kstars/doc/timezones.docbook

0001 <sect1 id="ai-timezones">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>Time Zones</title>
0009 <indexterm><primary>Time Zones</primary>
0010 </indexterm>
0011 <para>
0012 The Earth is round, and it is always half-illuminated by the Sun.  However,
0013 because the Earth is spinning, the half that is illuminated is always changing.
0014 We experience this as the passing of days wherever we are on the Earth's
0015 surface.  At any given instant, there are places on the Earth passing from the
0016 dark half into the illuminated half (which is seen as <emphasis>dawn</emphasis>
0017 on the surface).  At the same instant, on the opposite side of the Earth, points
0018 are passing from the illuminated half into darkness (which is seen as
0019 <emphasis>dusk</emphasis> at those locations).  So, at any given time, different
0020 places on Earth are experiencing different parts of the day.  Thus, Solar time
0021 is defined locally, so that the clock time at any location describes the part of
0022 the day consistently.
0023 </para><para>
0024 This localization of time is accomplished by dividing the globe into 24 vertical
0025 slices called <firstterm>Time Zones</firstterm>.  The Local Time is the same
0026 within any given zone, but the time in each zone is one Hour
0027 <emphasis>earlier</emphasis> than the time in the neighboring Zone to the East.
0028 Actually, this is an idealized simplification; real Time Zone boundaries are not
0029 straight vertical lines, because they often follow national boundaries and other
0030 political considerations.
0031 </para><para>
0032 Note that because the Local Time always increases by an hour when moving between
0033 Zones to the East, by the time you move through all 24 Time Zones, you are a
0034 full day ahead of where you started.  We deal with this paradox by defining the
0035 <firstterm>International Date Line</firstterm>, which is a Time Zone boundary in
0036 the Pacific Ocean, between Asia and North America.  Points just to the East of
0037 this line are 24 hours behind the points just to the West of the line.  This
0038 leads to some interesting phenomena.  A direct flight from Australia to
0039 California arrives before it departs.  Also, the islands of Fiji straddle the
0040 International Date Line, so if you have a bad day on the West side of Fiji, you
0041 can go over to the East side of Fiji and have a chance to live the same day all
0042 over again.
0043 </para>
0044 </sect1>