Julian year (symbol: a) is a time measurement in astronomyUnit of measurement, the value defined is 365.25 days, and every day isInternational unit86400 seconds, totaling 31557600 seconds.This value was used in the early Western societyJulian calendarThe average length of middle age and the name of this unit.However, because the Julian year is only a unit for measuring time and is not specific to a specific date, it is not associated with the Julian calendar or any other calendar, nor with the definition of many other forms of years.[1]
The Julian year is neither the basic unit of measurement nor theInternational System of Units(SI) Recognized unit.however,astronomerAnd other scientists use it to facilitate the measurement of long periods of time asdayExpress time units that are awkward and inconvenient.Because Julian year is the accumulation of days, it can be misunderstood when used. No wonder most people regard it as a year.For example,PlutoIts orbital period is 248 Julian years (248a), which is easier to express than 90590 days.For this reason,International Astronomical Union(IAU).
One hundred Julian years (36525 days) are calledJulian centuryOne thousand Julian years (365250 days) are called the Julian millennium.This unit is used to calculate the solar systemCalendar。[2]
Epoch
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The epoch is a specific moment in astronomy.For practical reasons, a new epoch standard will be selected every 50 years.
The epoch standard isJulian calendarYuan J2000.0.Its correct moment is TTGregorian calendar(Not the Julian calendar) 12:00 on January 1, 2000 (very closeGreenwichMidday, but not exactly).In the nameJIt means that other epochs before or after the Julian year are all 365.25 days apart.For example, the time interval of the future epoch J2100.0 is 36525 days from J200.0（Julian century）January 1, 2100 (the date is still the same becauseGreeThe number of days in the 2000-2100 century is the same as that in the Julian century).
Because Julian won'tGregorian calendarThe length of a year is the same, and the astronomical epoch will be different from the date of the Gregorian calendar hundreds of years later.
Newton (from 1642-12-25 to 1727-3-20 in the Julian calendar)
The positions and events of celestial bodies measured from the Earth will change over time.Therefore, when measuring or predicting the position of celestial bodies, it is necessary to assign and designate the appropriate epoch.[2]
Characteristics of the Julian Calendar
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Julian year is the same time interval, not changing in the long historyJulian calendarConfusion.The Julian year in astronomy has never been numbered. Astronomers also follow the resolution of the Calendar Congress as all over the world: on October 15, 1582Gregorian calendarAfter being introduced, the Gregorian calendar was adopted (or later due to different countries and regions), while the Julian calendar was used before.[3]
Characteristics of Julian Day
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Julian year should not be used in astronomyJulian Day(There is also Julian Day or JDN) confusion.Although the names are very similar, there is little connection between the two books.Julian DayIs specified from, calledStarting epochThe number of (cumulative) integer days since the reference date of.Julian day is a single day count that does not refer to the day, month or year in any calendar. It is special that only days and time under decimal system are counted.
