime meridian) to 360°. West longitudes are used if the rotation of the body is prograde (or 'direct', like Earth), meaning that its direction of rotation is the same as that of its orbit. East longitudes are used if the rotation is retrograde.[3]

  1. ^ Norgate & Norgate 2006.
  2. ^ "What is the Prime Meridian and why is it in Greenwich?". Royal Museums Greeenwich. Archived from the original on 13 December 2021. Retrieved 13 December 2021. The IRM is the only meridian that may now be described as the prime meridian of the world, as it defines 0° longitude by international agreement. The IRM passes 102.5 metres (112 yards) to the east of the historic Prime Meridian of the World at the latitude of the Airy Transit Circle here. The entire Observatory and the historic Prime Meridian now lie to the west of the true prime meridian.
  3. ^ Archimal, B. A. (2015), Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015 (PDF), p. 27 of 46, archived (PDF) from the original on 6 August 2019, retrieved 6 August 2019, [W]est longitudes are used when the rotation is direct, and east longitudes are used when the rotation is retrograde. ... The Earth, Sun, and Moon do not traditionally conform to this definition. Their rotations are direct and longitudes run both east and west 180°, or positive to the east 360°. [footnotes omitted]