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diff --git a/doc/kstars/leapyear.docbook b/doc/kstars/leapyear.docbook new file mode 100644 index 00000000..899115ae --- /dev/null +++ b/doc/kstars/leapyear.docbook @@ -0,0 +1,65 @@ +<sect1 id="ai-leapyear"> +<sect1info> +<author> +<firstname>Jason</firstname> +<surname>Harris</surname> +</author> +</sect1info> +<title>Leap Years</title> +<indexterm><primary>Leap Years</primary> +</indexterm> +<para> +The Earth has two major components of motion. First, it spins on its rotational +axis; a full spin rotation takes one <firstterm>Day</firstterm> to complete. +Second, it orbits around the Sun; a full orbital rotation takes one +<firstterm>Year</firstterm> to complete. +</para><para> +There are normally 365 days in one <emphasis>calendar</emphasis> year, but it +turns out that a <emphasis>true</emphasis> year (&ie;, a full orbit of the Earth +around the Sun; also called a <firstterm>tropical year</firstterm>) is a little +bit longer than 365 days. In other words, in the time it takes the Earth to +complete one orbital circuit, it completes 365.24219 spin rotations. Do not be +too surprised by this; there is no reason to expect the spin and orbital motions +of the Earth to be synchronized in any way. However, it does make marking +calendar time a bit awkward.... +</para><para> +What would happen if we simply ignored the extra 0.24219 rotation at the end of +the year, and simply defined a calendar year to always be 365.0 days long? The +calendar is basically a charting of the Earth's progress around the Sun. If we +ignore the extra bit at the end of each year, then with every passing year, the +calendar date lags a little more behind the true position of Earth around the +Sun. In just a few decades, the dates of the solstices and equinoxes will have +drifted noticeably. +</para><para> +In fact, it used to be that all years <emphasis>were</emphasis> defined to have +365.0 days, and the calendar <quote>drifted</quote> away from the true seasons +as a result. In the year 46 <abbrev>BCE</abbrev>, Julius Caeser established the +<firstterm>Julian Calendar</firstterm>, which implemented the world's first +<firstterm>leap years</firstterm>: He decreed that every 4th year would be 366 +days long, so that a year was 365.25 days long, on average. This basically +solved the calendar drift problem. +</para><para> +However, the problem wasn't completely solved by the Julian calendar, because a +tropical year isn't 365.25 days long; it's 365.24219 days long. You still have +a calendar drift problem, it just takes many centuries to become +noticeable. And so, in 1582, Pope Gregory XIII instituted the +<firstterm>Gregorian calendar</firstterm>, which was largely the same as the +Julian Calendar, with one more trick added for leap years: even Century years +(those ending with the digits <quote>00</quote>) are only leap years if they are divisible by +400. So, the years 1700, 1800, and 1900 were not leap years (though they would +have been under the Julian Calendar), whereas the year 2000 +<emphasis>was</emphasis> a leap year. This change makes the average length of a +year 365.2425 days. So, there is still a tiny calendar drift, but it amounts to +an error of only 3 days in 10,000 years. The Gregorian calendar is still used as +a standard calendar throughout most of the world. +</para> +<note> +<para> +Fun Trivia: When Pope Gregory instituted the Gregorian Calendar, the Julian +Calendar had been followed for over 1500 years, and so the calendar date had +already drifted by over a week. Pope Gregory re-synchronized the calendar by +simply <emphasis>eliminating</emphasis> 10 days: in 1582, the day after October +4th was October 15th! +</para> +</note> +</sect1> |