<sect1 id="ai-geocoords">
<sect1info>
<author>
<firstname>Jason</firstname>
<surname>Harris</surname>
</author>
</sect1info>
<title>Geographic Coordinates</title>
<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 center 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 center 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
representing 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 cricles, 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?  It's latitude
is 90 degrees North.
</para>
<para>
It 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>