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<sect1 id="ai-leapyear">
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<firstname>Jason</firstname>
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<surname>Harris</surname>
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<title>Leap Years</title>
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<indexterm><primary>Leap Years</primary>
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The Earth has two major components of motion. First, it spins on its rotational
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axis; a full spin rotation takes one <firstterm>Day</firstterm> to complete.
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Second, it orbits around the Sun; a full orbital rotation takes one
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<firstterm>Year</firstterm> to complete.
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There are normally 365 days in one <emphasis>calendar</emphasis> year, but it
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turns out that a <emphasis>true</emphasis> year (&ie;, a full orbit of the Earth
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around the Sun; also called a <firstterm>tropical year</firstterm>) is a little
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bit longer than 365 days. In other words, in the time it takes the Earth to
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complete one orbital circuit, it completes 365.24219 spin rotations. Do not be
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too surprised by this; there is no reason to expect the spin and orbital motions
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of the Earth to be synchronized in any way. However, it does make marking
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calendar time a bit awkward....
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What would happen if we simply ignored the extra 0.24219 rotation at the end of
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the year, and simply defined a calendar year to always be 365.0 days long? The
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calendar is basically a charting of the Earth's progress around the Sun. If we
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ignore the extra bit at the end of each year, then with every passing year, the
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calendar date lags a little more behind the true position of Earth around the
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Sun. In just a few decades, the dates of the solstices and equinoxes will have
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In fact, it used to be that all years <emphasis>were</emphasis> defined to have
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365.0 days, and the calendar <quote>drifted</quote> away from the true seasons
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as a result. In the year 46 <abbrev>BCE</abbrev>, Julius Caeser established the
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<firstterm>Julian Calendar</firstterm>, which implemented the world's first
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<firstterm>leap years</firstterm>: He decreed that every 4th year would be 366
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days long, so that a year was 365.25 days long, on average. This basically
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solved the calendar drift problem.
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However, the problem wasn't completely solved by the Julian calendar, because a
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tropical year isn't 365.25 days long; it's 365.24219 days long. You still have
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a calendar drift problem, it just takes many centuries to become
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noticeable. And so, in 1582, Pope Gregory XIII instituted the
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<firstterm>Gregorian calendar</firstterm>, which was largely the same as the
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Julian Calendar, with one more trick added for leap years: even Century years
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(those ending with the digits <quote>00</quote>) are only leap years if they are divisible by
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400. So, the years 1700, 1800, and 1900 were not leap years (though they would
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have been under the Julian Calendar), whereas the year 2000
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<emphasis>was</emphasis> a leap year. This change makes the average length of a
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year 365.2425 days. So, there is still a tiny calendar drift, but it amounts to
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an error of only 3 days in 10,000 years. The Gregorian calendar is still used as
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a standard calendar throughout most of the world.
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Fun Trivia: When Pope Gregory instituted the Gregorian Calendar, the Julian
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Calendar had been followed for over 1500 years, and so the calendar date had
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already drifted by over a week. Pope Gregory re-synchronized the calendar by
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simply <emphasis>eliminating</emphasis> 10 days: in 1582, the day after October
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removed
doc/leapyear.docbook
