# Julian year (astronomy)

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Julian year (astronomy)

In astronomy, a Julian year (symbol: a) is a unit of measurement of time defined as exactly 365.25 days of 86,400 SI seconds each, totalling 31,557,600 seconds. That is the average length of the year in the Julian calendar used in Western societies in previous centuries, and for which the unit is named. Nevertheless, because a Julian year measures duration rather than designating a date, the Julian year does not correspond to years in the Julian calendar or any other calendar. Nor does it correspond to the many other ways of defining a year (for which, "see Year").

Usage

The Julian year is not a fundamental unit of measurement, nor is it sanctioned in the International System of Units (SI). Nevertheless, astronomers and other scientists use it for convenience to measure lengthy durations, which would be unwieldy to express as a number of days. Since the Julian year corresponds to the duration of what most people think of as a year, its use also aids comprehension. For example, it is easier to express and to comprehend the orbital period of Pluto as 248 Julian years (248 a) than as 90,590 days (90,590 d). For this reason, its use is recommended by International Astronomical Union (IAU). [cite web
url = http://www.iau.org/Units.234.0.html
title = Recommendations Concerning Units
accessdate =
accessdaymonth =
accessmonthday = February 18
accessyear = 2007
author = International Astronomical Union
Reprinted from the "IAU Style Manual" by G.A. Wilkinson, Comm. 5, in IAU Transactions XXB (1987).
]

The Julian year is the basis of the definition of the light-year as a unit of measurement of distance.

One hundred Julian years (36,525 days) are called a "Julian century". One thousand Julian years (365,250 days) are called a "Julian millennium". These units are used in calculating Solar System ephemerides.

Epochs

In astronomy, an "epoch" specifies a precise moment in time. For practical reasons, a new standard epoch is chosen about every 50 years.

The standard epoch in use today is Julian epoch J2000.0. It is exactly 12:00 TT (close to but not exactly Greenwich mean noon) on January 1, 2000 in the Gregorian ("not" Julian) calendar. "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 on January 1, 2100 (the dates will still agree because the Gregorian century 2000–2100 will have the same number of days as a Julian century).

Because Julian years are not exactly the same length as years on the Gregorian calendar, astronomical epochs will diverge noticeably from the Gregorian calendar in a few hundred years.

The positions of celestial objects and events, as measured from earth, change over time. Therefore, when measuring or predicting celestial positions, the epoch to which they pertain must be specified.

Julian calendar distinguished

The "Julian year", being a uniform measure of duration, should not be confused with the variable length historical years in the Julian calendar. An astronomical Julian year is never individually numbered. Astronomers follow the same calendar conventions that are accepted in the world community: They use the Gregorian calendar for events since its introduction on 15 October 1582 (or later, depending on country), and the Julian calendar for events before that date.

Julian day distinguished

A "Julian year" should not be confused with the "Julian day" (also "Julian day number" or "JDN"), which is also used in astronomy. Despite the similarity of names, there is little connection between the two. It is a way of expressing a date as the integer number of days that have elapsed since a reference date called the initial epoch. The Julian day uniquely specifies a date without reference to its day, month, or year in any particular calendar. A specific time within a day is specified via a decimal fraction.

References

Notes

Other sources

* "Explanatory supplement to the Astronomical Almanac". P. Kenneth Seidelmann, editor. Mill Valley, Cal.: University Science Books, 1992. Pages 8, 696, 698-9, 704, 716, 730.

* [http://pweb.jps.net/~gangale3/other/allison2_frm.htm What is a "Year" (on Earth or Mars)?]

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