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Julian within its name indicates that other Julian epochs can be a number of Julian years of 365.25 days each before or after J2000.0. For example, the future epoch J2100.0 will be exactly 36,525 days (one Julian century) from J2000.0 at 12:00 TT on January 1, 2100 (the dates will still agree because the Gregorian century 2000–2100 will have ...
In astronomy, the Julian year is a unit of time defined as 365.25 days, each of exactly 86 400 seconds (SI base unit), totaling exactly 31,557,600 seconds in the Julian astronomical year. [ 4 ] [ 5 ] Revised Julian calendar
According to Āryabhaṭa, the duration of a year is 365 days 6 hours 12 minutes 30 seconds, that is, 365.25 days. Hence, as per Āryabhaṭa, the number days in a period of 3600 years is 1,314,931.25 days. Since a Julian year is 365.25, the number of Julian years in a period of 1,314,931.25 days is 3600 years 31.25 days.
A table with 197 Julian days ("Date in Mean Solar Days", one per century mostly) was included for the years –4713 to 2000 with no year 0, thus "–" means BC, including decimal fractions for hours, minutes, and seconds. [54]
Because of this, the table below does not include weeks, months, and years. Instead, the table uses the annum or astronomical Julian year (365.25 days of 86,400 seconds), denoted with the symbol a. Its definition is based on the average length of a year according to the Julian calendar, which has one leap year every four years.
Years affected are those which divide by 100 without remainder but do not divide by 400 without remainder (e.g., 1900 and 2100 but not 2000). No guidance is provided about conversion of dates before March 5, -500, or after February 29, 2100 (both being Julian dates).
Note that Julian days begin at noon (hour = 12) and thus hours 0-11 of a solar day are one Julian day earlier than hours 12-23. The value may extend outside of the normal range and is considered as additional number of julian days (a Julian day is 24 hours or 86400 seconds exactly, ignoring any adjustment of leap seconds within the UTC calendar).
TT differs from Geocentric Coordinate Time (TCG) by a constant rate. Formally it is defined by the equation = +, where TT and TCG are linear counts of SI seconds in Terrestrial Time and Geocentric Coordinate Time respectively, is the constant difference in the rates of the two time scales, and is a constant to resolve the epochs (see below).