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0001 <sect1 id="ai-utime">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>Universal Time</title>
0009 <indexterm><primary>Universal Time</primary>
0010 <seealso>Time Zones</seealso>
0011 </indexterm>
0012 <para>
0013 The time on our clocks is essentially a measurement of the current position of
0014 the Sun in the sky, which is different for places at different Longitudes
0015 because the Earth is round (see <link linkend="ai-timezones">Time Zones</link>).
0016 </para><para>
0017 However, it is sometimes necessary to define a global time, one that is the same
0018 for all places on Earth.  One way to do this is to pick a place on the Earth,
0019 and adopt the Local Time at that place as the <firstterm>Universal
0020 Time</firstterm>, abbreviated <abbrev>UT</abbrev>.  (The name is a bit of a
0021 misnomer, since Universal Time has little to do with the Universe.  It would
0022 perhaps be better to think of it as <emphasis>global time</emphasis>).
0023 </para><para>
0024 The geographic location chosen to represent Universal Time is Greenwich,
0025 England.  The choice is arbitrary and historical.  Universal Time became an
0026 important concept when European ships began to sail the wide open seas, far from
0027 any known landmarks.  A navigator could reckon the ship's longitude by comparing
0028 the Local Time (as measured from the Sun's position) to the time back at the
0029 home port (as kept by an accurate clock on board the ship).  Greenwich was home
0030 to England's Royal Observatory, which was charged with keeping time
0031 very accurately, so that ships in port could re-calibrate their clocks before
0032 setting sail.
0033 </para>
0034 <tip>
0035 <para>Exercise:</para>
0036 <para>
0037 Set the geographic location to <quote>Greenwich, England</quote> using the
0038 <guilabel>Set Location</guilabel> window
0039 (<keycombo action="simul">&Ctrl;<keycap>G</keycap></keycombo>).  Note that the
0040 Local Time (<abbrev>LT</abbrev>) and the Universal Time (<abbrev>UT</abbrev>) are
0041 now the same.
0042 </para><para>
0043 Further Reading: The history behind the construction of the first clock
0044 that was accurate and stable enough to be used on ships to keep Universal Time
0045 is a fascinating tale, and one told expertly in the book <quote>Longitude</quote>,
0046 by Dava Sobel.
0047 </para>
0048 </tip>
0049 </sect1>