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0001 <sect1 id="ai-leapyear">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>Leap Years</title>
0009 <indexterm><primary>Leap Years</primary>
0010 </indexterm>
0011 <para>
0012 The Earth has two major components of motion.  First, it spins on its rotational
0013 axis; a full spin rotation takes one <firstterm>Day</firstterm> to complete.
0014 Second, it orbits around the Sun; a full orbital rotation takes one
0015 <firstterm>Year</firstterm> to complete.
0016 </para><para>
0017 There are normally 365 days in one <emphasis>calendar</emphasis> year, but it
0018 turns out that a <emphasis>true</emphasis> year (&ie;, a full orbit of the Earth
0019 around the Sun; also called a <firstterm>tropical year</firstterm>) is a little
0020 bit longer than 365 days. In other words, in the time it takes the Earth to
0021 complete one orbital circuit, it completes 365.24219 spin rotations.  Do not be
0022 too surprised by this; there is no reason to expect the spin and orbital motions
0023 of the Earth to be synchronized in any way. However, it does make marking
0024 calendar time a bit awkward...
0025 </para><para>
0026 What would happen if we simply ignored the extra 0.24219 rotation at the end of
0027 the year, and simply defined a calendar year to always be 365.0 days long?  The
0028 calendar is basically a charting of the Earth's progress around the Sun.  If we
0029 ignore the extra bit at the end of each year, then with every passing year, the
0030 calendar date lags a little more behind the true position of Earth around the
0031 Sun.  In just a few decades, the dates of the solstices and equinoxes will have
0032 drifted noticeably.
0033 </para><para>
0034 In fact, it used to be that all years <emphasis>were</emphasis> defined to have
0035 365.0 days, and the calendar <quote>drifted</quote> away from the true seasons
0036 as a result. In the year 46 <abbrev>BCE</abbrev>, Julius Caeser established the
0037 <firstterm>Julian Calendar</firstterm>, which implemented the world's first
0038 <firstterm>leap years</firstterm>: He decreed that every 4th year would be 366
0039 days long, so that a year was 365.25 days long, on average.  This basically
0040 solved the calendar drift problem.
0041 </para><para>
0042 However, the problem wasn't completely solved by the Julian calendar, because a
0043 tropical year isn't 365.25 days long; it's 365.24219 days long.  You still have
0044 a calendar drift problem, it just takes many centuries to become
0045 noticeable.  And so, in 1582, Pope Gregory XIII instituted the
0046 <firstterm>Gregorian calendar</firstterm>, which was largely the same as the
0047 Julian Calendar, with one more trick added for leap years: even Century years
0048 (those ending with the digits <quote>00</quote>) are only leap years if they are divisible by
0049 400. So, the years 1700, 1800, and 1900 were not leap years (though they would
0050 have been under the Julian Calendar), whereas the year 2000
0051 <emphasis>was</emphasis> a leap year. This change makes the average length of a
0052 year 365.2425 days.  So, there is still a tiny calendar drift, but it amounts to
0053 an error of only 3 days in 10,000 years. The Gregorian calendar is still used as
0054 a standard calendar throughout most of the world.
0055 </para>
0056 <note>
0057 <para>
0058 Fun Trivia:  When Pope Gregory instituted the Gregorian Calendar, the Julian
0059 Calendar had been followed for over 1500 years, and so the calendar date had
0060 already drifted by over a week.  Pope Gregory re-synchronized the calendar by
0061 simply <emphasis>eliminating</emphasis> 10 days:  in 1582, the day after October
0062 4th was October 15th!
0063 </para>
0064 </note>
0065 </sect1>