In astronomy, a Julian year is a unit of time defined as exactly 365.25 days or 31,557,600 seconds. The name derives from the fact that this corresponds to the average length of the year in the Julian calendar formerly used in Western societies in previous centuries. However, it is fundamentally just a convenient intuitive way to measure large intervals of days (not "real" years such as tropical years or sidereal years), and there is no connection with year-month-day calendar timekeeping in the Julian or any other calendar. Astrometry: the study of the position of objects in the sky and their changes of position. ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... The Julian calendar was introduced in 46 BC by Julius Caesar and took force in 45 BC (709 ab urbe condita). ... A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... The sidereal year is the time for the Sun to return to the same position in respect to the stars of the celestial sphere. ...

Julian years are used primarily for convenience in ephemeris work where stating a number of days would be unwieldy (for instance, it is easier to express the orbital period of Pluto as 248 Julian years rather than 90,590 days). It is an intuitively understood unit whose value is very close to the actual length of a year, yet intervals measured in Julian years can easily be converted to an interval in days without awkward long decimal fraction arithmetic. An ephemeris (plural: ephemerides) (from the Greek word ephemeros= daily) was, traditionally, a table providing the positions (given in a Cartesian coordinate system, or in right ascension and declination or, for astrologers, in longitude along the zodiacal ecliptic), of the Sun, the Moon, and the planets in the sky at... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ...

Julian years are not to be confused with the Julian day (or Julian date), which is also used in astronomy. Despite the similarity of names, there is no connection between the two. A Julian year is not 365.25 "Julian days", it is simply 365.25 days. A Julian day (or Julian date) is not a unit of time, but simply a running count of days with an arbitrarily chosen starting point in the distant past, with each day number one greater than the previous, a way to specify a date without reference to months or years. On the other hand, a Julian year is a unit of time and not a running count of years. Astronomers would not say something like "this year is Julian year 2005" (which would be like saying "this hour is hour number 245"); rather they would use it in sentences like "the sidereal period of Pluto is 248.0208 Julian years". The Julian day or Julian day number (JDN) is the number of days that have elapsed since 12 noon Greenwich Mean Time (UT or TT) on Monday, January 1, 4713 BC in the proleptic Julian calendar . ...

Specifying and naming epochs

Note that Julian years in astronomy are purely a unit for measuring time intervals and durations, and are not used as any kind of calendar or timekeeping system. Astronomers do not use the Julian calendar for modern events: when they need to mention a particular date (the date of a solar eclipse for instance), they use the Gregorian calendar like everyone else. However, for events before the introduction of the Gregorian calendar on 15 October 1582, the Julian calendar with a 1 January to 31 December historical year is used. This article needs to be cleaned up to conform to a higher standard of quality. ... The Gregorian calendar is the calendar that is used nearly everywhere in the world. ... October 15 is the 288th day of the year (289th in Leap years). ... Events January 15 - Russia cedes Livonia and Estonia to Poland February 24 - Pope Gregory XIII implements the Gregorian Calendar. ... January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ... December 31 is the 365th day of the year (366th in leap years) in the Gregorian Calendar. ...

Nevertheless, Julian years are the basis for naming standard epochs used in astronomy. An epoch simply specifies a precise moment in time. The Julian epoch J2000.0 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, and future epochs can be calculated and named according to the number of days since then, divided by 365.25. Thus the future epoch "J2100.0" will be exactly 36525 days from J2000. In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. ... A Julian epoch is an epoch that is based on Julian years of exactly 365. ... The J2000. ... This article is about terrestrial time; for other meanings of TT, see TT (disambiguation). ... Greenwich Mean Time (GMT) is mean solar time at the Royal Greenwich Observatory in Greenwich, London, England, which by convention is at 0 degrees geographic longitude. ... January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ... This article is about the year 2000. ...

However, this is not really of any practical use as a calendar system (it is synchronized to the Gregorian calendar date for January 1, 2000 and not the Julian calendar date with the same designation, which differ by nearly two weeks, yet it uses Julian years so it will diverge from the Gregorian calendar in a few hundred years). Rather, it is simply a convenient way to specify and name epochs. January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ... This article is about the year 2000. ...

In astronomy, positions of stars (right ascension and declination, corresponding to geographic longitude and latitude) gradually change due to precession, so when specifying the coordinates of a star it is necessary to mention what epoch those coordinates apply to. Also the orbital elements of planets change slightly on a continuous basis due to the effects of gravitational perturbations, so when these are specified it is also necessary to specify what epoch applies. The standard epoch in use today is J2000, but for practical reasons a new standard epoch is chosen about every 50 years or so. Right ascension (RA; symbol α: Greek letter alpha) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. ... In astronomy, declination (dec) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. ... Map of Earth showing lines of longitude, which appear curved and vertical in this projection, but are actually halves of great circles Longitude, sometimes denoted by the Greek letter λ, describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ... Latitude, sometimes denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ... There are two types of precession, torque-free and torque-induced, the latter being discussed here in more detail. ... The elements of an orbit are the parameters needed to specify that orbit uniquely, given a model of two ideal masses obeying the Newtonian laws of motion and the inverse-square law of gravitational attraction. ... A planet in common parlance is a large object in orbit around a star that is not a star itself. ... The J2000. ...

Reference

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

See also

• Anomalistic year
• Sidereal year
• Tropical year

A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... The sidereal year is the time for the Sun to return to the same position in respect to the stars of the celestial sphere. ... A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ...

External link

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