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Julian year (astronomy)
 
 
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In astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
, a Julian year (symbol: a) is a unit of measurement
Units of measurement

The definition, agreement and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day....
 of time
Time

Time is a component of the measurement used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects....
 defined as exactly 365.25 day
Day

A day is a units of measurement of time equivalent to approximately 24 hours. It is not an International System of Units unit but it is accepted for use with SI....
s of 86,400 SI
International System of Units

The International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten....
 second
Second

The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time....
s each, totalling 31,557,600 seconds. That is the average length of the year
Year

A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which Mars completes its own orbit....
 in the Julian calendar
Julian calendar

The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC, and came into force in 45 BC . It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus....
 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
Julian calendar

The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC, and came into force in 45 BC . It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus....
 or any other calendar
Calendar

A calendar is a system of organize days for a social, religious, commercial or administrative purpose. This organization is done by giving names to periods of time ? typically days, weeks, months and years....
.






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In astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
, a Julian year (symbol: a) is a unit of measurement
Units of measurement

The definition, agreement and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day....
 of time
Time

Time is a component of the measurement used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects....
 defined as exactly 365.25 day
Day

A day is a units of measurement of time equivalent to approximately 24 hours. It is not an International System of Units unit but it is accepted for use with SI....
s of 86,400 SI
International System of Units

The International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten....
 second
Second

The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time....
s each, totalling 31,557,600 seconds. That is the average length of the year
Year

A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which Mars completes its own orbit....
 in the Julian calendar
Julian calendar

The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC, and came into force in 45 BC . It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus....
 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
Julian calendar

The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC, and came into force in 45 BC . It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus....
 or any other calendar
Calendar

A calendar is a system of organize days for a social, religious, commercial or administrative purpose. This organization is done by giving names to periods of time ? typically days, weeks, months and years....
. Nor does it correspond to the many other ways of defining a year (for which, see Year
Year

A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which Mars completes its own orbit....
).

Usage


The Julian year is not a fundamental unit of measurement, nor is it sanctioned in the International System of Units
International System of Units

The International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten....
 (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
Year

A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which Mars completes its own orbit....
, its use also aids comprehension. For example, it is easier to express and to comprehend the orbital period
Orbital period

The orbital Periodicity is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars....
 of Pluto
Pluto

Pluto , Minor planet names Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun....
 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
International Astronomical Union

The International Astronomical Union is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy....
 (IAU).

The Julian year is the basis of the definition of the light-year
Light-year

A light-year or light year is a Units of measurement of length, equal to just under ten orders_of_magnitude_%28numbers%29#1012 kilometres....
 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
Solar System

The Solar System consists of the Sun and those Astronomical object bound to it by gravity: the eight planets and five dwarf planets, their 173 known Natural satellite, and billions of Small Solar System body....
 ephemerides
Ephemeris

An ephemeris is a table of values that gives the positions of astronomical objects in the sky at a given time or times. Different kinds are used for astronomy and astrology....
.

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
Terrestrial Time

Terrestrial Time is the modern astronomical time standard for the passage of time on the surface of the Earth . Since time moves at different rates for observers in different locations , and "the surface of the Earth" is not a single point in space, TT is a theoretical ideal; its measurement is approximated by the International Atomic Time...
 (close to but not exactly Greenwich mean
Greenwich Mean Time

Greenwich Mean Time is a term originally referring to solar time at the Royal Observatory, Greenwich in Greenwich, London. It is regularly used to refer to Coordinated Universal Time when this is viewed as a time zone, especially by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, the Met Office an...
 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
Gregorian calendar

The Gregorian calendar is the internationally accepted civil calendar. It was first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, after whom it was named, on 24 February 1582 by the papal bull Inter gravissimas....
 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
Calendar date

A date in a calendar is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified....
 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.

See also


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.


External links