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+<sect1 id="ai-geocoords">
+<sect1info>
+<author
+><firstname
+>Jason</firstname
+> <surname
+>Harris</surname
+> </author>
+</sect1info>
+<title
+>Geographic Coordinates</title>
+<indexterm
+><primary
+>Geographic Coordinate System</primary
+></indexterm>
+<indexterm
+><primary
+>Longitude</primary
+><see
+>Geographic Coordinate System</see
+></indexterm>
+<indexterm
+><primary
+>Latitude</primary
+><see
+>Geographic Coordinate System</see
+></indexterm>
+<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>
+<tip>
+<para
+>Exercise:</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>
+</tip>
+</sect1>