The geographic (earth-mapping) coordinate system is a spherical coordinate system which is aligned with the spin axis of the Earth. It defines two angles measured from the center of the Earth. One angle, called the latitude, measures the angle between any point and the equator. The other angle, called the longitude, measures the angle along the Equator from an arbitrary point on the Earth. Greenwich, England is the accepted zero-longitude point in most modern societies.
By combining these two angles, any location on Earth can be specified. For example, Baltimore, Maryland (USA) has a latitude of 39.3 degrees North, and a longitude of 76.6 degrees West. So, a vector drawn from the center of the Earth to a point 39.3 degrees above the equator and 76.6 degrees west of Greenwich, England will pass through Baltimore.
The equator is obviously an important part of this coordinate system, it representing the zeropoint of the latitude angle, and the halfway point between the poles. The equator is the fundamental plane of the geographic coordinate system. All spherical coordinate systems define such a fundamental plane.
Lines of constant latitude are called parallels. They trace circles on the surface of the Earth, but the only parallel that is a great circle is the equator (latitude=0 degrees). Lines of constant longitude are called meridians. The meridian passing through Greenwich is the Prime Meridian (longitude=0 degrees). Unlike parallels, all meridians are great cricles, and meridians are not parallel: they intersect at the north and south poles.
This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml
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