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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>