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diff --git a/doc/kstars/geocoords.docbook b/doc/kstars/geocoords.docbook new file mode 100644 index 00000000..5dd04a8c --- /dev/null +++ b/doc/kstars/geocoords.docbook @@ -0,0 +1,56 @@ +<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 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 +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> |