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This article is about the time interval used in astronomy. For years in the Julian calendar, see Julian calendar.

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 designates 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).

Contents 1 Usage 2 Epochs 3 Julian calendar distinguished 4 Julian day distinguished 5 References 5.1 Notes 5.2 Other sources 6 External links

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).^[1]

The Julian year is the basis of the definition of 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.

Main article: Epoch (astronomy)

In astronomy, an epoch specifies a precise moment in time. Julian years are the basis for naming so-called standard epochs. 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 synchronized to exactly 12:00 TT (close to but not exactly Greenwich mean noon) on January 1, 2000 in the Gregorian (not Julian) calendar. Future epochs can be calculated and named according to the number of days since then divided by 365.25. For example, the future epoch J2100.0 will be exactly 36,525 days (one Julian century) from J2000.0, January 1, 2100 (the dates will still agree, because 2000-2100 is one of the few julian centuries with the same number of days as the gregorian calendar).

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.

Main article: Julian calendar

The Julian year, being a measure of duration, should not be confused with historical years in the Julian calendar. In ordinary writing, 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.

Main article: Julian day

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 no connection between the two. A Julian day is not a unit of time. Rather, 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.

1. ^ International Astronomical Union. Recommendations Concerning Units. Retrieved on February 18, 2007. Reprinted from the "IAU Style Manual" by G.A. Wilkinson, Comm. 5, in IAU Transactions XXB (1987).

• 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.

• What is a "Year" (on Earth or Mars)?