No astronomical year has an integer number of days or lunar months, so any calendar that follows an astronomical year must have a system of intercalation such as leap years. Financial and scientific calculations often use a 365-day calendar to simplify daily rates.
The Gregorian calendar attempts to keep the vernal equinox on or soon before March 21, hence it follows the vernal equinox year. The average length of this calendar's year is 365.2425 days (as 97 out of 400 years are leap years); this is within one ppm of the current length of the mean tropical year (365.24219 days). It is estimated that, by the year 4000, the vernal equinox will fall back by one day in the Gregorian calendar, not because of this difference, but because of the slowing down of the Earth's rotation and the associated lengthening of the sidereal day.
The Revised Julian calendar, as used in some Eastern Orthodox Churches, also tries to synchronize with the tropical year. The average length of this calendar's year is 365.242222 days (as 218 out of 900 years are leap years). Gregorian and Revised Julian calendars will start to differ in 2800.
The Persian calendar, in use in Afghanistan and Iran, has its year begin on the day of the vernal equinox as determined by astronomical computation (for the time zone of Tehran), as opposed to using an algorithmic system of leap years.
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Famous quotes containing the words year and/or calendar:
“We hold on to hopes for next year every year in western Dakota: hoping that droughts will end; hoping that our crops wont be hailed out in the few rainstorms that come; hoping that it wont be too windy on the day we harvest, blowing away five bushels an acre; hoping ... that if we get a fair crop, well be able to get a fair price for it. Sometimes survival is the only blessing that the terrifying angel of the Plains bestows.”
—Kathleen Norris (b. 1947)
“To divide ones life by years is of course to tumble into a trap set by our own arithmetic. The calendar consents to carry on its dull wall-existence by the arbitrary timetables we have drawn up in consultation with those permanent commuters, Earth and Sun. But we, unlike trees, need grow no annual rings.”
—Clifton Fadiman (b. 1904)