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45 lines
2.3 KiB
45 lines
2.3 KiB
15 years ago
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<sect1 id="ai-timezones">
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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>Time Zones</title>
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<indexterm><primary>Time Zones</primary>
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</indexterm>
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<para>
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The Earth is round, and it is always half-illuminated by the Sun. However,
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because the Earth is spinning, the half that is illuminated is always changing.
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We experience this as the passing of days wherever we are on the Earth's
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surface. At any given instant, there are places on the Earth passing from the
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dark half into the illuminated half (which is seen as <emphasis>dawn</emphasis>
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on the surface). At the same instant, on the opposite side of the Earth, points
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are passing from the illuminated half into darkness (which is seen as
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<emphasis>dusk</emphasis> at those locations). So, at any given time, different
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places on Earth are experiencing different parts of the day. Thus, Solar time
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is defined locally, so that the clock time at any location describes the part of
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the day consistently.
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</para><para>
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This localization of time is accomplished by dividing the globe into 24 vertical
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slices called <firstterm>Time Zones</firstterm>. The Local Time is the same
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within any given zone, but the time in each zone is one Hour
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<emphasis>earlier</emphasis> than the time in the neighboring Zone to the East.
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Actually, this is a idealized simplification; real Time Zone boundaries are not
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straight vertical lines, because they often follow national boundaries and other
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political considerations.
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</para><para>
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Note that because the Local Time always increases by an hour when moving between
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Zones to the East, by the time you move through all 24 Time Zones, you are a
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full day ahead of where you started. We deal with this paradox by defining the
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<firstterm>International Date Line</firstterm>, which is a Time Zone boundary in
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the Pacific Ocean, between Asia and North America. Points just to the East of
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this line are 24 hours behind the points just to the West of the line. This
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leads to some interesting phenomena. A direct flight from Australia to
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California arrives before it departs. Also, the islands of Fiji straddle the
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International Date Line, so if you have a bad day on the West side of Fiji, you
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can go over to the East side of Fiji and have a chance to live the same day all
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over again.
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</para>
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</sect1>
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