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33 lines
1.5 KiB
Markdown
33 lines
1.5 KiB
Markdown
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# Julian Calendar
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## What is this?
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The [Julian calendar][0] is a calendar introduced by Julius Caesar in 46 BC. A
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reformed version, the [Gregorian calendar][1], forms the basis of civil calendar
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in most countries.
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Julius Caesar (with aid of Sosigenes of Alexandria) introduced this calendar to
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replace the earlier Roman calendar, which required manual addition of leap
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months to keep it in synchrony with the seasons. Unfortunately, it had leap
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days every four years without question, resulting in the average year having
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365.25 days. However, the actual tropical year is roughly 365.2422 days,
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resulting in the Julian calendar gaining a day every 129 years. This means
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that seasons drift, starting earlier and earlier in the year.
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In 1582, Pope Gregory XIII decided to fix this drift by reducing the number
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of leap days, motivated by a desire to keep the March equinox on March 21st,
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since that value was hardcoded in the calculation for the date of Easter. To
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achieve this, he made years divisible by 100 but not by 400 non-leap years,
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resulting in 97 leap years every 400 years, resulting in the Gregorian calendar.
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To bring the equinox back in alignment, October 5th to October 14th in 1582
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were deleted. The result is the Gregorian calendar.
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This website extends the Julian calendar indefinitely into the future for
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reference reasons. Note that in the 20th and 21st centuries, the Julian
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calendar is 13 days behind the Gregorian calendar.
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[0]: https://en.wikipedia.org/wiki/Julian_calendar
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[1]: https://en.wikipedia.org/wiki/Gregorian_calendar
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