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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>