Julian year (astronomy)
Inastronomy,aJulian year(symbol:aoraj) is aunit of measurementoftimedefined as exactly 365.25daysof86400SIsecondseach.[1][2][3][4]The length of the Julian year is the average length of theyearin theJulian calendarthat was used in Western societiesuntil 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[edit]
The Julian year is not a unit of measurement in theInternational System of Units(SI), but it is recognized by theInternational Astronomical Union(IAU) as a non-SI unit for use in astronomy.[3]Before 1984, both the Julian year and the meantropical yearwere used by astronomers. In 1898,Simon Newcombused both in hisTables of the Sunin the form of the Julian century (36 525 days) and the "solar century" (36524.22days), a rounded form of 100 meantropical yearsof365.24219879deach according to Newcomb.[5]However, the mean tropical year is not suitable as a unit of measurement because it varies from year to year by a small amount,6.14×10−8days according to Newcomb.[5]In contrast, the Julian year is defined in terms of the SI unit onesecond,so is as accurate as that unit and is constant. It approximates both thesidereal yearand the tropical year to about ±0.008 days. The Julian year is the basis of the definition of thelight-yearas a unit of measurement of distance.[2]
Epochs[edit]
In astronomy, anepochspecifies a precise moment in time. The positions of celestial objects and events, as measured fromEarth,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 isJulian epoch J2000.0.It is exactly 12:00TT(close to but not exactlyGreenwich meannoon) onJanuary 1, 2000in theGregorian(notJulian) calendar.Julianwithin 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 onJanuary 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. 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 beJanuary 8, 3000 12:00 TT.
Julian calendar distinguished[edit]
TheJulian 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 usingJulian day numbers(see next§), astronomers follow the same conventional calendars that are accepted in the world community: They use theGregorian calendarfor events since its introduction onOctober 15, 1582(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[edit]
AJulian yearshould not be confused with theJulian day,which is also used in astronomy (more properly called theJulian day numberorJDN). 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 theJulian day numbersandJulian 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 eachdateas 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 usingUTC,is specified via a decimal fraction.
References[edit]
- ^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.
- ^ab"Measuring the Universe".International Astronomical Union.RetrievedMarch 22,2012.
- ^abInternational Astronomical Union."Recommendations Concerning Units".Archived fromthe originalon February 16, 2007.RetrievedFebruary 18,2007.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.
- ^abSimon Newcomb,Tables of the Four Inner Planets,vol. 6 ofAstronomical Papers Prepared for the Use of the American Ephemeris and Nautical Almanac(Washington, DC: 1898), pp. 10–11.
External links[edit]
- Michael Allison (@ Goddard Institute for Space Studies) (2001)."What is a" Year "(on Earth or Mars)?".pweb.jps.net.Archived fromthe originalon 2011-07-24.Retrieved2011-07-26.