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Julian and Gregorian Calendars (view source)
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The official | The official [http://astro.nmsu.edu/~lhuber/leaphist.html calendar] used in most of the modern world today is the Gregorian Calendar, named after Pope Gregory. It is based on a standard year of 365 days with modifications to keep it consistent with the earth’s movement around the sun. Though not perfect, the Gregorian Calendar will take 3300 years before being one day off. It replaced the old Julian Calendar instituted in 46 BC and named after Julius Caesar. | ||
The old Julian Calendar assumed the earth went around the sun in 365.25 days. For this calendar to follow the earth’s movement, this rule was used - Every year that is divisible by 4 was made a leap year of 366 days, otherwise it was a standard year of 365 days. But in actual fact, the earth travels around the sun in 365.2425 days, about 11 minuets shorter than the old Julian Calendar accounted for. This discrepancy accumulated about 3 days short every 4 centuries. By 1582, the spring equinox was happening 10 days early on 11 March. | The old Julian Calendar assumed the earth went around the sun in 365.25 days. For this calendar to follow the earth’s movement, this rule was used - Every year that is divisible by 4 was made a leap year of 366 days, otherwise it was a standard year of 365 days. But in actual fact, the earth travels around the sun in 365.2425 days, about 11 minuets shorter than the old Julian Calendar accounted for. This discrepancy accumulated about 3 days short every 4 centuries. By 1582, the spring equinox was happening 10 days early on 11 March. | ||