======================
Geographic Coordinates
======================

         **Jason Harris**

         Locations on Earth can be specified using a spherical
         coordinate system. The geographic (“earth-mapping”) coordinate
         system 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 represents the *zeropoint* of the latitude angle,
         and the halfway point between the poles. The Equator is the
         *Fundamental Plane* of the geographic coordinate system. :doc:`All
         Spherical Coordinate Systems  <ai-skycoords>` 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 :doc:`Great Circle  <ai-greatcircle>` 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 circles, and Meridians are not parallel:
         they intersect at the north and south poles.

         .. tip::

            Exercise:

            What is the longitude of the North Pole? Its latitude is 90
            degrees North.

            This is a trick question. The Longitude is meaningless at
            the north pole (and the south pole too). It has all
            longitudes at the same time.