astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

, a Julian year (symbol: a or a_j) is a unit of measurement of

time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

defined as exactly 365.25 days of SI seconds each.P. Kenneth Seidelmann, ed.
''The explanatory supplement to the Astronomical Almanac''
(Mill Valley, Cal.: University Science Books, 1992), pp. 8, 696, 698–9, 704, 716, 730. Reprinted from the "IAU Style Manual" by G.A. Wilkinson, Comm. 5, in IAU Transactions XXB (1987).Harold Rabinowitz and Suzanne Vogel
''The manual of scientific style''
(Burlington, MA: Academic Press, 2009) 369. The length of the Julian year is the average length of the

year A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the h ...

in the

Julian calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandri ...

that was used in Western societies until the adoption of the Gregorian Calendar, and from which the unit is named. Nevertheless, because astronomical Julian years are measuring duration rather than designating dates, this 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.

Usage

The Julian year is not a unit of measurement in the International System of Units (SI), but it is recognized by the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

(IAU) as a non-SI unit for use in astronomy. Before 1984, both the Julian year and the mean

tropical year A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time ...

were used by astronomers. In 1898, Simon Newcomb used both in his '' Tables of the Sun'' in the form of the Julian century (36 525 days) and the "solar century" ( days), a rounded form of 100 mean

s of each according to Newcomb.Simon Newcomb
''Tables of the Four Inner Planets''
vol. 6 of ''Astronomical Papers Prepared for the Use of the American Ephemeris and Nautical Almanac'' (Washington, DC: 1898), pp. 10–11. However, the mean tropical year is not suitable as a unit of measurement because it varies from year to year by a small amount, days according to Newcomb. In contrast, the Julian year is defined in terms of SI units so is as accurate as those units and is constant. It approximates both the sidereal year and the tropical year to about ±0.008 days. The Julian year is the basis of the definition of the light-year as a unit of measurement of distance.

Epochs

In astronomy, an ''epoch'' specifies a precise moment in time. The positions of celestial objects and events, as measured from

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

, change over time, so when measuring or predicting celestial positions, the epoch to which they pertain must be specified. 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 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 at 12:00 TT on (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. For example, in the next 1000 years, seven days will be dropped from the Gregorian calendar but not from 1000 Julian years, so J3000.0 will be .

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. When not using Julian day numbers (''see next'' §), astronomers follow the same conventional calendars that are accepted in the world community: They use the

Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years d ...

for events since its introduction on (or later, depending on country), and the Julian calendar for events before that date, and occasionally other, local calendars when appropriate for a given publication.

Julian day distinguished

A ''Julian year'' should not be confused with the ''Julian day'', which is also used in astronomy (more properly called the ''Julian day number'' or ''JDN''). The JDN uniquely specifies a place in time, without becoming bogged down in its date-in-month, week, month, or year in any particular calendar. Despite the similarity of names, there is almost no connection between the ''Julian day numbers'' and ''Julian years''. The Julian day number is a simplified time-keeping system originally intended to ease calculation with historical dates which involve a diversity of local, idiosyncratic calendars. It was adopted by astronomers in the mid-1800s, and identifies each date as the integer number of days that have elapsed since a reference date ( "epoch"), chosen to precede most, if not all, historical records. A specific time within a day, always using UTC, is specified via a decimal fraction.

References

External links

* {{DEFAULTSORT:Julian Year (Astronomy) Types of year Time in astronomy

Year A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the h ...