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refimprove|date=December 2011Use mdy dates|date=October 2011pp-move-indefWiktionaryA year (from Old English language|Old English Jeram|gear ) is the orbital period of the Earth moving around the Sun . For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic .
In astronomy, the Julian year (astronomy)|Julian year is a unit of time , defined as 365.25 day s of gaps|86|400 second|SI second s each. International Astronomical Union " http://www.iau.org/science/publications/proceedings_rules/units/ SI units" accessed February 18, 2010. (See Table 5 and section 5.15.) Reprinted from George A. Wilkins & IAU Commission 5, http://www.iau.org/static/publications/stylemanual1989.pdf "The IAU Style Manual (1989)" (PDF file) in IAU Transactions Vol. XXB
There is no universally accepted symbol for the year as a unit of time. The International System of Units does not propose one. A common abbreviation in international use is a (for Latin :wikt:annus|annus ), in English also y or yr .
Due to the Earth's axial tilt , the course of a year sees the passing of the season s, marked by changes in weather , hours of daylight , and consequently vegetation and Fertility (soil)|fertility . In temperate and Subpolar climate|subpolar regions, generally four seasons are recognized: Spring (season)|spring , summer , autumn and winter , astronomically marked by the Sun reaching the points of equinox and solstice , although the climatic seasons Seasonal lag|lag behind their astronomical markers. In some tropics|tropical and subtropical regions it is more common to speak of the wet season|rainy (or wet, or monsoon ) season versus the dry season .
A calendar year is an approximation of the Earth's orbital period in a given calendar . A calendar year in the Gregorian calendar (as well as in the Julian calendar ) has either 365 ( common year s) or 366 ( leap year s) days.
The word "year" is also used of periods loosely associated but not strictly identical with either the astronomical or the calendar year, such as the seasonal year , the fiscal year or the academic year , etc. By extension, the term year can mean the orbital period of any planet : for example, a "Martian year" is the time in which Mars completes its own orbit. The term is also applied more broadly to any long period or cycle, such as the " Great Year ". OED , s.v. "year", entry 2.b.: " transf. Applied to a very long period or cycle (in chronology or mythology, or vaguely in poetic use)."
Etymology
Further|Jeran West Saxon gear (IPA|j?ar), Anglian dialects|Anglian ger continues Proto-Germanic *j?ram ( *j Eihwaz|e2ram ). Cognates are German language|German Jahr , Old High German jar , Old Norse ár and Gothic language|Gothic jer , all from a PIE *yerom "year, season". Cognates outside of Germanic are Avestan yare "year", Ancient Greek|Greek lang|grc|??a "year, season, period of time" (whence " hour "), Old Church Slavonic jaru and Latin hornus "of this year".
Latin :wikt:annus|Annus (a 2nd declension masculine noun; annum is the accusative case|accusative singular ; anni is genitive singular and nominative plural; anno the dative and ablative singular) is from a PIE noun PIE|*at-no- , which also yielded Gothic language|Gothic aþnam "year".
Both *ye-ro- and *at-no- are based on verbal roots expressing movement, *at- and *ey- respectively, both meaning "to go" generally.
The Greek word for "year", lang|grc|?t??, is cognate with Latin vetus "old", from PIE *wetus- "year", also preserved in this meaning in Sanskrit IAST|vat-sa- "yearling (calf)" and IAST|vat-sa-ras "year".
Derived from Latin annus are a number of English words, such as wikt:annual|annual , :wikt:annuity|annuity , :wikt:anniversary|anniversary , etc.; :wikt:per annum|per annum means "each year".
Seasonal year
Further|Effect of sun angle on climateA seasonal year is the time between successive recurrences of a season al event such as the flooding of a river, the migration of a species of bird, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than a month from year to year.
Calendar year
A calendar year is the time between two dates with the same name in a calendar . A half year (one half of a year) may run from January to June, or July to December.
No astronomical year has an integer number of days or lunar months, so any calendar that follows an astronomical year must have a system of intercalation such as leap year s. Financial and scientific calculations often use a 365-day calendar to simplify daily rates.
In the Julian calendar , the average length of a year is 365.25 days. In a non-leap year, there are 365 days, in a leap year there are 366 days. A leap year occurs every 4 years.
The Gregorian calendar attempts to keep the vernal equinox on or soon before March 21, hence it follows the tropical year|vernal equinox year . The average length of this calendar's year is gaps|365.242|5 mean solar days (as 97 out of 400 years are leap years); this is within one Parts-per notation|ppm of the current length of the mean tropical year (gaps|365.242|19 days). It is estimated that, by the year 4000, the vernal equinox will fall back by one day in the Gregorian calendar, not because of this difference, but because of the slowing down of the Earth's rotation and the associated lengthening of the sidereal day .
The Revised Julian calendar , as used in some Eastern Orthodox Church es, also tries to synchronize with the tropical year. The average length of this calendar's year is gaps|365.242|222 mean solar days (as 218 out of 900 years are leap years). Gregorian and Revised Julian calendars will start to differ in 2800.
The Iranian calendar|Persian calendar , in use in Afghanistan and Iran , has its year begin on the day of the vernal equinox as determined by astronomical computation (for the time zone of Tehran ), as opposed to using an algorithmic system of leap years.
Numbering calendar years
A calendar era is used to assign a number to individual years, using a reference point in the past as the beginning of the era. In many countries, the most common era is from the traditional (though now believed incorrect) year of the birth of Jesus . Dates in this era are designated Anno Domini (Latin for "in the year of the Lord"), abbreviated AD, or CE (for " common era "). The year before 1 AD or CE is designated 1 Before Christ (BC) or Before the Common Era (BCE), the year before that 2 BC/BCE, etc. Hence there was no year 0 AD/CE.
Other eras are also used to enumerate the years in different cultural, religious or scientific contexts.
Other annual periods
Fiscal year
A fiscal year or financial year is a 12-month period used for calculating annual financial statements in businesses and other organizations. In many jurisdictions, regulations regarding accounting require such reports once per twelve months, but do not require that the twelve months constitute a calendar year.
For example, the federal government of the U.S. has a fiscal year that starts on October 1 instead of January 1. In India the fiscal year is between April 1 and March 31. In the United Kingdom and Canada , the financial year runs from April 6 and April 1 respectively, and in Australia it runs from July 1.
Academic year
An academic year is the annual period during which a student attends an educational institution . The academic year may be divided into academic term s, such as semesters or quarters.
Some schools in the UK and USA divide the academic year into three roughly equal-length terms (called "trimesters" or "quarters" in the USA), roughly coinciding with autumn, winter, and spring. At some, a shortened summer session, sometimes considered part of the regular academic year, is attended by students on a voluntary or elective basis.
Other schools break the year into two main semesters, a first (typically August through December) and a second semester (January through May). Each of these main semesters may be split in half by mid-term exams, and each of the halves is referred to as a "quarter" (or "term" in some countries). There may also be an elective summer session and/or a short January session.
Some other schools, including some in the United States, have four marking periods. The school year in many countries starts in August or September and ends in May, June or July.
Some schools in the United States, notably Boston Latin School , may divide the year into five or more marking periods. Some state in defense of this that there is perhaps a positive correlation between report frequency and academic achievement.
There are typically 180 days of teaching each year in schools in the USA, excluding weekends and breaks, while 190 days for pupils in state schools in the United Kingdom, New Zealand and Canada.
In India the academic year normally starts from June 1 and ends on May 31. Though schools start closing from mid-March, the actual academic closure is on May 31 and in Nepal it starts from July 15.Citation needed|date=June 2010 Schools and universities in Australia typically have academic years that roughly align with the calendar year (i.e. starting in February or March and ending in October to December), as the southern hemisphere experiences summer from December to February.
In Israel the academic year begins around October or November, aligned with the second month of the Hebrew Calendar .
Astronomical years
Julian year
Main|Julian year (astronomy)The Julian year (astronomy)|Julian year , as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days. This is the normal meaning of the unit "year" (symbol "a" from the Latin annus ) used in various scientific contexts. The Julian century of gaps|36|525 days and the Julian millennium of gaps|365|250 days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify how many days (not how many "real" years), for long time intervals where stating the number of days would be unwieldy and unintuitive. By convention, the Julian year is used in the computation of the distance covered by a light-year .
In the Unified Code for Units of Measure , the symbol a (without subscript) always refers to the Julian year aj of exactly gaps|31|557|600 second s.
365.25 days of gaps|86|400 seconds = 1 a = 1 aj = gaps|31.557|6 Ms
The SI multiplier prefixes may be applied to it to form ka (kiloannum), Ma (megaannum) etc.
Sidereal, tropical, and anomalistic years
: The relations among these are considered more fully in Axial precession (astronomy) .
Each of these three years can be loosely called an 'astronomical year'.
The sidereal year is the time taken for the Earth to complete one revolution of its orbit, as measured against a fixed frame of reference (such as the fixed stars, Latin sidera , singular sidus ). Its average duration is gaps|365.256|363|004 mean solar days (365 d 6 h 9 min 9.76 s) (at the epoch J2000.0 = January 1, 2000, 12:00:00 Terrestrial Time|TT ).International Earth Rotation and Reference System Service. (2010). http://hpiers.obspm.fr/eop-pc/models/constants.html IERS EOP PC Useful constants.
The tropical year is the period of time for the ecliptic longitude of the Sun to increase by 360 degrees. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of the seasons; because of the economic importance of the seasons, the tropical year is the basis of most calendar s. The tropical year is often defined as the time between southern solstice s, or between northward equinox es. Because of the Earth's axial precession (astronomy)|axial precession , this year is about 20 minutes shorter than the sidereal year. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds cite book|url= http://asa.usno.navy.mil/SecM/Glossary.html#y |title=Astronomical Almanac for the Year 2011 |year=2009 |publisher = U.S. Government Printing Office and the U.K. Hydrographic Office |location = Washington and Taunton |page = M18 (Glossary) (= gaps|365.242|19 days).
The anomalistic year is the time taken for the Earth to complete one revolution with respect to its Apsis|apsides . The orbit of the Earth is elliptical; the extreme points, called apsides, are the perihelion , where the Earth is closest to the Sun (January 3 in 2011), and the aphelion , where the Earth is farthest from the Sun (July 4 in 2011). The anomalistic year is usually defined as the time between perihelion passages. Its average duration is gaps|365.259|636 days (365 d 6 h 13 min 52.6 s) (at the epoch J2011.0).cite book |title=Astronomical Almanac for the Year 2011 |year=2009 |publisher = U.S. Government Printing Office and the U.K. Hydrographic Office |location = Washington and Taunton |pages = A1, C2
If Earth moved in an ideal Kepler orbit , i.e. a perfect ellipse with the Sun fixed at one focus, each kind of year would always have the same duration, and the sidereal and anomalistic years would be equal. Because of Perturbation (astronomy)|perturbation s by the gravity of other planets, Earth's motion varies slightly, causing the sidereal and tropical years to vary in length by about 25 minutes (see table below). Both are affected in the same way, so that the sidereal year is consistently 20 minutes longer than the tropical year, provided that they are measured in the same way.
nowrap>365.242
2007-12-22 06:04:04.2
+10.51 minutes
2008-12-21 12:03:19.7
-11.86 minutes
2009-12-21 17:40:13.2
+15.91 minutes
2010-12-21 23:44:53.2
-11.94 minutes
2011-12-22 05:21:41.8
+3.58 minutes
2012-12-21 11:14:01.9
+2.85 minutes
2013-12-21 17:05:38.3
+0.86 minutes
2014-12-21 22:55:15.2
+0.48 minutes
An example of a year that will have a duration exceeding the average value of gaps|365.242|19 SI days with as much as 24.23 minutes is the one that will begin at winter solstice December 21, 2042 17:47:45.5 (Atomic time).
Draconic year
The draconic year , draconitic year , eclipse year , or ecliptic year is the time taken for the Sun (as seen from the Earth) to complete one revolution with respect to the same lunar node (a point where the Moon's orbit intersects the ecliptic). This period is associated with eclipse s: these occur only when both the Sun and the Moon are near these nodes; so eclipses occur within about a month of every half eclipse year. Hence there are two eclipse season s every eclipse year. The average duration of the eclipse year is :gaps|346.620|075|883 days (346 d 14 h 52 min 54 s) (at the epoch J2000.0).
This term is sometimes erroneously used for the draconic or nodal period of lunar precession , that is the period of a complete revolution of the Moon's ascending node around the ecliptic: gaps|18.612|815|932 Julian years (gaps|6|798.331|019 days; at the epoch J2000.0).
Full moon cycle
The full moon cycle is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the perigee of the Moon's orbit. This period is associated with the apparent size of the full moon , and also with the varying duration of the month|synodic month . The duration of one full moon cycle is: :gaps|411.784|430|29 days (411 d 18 h 49 min 34 s) (at the epoch J2000.0).
Lunar year
The lunar year comprises twelve full cycles of the phases of the Moon, as seen from Earth. It has a duration of approximately 354.37 days. Muslims use this for celebrating their Eid s and for marking the start of the fasting month of Ramadan .
Vague year
The vague year , from annus vagus or wandering year, is an integral approximation to the year equaling 365 days, which wanders in relation to more exact years. Typically the vague year is divided into 12 Wiktionary:schematic|schematic months of 30 days each plus 5 Intercalation|epagomenal days. The vague year was used in the calendars of Egyptian calendar|Ancient Egypt , Iranian calendar|Iran , Armenian calendar|Armenia and in Mesoamerican calendars|Mesoamerica among the Aztec calendar|Aztecs and Haab'|Maya . http://www.mayacalendar.com/descripcion.html Calendar Description and Coordination Maya World Studies Center
Heliacal year
A heliacal year is the interval between the heliacal rising s of a star. It differs from the sidereal year for stars away from the ecliptic due mainly to the precession of the equinoxes .
Sothic year
The Sothic cycle|Sothic year is the interval between heliacal risings of the star Sirius . It is presently less than the #Sidereal, tropical, and anomalistic years|sidereal year and its duration is very close to the mean Julian year of 365.25 days.
Gaussian year
The Gaussian year is the sidereal year for a planet of negligible mass (relative to the Sun) and unperturbed by other planets that is governed by the Gaussian gravitational constant . Such a planet would be slightly closer to the Sun than Earth's mean distance. Its length is: :gaps|365.256|898|3 days (365 d 6 h 9 min 56 s).
Besselian year
The Besselian epoch|Besselian year is a tropical year that starts when the (fictitious) mean Sun reaches an ecliptic longitude of 280°. This is currently on or close to January 1. It is named after the 19th century German astronomer and mathematician Friedrich Bessel . The following equation can be used to compute the current Besselian epoch (in years): cite book|title= Astronomical Almanac|Astronomical Almanac for the Year 2010 |year=2008 |publisher = U.S. Government Printing Office and the U.K. Hydrographic Office |location = Washington and Taunton |page = B3
: B = 1900.0 + (Julian dateTT - gaps|2|415|020.313|52) / gaps|365.242|198|781 The TT subscript indicates that for this formula, the Julian date should use the Terrestrial Time scale, or its predecessor, ephemeris time .
Variation in the length of the year and the day
The exact length of an astronomical year changes over time. The main sources of this change are:
The precession of the equinoxes changes the position of astronomical events with respect to the apsides of Earth's orbit. An event moving toward perihelion recurs with a decreasing period from year to year; an event moving toward aphelion recurs with an increasing period from year to year (though this effect does not change the average value of the length of the year).
Each planet's movement is perturbed by the gravity of every other planet.
Tidal drag between the Earth and the Moon and Sun increases the length of the day and of the month (by transferring angular momentum from the rotation of the Earth to the revolution of the Moon); since the apparent mean solar day is the unit with which we measure the length of the year in civil life, the length of the year appears to change. Tidal drag in turn depends on factors such as post-glacial rebound and sea level rise .
Changes in the effective mass of the Sun, caused by solar wind and radiation of energy generated by nuclear fusion and radiated by its surface, will affect the Earth's orbital period over a long time (approximately an extra 1.25& nbsp; microsecond per year).Solar mass is ~2×1030& nbsp;kg, decreasing at ~5×109& nbsp;kg/s, or ~8×10-14 solar mass per year. The period of an orbiting body is proportional to , where M is the mass of the primary.
The Poynting–Robertson effect shortens the year by about 30 nanosecond s per year.
Gravitational radiation shortens the year by about 165& nbsp; attosecond s per year.~300& nbsp;W of radiation produces ~9.5×109& nbsp;J orbital energy decrease per year; this varies as 1/R, and period varies as R1.5
Summary
346.62 days: a Year#Draconic year|draconitic year .
353, 354 or 355 days: the lengths of common years in some lunisolar calendar s.
354.37 days (12 lunar months): the average length of a year in lunar calendar s, notably the Muslim calendar.
365 days: a vague year and a common year in many solar calendar s.
gaps|365.242|19 days: a mean tropical year (rounded to five decimal places) for the epoch 2000.
gaps|365.242|4 days: a vernal equinox year (rounded to four decimal places) for the epoch 2000.
gaps|365.242|5 days: the average length of a year in the Gregorian calendar.
365.25 days: the average length of a year in the Julian calendar.
gaps|365.256|4 days: a sidereal year.
366 days: a leap year in many solar calendars.
383, 384 or 385 days: the lengths of leap years in some lunisolar calendars.
383.9 days (13 lunar months): a leap year in some lunisolar calendars.
An average Gregorian year is gaps|365.242|5 day s = gaps|52.177|5 week s = gaps|8|765.82 hour s = gaps|525|949.2 minute s = gaps|31|556|952 second s (mean solar, not SI).
A common year is 365 days = gaps|8|760 hours = gaps|525|600 minutes = gaps|31|536|000 seconds.
A leap year is 366 days = gaps|8|784 hours = gaps|527|040 minutes = gaps|31|622|400 seconds.
The 400-year cycle of the Gregorian calendar has gaps|146|097 days and hence exactly gaps|20|871 weeks.
See also Gregorian calendar#Leap seconds and other aspects|Leap seconds and other aspects of the Gregorian calendar .
Symbol
There is no universally accepted symbol for the year as a unit of time . The International System of Units does not propose one. NIST SP811 cite journal|author=Ambler Thompson, Barry N. Taylor |url= http://physics.nist.gov/Document/sp811.pdf |format=PDF |publisher= National Institute of Standards and Technology (NIST) |title=Special Publication 811: Guide for the Use of the International System of Units (SI) |paragraph=8.1 |year=2008 and ISO 80000-3:2006 cite web|publisher= International Organization for Standardization |url= http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm? csnumber=31888 |title=ISO 80000-3:2006, Quantities and units & ndash; Part 3: Space and time |location=Geneva, Switzerland |year=2006
suggest the symbol a (year)|a is taken from the Latin word annus .cite web |work=How Many? A Dictionary of Units of Measurement |author=Russ Rowlett |publisher=University of North Carolina |title=Units: A |url= http://www.unc.edu/~rowlett/units/dictA.html |accessdate=January 9, 2009
In English, the abbreviations y or yr are sometimes used, specifically in geology and paleontology , where kyr, myr, byr (thousands, millions, and billions of years, respectively) and similar abbreviations are used to denote intervals of time remote from the present. cite web|url= http://www.agu.org/pubs/style_guide_intro.html |publisher=American Geophysical Union |title=AGU Editorial Style Guide for Authors |date=September 21, 2007 |accessdate=2009-01-09 |archiveurl = http://web.archive.org/web/20080714134306/ http://www.agu.org/pubs/style_guide_intro.html |archivedate = 2008-07-14 cite journal|author=North American Commission on Stratigraphic Nomenclature |url= http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13 |title=North American Stratigraphic Code |journal=The American Association of Petroleum Geologists Bulletin |volume=89 |issue=11 |month=November |year=2005 |pages=1547–1591 |doi= |edition=Article 13 (c)
Symbol a
NIST SP811cite web |author=Ambler Thompson, Barry N. Taylor |url= http://physics.nist.gov/Document/sp811.pdf |publisher=National Institute of Standards and Technology (NIST) |title=Special Publication 811 & ndash; Guide for the Use of the International System of Units (SI) |at=para 8.1 |year=2008 and ISO 80000-3:2006cite web |publisher=International Organization for Standardization |url= http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm? csnumber=31888 |title=ISO 80000-3:2006, Quantities and units |at=Part 3: Space and time |location=Geneva |year=2006 suggest the symbol a (in the International System of Units , although a is also the symbol for the Hectare#Are|are , the unit of area used to measure land area, but context is usually enough to disambiguate). In English, the abbreviations y and yr are also used.
The Unified Code for Units of Measure cite web |author=Gunther Schadow, Clement J. McDonald |url= http://aurora.rg.iupui.edu/ucum |title=Unified Code for Units of Measure disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 http://aurora.regenstrief.org/~ucum/ucum.html#para-31 by using
: ar for Hectare#Are|are and:
: at = a_t = gaps|365.242|19 days for the mean tropical year
: aj = a_j = 365.25 days for the mean Julian year
: ag = a_g = gaps|365.242|5 days for the mean Gregorian year
: a = 1 aj year (without further qualifier)
A definition jointly adopted by the International Union of Pure and Applied Chemistry and the International Union of Geological Sciences is to use annus , with symbol a , for year, defined as the length of the tropical year in the year 2000:cite journal |author=Norman E. Holden, Mauro L. Bonardi, Paul De Bièvre, Paul R. Renne, and Igor M. Villa |title=IUPAC-IUGS common definition and convention on the use of the year as a derived unit of time (IUPAC Recommendations 2011) |journal= Pure and Applied Chemistry |year=2011 |volume=83 |pages=1159–1162 |doi=10.1351/PAC-REC-09-01-22 |issue=5cite journal |author=Celeste Biever |title=Push to define year sparks time war |journal= New Scientist |url= http://www.newscientist.com/article/dn20423-push-to-define-year-sparks-time-war.html |date=April 27, 2011 |accessdate=April 28, 2011
: a = gaps|365.242|192|65 days = gaps|31|556|925.445 seconds
The notation has proved controversial; it conflicts with an earlier convention among geoscientists to use a specifically for "years ago", and y or yr for a one-year time period.
SI prefix multipliers
ka (for kiloannum ), is a millennium|unit of time equal to one thousand (103) years. This is typically used in geology , paleontology , and archaeology for Holocene and Pleistocene periods where a non radiocarbon dating technique, i. e., ice core dating, dendrochronology , uranium-thorium dating , or varve analysis, is used as the primary dating method for age determination. If age is primarily determined by radiocarbon dating, then the age should be expressed in either radiocarbon or calendar (calibrated) years Before Present .
Ma (for megaannum ), is a unit of time equal to one million (106) years. It is commonly used in scientific disciplines such as geology , paleontology , and celestial mechanics to signify very long time periods into the past or future. For example, the dinosaur species Tyrannosaurus|Tyrannosaurus rex was abundant approximately 65 Ma (65 million years) ago ( ago may not always be mentioned; if the quantity is specified while not explicitly discussing a duration, one can assume that "ago" is implied; the alternative but deprecated " mya (unit)|mya " unit includes "ago" explicitly.). In astronomical applications, the year used is the Julian year (astronomy)|Julian year of precisely 365.25 days. In geology and paleontology, the year is not so precise and varies depending on the author.
Ga (for gigaannum ), is a unit of time equal to 109 years (one 1000000000 (number)|billion on the long and short scales|short scale , one milliard on the long and short scales|long scale ). It is commonly used in scientific disciplines such as physical cosmology|cosmology and geology to signify extremely long time periods in the past. For example, the formation of the Earth occurred approximately 4.57 Ga (4.57 billion years) ago.
Ta (for teraannum ), is a unit of time equal to 1012 years (one 1000000000000 (number)|trillion on the short scale, one billion on the long scale). It is an extremely long unit of time, about 70 times as long as the age of the universe . It is the same order of magnitude as the expected life span of a small red dwarf star .
Pa (for petaannum ), is a unit of time equal to 1015 years (one Orders of magnitude (numbers)#1015|quadrillion on the short scale, one billiard on the long scale). The half-life of the nuclide cadmium -113 is about 8 Pa.cite journal| author = P. Belli et al. | title = Investigation of ß decay of 113Cd| journal = Phys. Rev. C |volume = 76|page = 064603|year = 2007 |doi = 10.1103/PhysRevC.76.064603 This symbol coincides with that for the pascal (unit)|pascal without a multiplier prefix, though both are infrequently used and context will normally be sufficient to distinguish time from pressure values.
Ea (for exaannum ), is a unit of time equal to 1018 years (one quintillion on the short scale, one trillion on the long scale). The half-life of tungsten -180 is 1.8 Ea.cite journal| author = F. A. Danevich et al. | title = a activity of natural tungsten isotopes| journal = Phys. Rev. C |volume = 67|page = 014310|year = 2003| arxiv = nucl-ex/0211013|doi = 10.1103/PhysRevC.67.014310
Symbols y and yr
In astronomy , geology , and paleontology , the abbreviation yr for "years" and ya for "years ago" are sometimes used, combined with prefixes for "thousand", "million", or "billion".name="AGUStyle" />name="NASC">cite journal |author=North American Commission on Stratigraphic Nomenclature|url= http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13 |title=North American Stratigraphic Code|chapter=Article 13 (c) |quote=(c) Convention and abbreviations. – The age of a stratigraphic unit or the time of a geologic event, as commonly determined by numerical dating or by reference to a calibrated time-scale, may be expressed in years before the present. The unit of time is the modern year as presently recognized worldwide. Recommended (but not mandatory) abbreviations for such ages are SI (International System of Units) multipliers coupled with "a" for annum: ka, Ma, and Ga for kilo-annum (103 years), Mega-annum (106 years), and Giga-annum (109 years), respectively. Use of these terms after the age value follows the convention established in the field of C-14 dating. The "present" refers to 1950 AD, and such qualifiers as "ago" or "before the present" are omitted after the value because measurement of the duration from the present to the past is implicit in the designation. In contrast, the duration of a remote interval of geologic time, as a number of years, should not be expressed by the same symbols. Abbreviations for numbers of years, without reference to the present, are informal (e.g., y or yr for years; my, m.y., or m.yr. for millions of years; and so forth, as preference dictates). For example, boundaries of the Late Cretaceous Epoch currently are calibrated at 63 Ma and 96 Ma, but the interval of time represented by this epoch is 33 m.y.
They are not SI units, using y to abbreviate English :wikt:year|year , but following ambiguous international recommendations, use either the standard English first letters as prefixes (t, m, and b) or metric prefix es (k, M, and G) or variations on metric prefixes (k, m, g). These abbreviations include:
non-SI abbreviation
kyr
Thousand years
myr
Million years
byr
Billion years ( Long and short scales|thousand million years)
tya or kya
Appearance of Homo sapiens , ca. 200 tya
Out-of-Africa migration , ca. 60 tya
Last Glacial Maximum , ca. 20 tya
Neolithic Revolution , ca. 10 tya
mya
Pliocene , 5.3 to 2.6 mya
* The Brunhes–Matuyama reversal|last geomagnetic reversal was 0.78 myacite journal|author=Bradford M. Clement|title=Dependence of the duration of geomagnetic polarity reversals on site latitude|date=April 8, 2004|journal=Nature|volume=428|pmid=15071591|doi=10.1038/nature02459 |issue=6983 |pages=637–40
Use of "mya" and "bya" is deprecated in modern geophysics, the recommended usage being "Ma" and "Ga" for dates Before Present , but "m.y." for the duration of epochs. This ad hoc distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America.cite web|publisher=Geological Society of America|title=Time Units|url= http://www.geosociety.org/TimeUnits/|accessdate=February 17, 2010
Note that on graphs using "ya" units on the horizontal axis time flows from right to left, which may seem counter-intuitive. If the "ya" units are on the vertical axis, time flows from top to bottom which is probably easier to understand than conventional notation.
"Great years"
Equinoctial cycle
The Great year , or Equinoctial cycle corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,700 years, and cannot be determined precisely as the astronomical precession|precession speed is variable.
Galactic year
The Galactic year is the time it takes Earth's solar system to revolve once around the galactic center . It comprises roughly 230 million Earth years.cite web | url= http://www.orau.gov/SCIENCEBOWL/teams/files/astrset2.pdf | title=Science Bowl Questions, Astronomy, Set 2 | work=Science Bowl Practice Questions | year=2009 | publisher=Oak Ridge Associated Universities | accessdate=December 9, 2009
See also
Portal|Timediv col|2
CURRENTYEAR
Astronomical year numbering
ISO 8601 : standard for representation of dates and times
Jeran
List of calendars
List of years
Man-hour
Orders of magnitude (time)
Unit of time
div col end
References
Notes
Reflist
Further reading
Cite book
| title=Time, the Familiar Stranger | last=Fraser | first=Julius Thomas | edition=illustrated | location=Amherst | publisher= University of Massachusetts Press | year=1987 | isbn=0-87023-576-1 | oclc=15790499
Cite book
| title=What is Time? | last=Whitrow | first=Gerald James | location=Oxford | publisher= Oxford University Press | year=2003 | isbn=0-19-860781-4 | oclc=265440481 Time TopicsTime measurement and standards Category:Orders of magnitude (time) Category:Units of time
