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A tropical year or solar year ... the calendar year is an approximation of the solar year: the Gregorian calendar (with its rules for catch-up leap days) ...
A leap year (also known as an intercalary year or bissextile year) is a calendar year that contains an additional day (or, in the case of a lunisolar calendar, a month) compared to a common year. The 366th day (or 13th month) is added to keep the calendar year synchronised with the astronomical year or seasonal year . [ 1 ]
The mean calendar year of such a calendar approximates the sidereal year. Leaping from one lunation to another, but one Sidereal year is the period between two occurrences of the sun, as measured by the stars' solar calendar, which is derived from the Earth's orbit around the sun every 28 years. [3]
The Gregorian calendar, or modern calendar, presents its calendar year to be either a common year of 365 days or a leap year of 366 days, as do the Julian calendars. For the Gregorian calendar, the average length of the calendar year (the mean year) across the complete leap cycle of 400 years is 365.2425 days (97 out of 400 years are leap years ...
the fraction 1/31 556 925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time. was obtained from the linear time-coefficient in Newcomb's expression for the solar mean longitude (above), taken and applied with the same meaning for the time as in formula (3) above. The relation with Newcomb's coefficient can be seen from:
A year has about 365.24 solar days but 366.24 sidereal days. Therefore, there is one fewer solar day per year than there are sidereal days, similar to an observation of the coin rotation paradox. [5] This makes a sidereal day approximately 365.24 / 366.24 times the length of the 24-hour solar day.
The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day "tropical" or "solar" year that is determined by the Earth's revolution around the Sun. The rule for leap years is:
The Lunar Hijri calendar ("Islamic calendar") is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. [a] The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the calendar year in most calendars. A year can also be ...