<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>
