<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>