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+<sect1 id="ai-utime">
+<sect1info>
+<author>
+<firstname>Jason</firstname>
+<surname>Harris</surname>
+</author>
+</sect1info>
+<title>Universal Time</title>
+<indexterm><primary>Universal Time</primary>
+<seealso>Time Zones</seealso>
+</indexterm>
+<para>
+The time on our clocks is essentially a measurement of the current position of
+the Sun in the sky, which is different for places at different Longitudes
+because the Earth is round (see <link linkend="ai-timezones">Time Zones</link>).
+</para><para>
+However, it is sometimes necessary to define a global time, one that is the same
+for all places on Earth. One way to do this is to pick a place on the Earth,
+and adopt the Local Time at that place as the <firstterm>Universal
+Time</firstterm>, abbreviated <abbrev>UT</abbrev>. (The name is a bit of a
+misnomer, since Universal Time has little to do with the Universe. It would
+perhaps be better to think of it as <emphasis>global time</emphasis>).
+</para><para>
+The geographic location chosen to represent Universal Time is Greenwich,
+England. The choice is arbitrary and historical. Universal Time became an
+important concept when European ships began to sail the wide open seas, far from
+any known landmarks. A navigator could reckon the ship's longitude by comparing
+the Local Time (as measured from the Sun's position) to the time back at the
+home port (as kept by an accurate clock on board the ship). Greenwich was home
+to England's Royal Observatory, which was charged with keeping time
+very accurately, so that ships in port could re-calibrate their clocks before
+setting sail.
+</para>
+<tip>
+<para>Exercise:</para>
+<para>
+Set the geographic location to <quote>Greenwich, England</quote> using the
+<guilabel>Set Location</guilabel> window
+(<keycombo action="simul">&Ctrl;<keycap>G</keycap></keycombo>). Note that the
+Local Time (<abbrev>LT</abbrev>)and the Universal Time (<abbrev>UT</abbrev>) are
+now the same.
+</para><para>
+Further Reading: The history behind the construction of the first clock
+that was accurate and stable enough to be used on ships to keep Universal Time
+is a fascinating tale, and one told expertly in the book
+<quote>Longitude</quote>, by Dava Sobel.
+</para>
+</tip>
+</sect1>