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Julian calendar

(calendar of 365 days and a leap day once every four years)

The Julian calendar is the calendar introduced in Rome by Julius Caesar, in which a year has 365 days plus a leap day (an extra day, such a year termed a leap year with 366 days) each four years, for an average of 365.25 days per year. The Gregorian calendar, which is generally used now, is a subsequent development that more closely matches the tropical year (aka solar year, the time from an equinox to its subsequent occurrence) by skipping the leap days three times each four hundred years to give it an average year of 325.2425, which better matches the tropical-year length which varies slightly, averaging around 325.2422. Both calendars were adjustments to the calendars of the time, and in both cases, at the time of adoption, the calendar date was recalibrated to restore the approximate dates of the solstices and equinoxes by adding or skipping some days.

The term Julian date is used for various types of day counts and the term Julian year means 365.25 days.


(time,measure)
Further reading:
http://en.wikipedia.org/wiki/Julian_calendar
http://en.wikipedia.org/wiki/Gregorian_calendar
http://en.wikipedia.org/wiki/Tropical_year
https://astronomy.swin.edu.au/cosmos/J/Julian+Calendar
https://astronomy.swin.edu.au/cosmos/G/Gregorian+Calendar
https://astronomy.swin.edu.au/cosmos/T/Tropical+Year

Referenced by pages:
Julian date (JD)

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