Julian year (astronomy)
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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. The Julian year 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).
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[edit] Usage
The Julian year is not a fundamental unit of measurement, nor is it sanctioned in the International System of Units (SI), nor does it correspond very exactly to the length of a solar year. Nevertheless, astronomers and other scientists use it for convenience to measure lengthy durations, which would be unwieldy to express as a number of seconds (the SI unit of time). Astronomers before the mid-20th century used the so called Besselian year instead, which corresponds much more closely to the real length of a solar year, but which is also not totally exact, and which has the disadvantage of being much harder to determine than the simple 365.25 day Julian year. Simplicity is also the reason why the Julian year is used instead of a "Gregorian Year" of 365.2425 days.
Since the Julian year corresponds roughly 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 7828 million seconds (7.828 × 109 s). For this reason, its use is recommended by International Astronomical Union (IAU).[1]
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.
[edit] 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.
[edit] 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 October 15, 1582 (or later, depending on country), and the Julian calendar for events before that date.
[edit] 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 or 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.
[edit] References
[edit] Notes
- ^ "Recommendations Concerning Units". http://www.iau.org/Units.234.0.html. Retrieved February 18, 2007 author = International Astronomical Union. Reprinted from the "IAU Style Manual" by G.A. Wilkinson, Comm. 5, in IAU Transactions XXB (1987).
[edit] 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.
