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