<para>Locations on Earth can be specified using a spherical coordinate system. The geographic (<quote>earth-mapping</quote>) coordinate system is aligned with the spin axis of the Earth. It defines two angles measured from the centre of the Earth. One angle, called the <firstterm>Latitude</firstterm>, measures the angle between any point and the Equator. The other angle, called the <firstterm>Longitude</firstterm>, measures the angle <emphasis>along</emphasis> the Equator from an arbitrary point on the Earth (Greenwich, England is the accepted zero-longitude point in most modern societies). </para><para>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 centre of the Earth to a point 39.3 degrees above the Equator and 76.6 degrees west of Greenwich, England will pass through Baltimore. </para><para>The Equator is obviously an important part of this coordinate system; it represents the <emphasis>zeropoint</emphasis> of the latitude angle, and the halfway point between the poles. The Equator is the <firstterm>Fundamental Plane</firstterm> of the geographic coordinate system. <link linkend="ai-skycoords">All Spherical Coordinate Systems</link> define such a Fundamental Plane. </para><para>Lines of constant Latitude are called <firstterm>Parallels</firstterm>. They trace circles on the surface of the Earth, but the only parallel that is a <link linkend="ai-greatcircle">Great Circle</link> is the Equator (Latitude=0 degrees). Lines of constant Longitude are called <firstterm>Meridians</firstterm>. The Meridian passing through Greenwich is the <firstterm>Prime Meridian</firstterm> (longitude=0 degrees). Unlike Parallels, all Meridians are great circles, and Meridians are not parallel: they intersect at the north and south poles. </para>
<para>What is the longitude of the North Pole? Its latitude is 90 degrees North. </para>
<para>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. </para>
