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0001 <sect1 id="ai-geocoords"> 0002 <sect1info> 0003 <author> 0004 <firstname>Jason</firstname> 0005 <surname>Harris</surname> 0006 </author> 0007 </sect1info> 0008 <title>Geographic Coordinates</title> 0009 <indexterm><primary>Geographic Coordinate System</primary></indexterm> 0010 <indexterm><primary>Longitude</primary><see>Geographic Coordinate System</see></indexterm> 0011 <indexterm><primary>Latitude</primary><see>Geographic Coordinate System</see></indexterm> 0012 <para> 0013 Locations on Earth can be specified using a spherical coordinate system. 0014 The geographic (<quote>earth-mapping</quote>) coordinate system is aligned 0015 with the spin axis of the Earth. It defines two angles measured from 0016 the center of the Earth. One angle, called the <firstterm>Latitude</firstterm>, 0017 measures the angle between any point and the Equator. The other angle, called 0018 the <firstterm>Longitude</firstterm>, measures the angle 0019 <emphasis>along</emphasis> the Equator from an arbitrary point on the Earth 0020 (Greenwich, England is the accepted zero-longitude point in most modern 0021 societies). 0022 </para><para> 0023 By combining these two angles, any location on Earth can be specified. 0024 For example, Baltimore, Maryland (USA) has a latitude of 39.3 degrees 0025 North, and a longitude of 76.6 degrees West. So, a vector drawn from 0026 the center of the Earth to a point 39.3 degrees above the Equator and 0027 76.6 degrees west of Greenwich, England will pass through Baltimore. 0028 </para><para> 0029 The Equator is obviously an important part of this coordinate system; it 0030 represents the <emphasis>zeropoint</emphasis> of the latitude angle, and the 0031 halfway point between the poles. The Equator is the <firstterm>Fundamental 0032 Plane</firstterm> of the geographic coordinate system. <link 0033 linkend="ai-skycoords">All Spherical Coordinate Systems</link> define such a 0034 Fundamental Plane. 0035 </para><para> 0036 Lines of constant Latitude are called <firstterm>Parallels</firstterm>. They 0037 trace circles on the surface of the Earth, but the only parallel that is a <link 0038 linkend="ai-greatcircle">Great Circle</link> is the Equator (Latitude=0 0039 degrees). Lines of constant Longitude are called 0040 <firstterm>Meridians</firstterm>. The Meridian passing through Greenwich is the 0041 <firstterm>Prime Meridian</firstterm> (longitude=0 degrees). Unlike Parallels, 0042 all Meridians are great circles, and Meridians are not parallel: they intersect 0043 at the north and south poles. 0044 </para> 0045 <tip> 0046 <para>Exercise:</para> 0047 <para> 0048 What is the longitude of the North Pole? Its latitude 0049 is 90 degrees North. 0050 </para> 0051 <para> 0052 This is a trick question. The Longitude is meaningless at the north pole (and the 0053 south pole too). It has all longitudes at the same time. 0054 </para> 0055 </tip> 0056 </sect1>