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A year (from Old English gar) 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 is a unit of time defined as 365.25 days of 86400 SI seconds each.1

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 annus) in English also y or yr.

Due to the Earth's axial tilt the course of a year sees the passing of the seasons marked by changes in weather hours of daylight and consequently vegetation and fertility. In temperate and subpolar regions generally four seasons are recognized: spring summer autumn and winter astronomically marked by the Sun reaching the points of equinox and solstice although the climatic seasons lag behind their astronomical markers. In some tropical and subtropical regions it is more common to speak of the 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 years) or 366 (leap years) 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".2 Contents 1 Etymology 2 Seasonal year 3 Calendar year 3.1 Numbering calendar years 4 Other annual periods 4.1 Fiscal year 4.2 Academic year 5 Astronomical years 5.1 Julian year 5.2 Sidereal tropical and anomalistic years 5.3 Draconic year 5.4 Full moon cycle 5.5 Lunar year 5.6 Vague year 5.7 Heliacal year 5.8 Sothic year 5.9 Gaussian year 5.10 Besselian year 5.11 Variation in the length of the year and the day 5.12 Summary 6 Symbol 6.1 Symbol a 6.1.1 SI prefix multipliers 6.2 Symbols y and yr 7 "Great years" 7.1 Equinoctial cycle 7.2 Galactic year 8 See also 9 References 9.1 Notes 10 Further reading Etymology Further information: Jran

West Saxon gear (jar) Anglian gr continues Proto-Germanic *jram (*j2ram). Cognates are German Jahr Old High German jar Old Norse r and Gothic jer all from a PIE *yrom "year season". Cognates outside of Germanic are Avestan yare "year" Greek "year season period of time" (whence "hour") Old Church Slavonic jaru and Latin hornus "of this year".

Latin Annus (a 2nd declension masculine noun; annum is the accusative singular; anni is genitive singular and nominative plural; anno the locative singular) is from a PIE noun *at-no- which also yielded Gothic anam "year".

Both *y-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" is cognate to Latin vetus "old" from PIE *wetus- "year" also preserved in this meaning in Sanskrit vat-sa- "yearling (calf)" and vat-sa-ras "year". Derived from Latin annus are a number of English words such as annual annuity anniversary etc.; per annum means "yearly". Seasonal year Further information: Effect of sun angle on climate A seasonal year is the time between successive recurrences of a seasonal 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 years. 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 vernal equinox year. The average length of this calendar's year is 365.2425 mean solar days (as 97 out of 400 years are leap years); this is within one ppm of the current length of the mean tropical year (365.24219 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 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 estimated date of the birth of Jesus Christ; dates in this era are designated anno Domini ("in the year of the Lord" abbreviated A.D.) or C.E. (common era). 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 refers to the annual period during which a student attends school college or university. The school year can be divided up in various ways two of which are most common in North American educational systems. Some schools in the UK and USA divide the academic year into three roughly equal-length terms (called trimesters in the USA) more or less 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 (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 180 days of teaching each year in schools in the USA excluding weekends and breaks 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 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. Astronomical years Julian year Main article: Julian year (astronomy) The 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 36525 days and the Julian millennium of 365250 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 31557600 seconds. 365.25 days of 86400 seconds 1 a 1 aj 31.5576 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 Precession (astronomy). Each of these three years can be loosely called an 'astronomical year'. The mean 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 duration is 365.256 363 004 mean solar days (365 d 6 h 9 min 9.76 s) (at the epoch J2000.0 2000 January 1 12:00:00 TT).3 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. The mean tropical year is approximated by 365 days 5 hours 48 minutes 45 seconds."4 ( 365.24219 days) The tropical year is shorter than the sidereal year because of the precession of the equinoxes. The anomalistic year is the time taken for the Earth to complete one revolution with respect to its 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 two successive perihelion passages. Its average duration is 365.259 636 days (365 d 6 h 13 min 52.6 s) (at the epoch J2011.0).5 The anomalistic year is slightly longer than the sidereal year because of the precession of the apsides (also known as anomalistic precession orbital precession and perihelion precession.) 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 eclipses: 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 seasons every eclipse year. The average duration of the eclipse year is: 346.620 075 883 days (346 d 14 h 52 min 54 s) (at the epoch J2000.0). This term is sometimes erroneously used to designate the draconic or nodal period of lunar precession that is the time it takes for a complete revolution of the Moon's ascending node around the ecliptic: 18.612 815 932 Julian years (6798.331 019 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 synodic month. The duration of one full moon cycle is: 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. 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 schematic months of 30 days each plus 5 epagomenal days. The vague year was used in the calendars of Ancient Egypt Iran Armenia and in Mesoamerica among the Aztecs and Maya.6 Heliacal year A heliacal year is the interval between the heliacal risings of a star. It differs from the sidereal year for stars away from the ecliptic due mainly to the precession of the equinoxes. (To visualise: the constellation Crux which rose and set as seen from the Mediterranean in ancient Greek times is never seen above the horizon there in modern times.) Sothic year The Sothic year is the interval between heliacal risings of the star Sirius. It is equal to the 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: 365.256 898 3 days (365 d 6 h 9 min 56 s). Besselian year The 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 1 January. It is named after the 19th century German astronomer and mathematician Friedrich Bessel. A formula7 to compute the current Besselian epoch (in years): B 1900.0 + (Julian dateTT 2415020.31352) / 365.242198781 The TT subscript indicates 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 microsecond per year.8 The PoyntingRobertson effect shortens the year by about 30 nanoseconds per year. Gravitational radiation shortens the year by about 165 attoseconds per year.9 Summary 346.62 days: a draconitic year. 353 354 or 355 days: the lengths of common years in some lunisolar calendars. 354.37 days (12 lunar months): the average length of a year in lunar calendars notably the Muslim calendar. 365 days: a vague year and a common year in many solar calendars. 365.24219 days: a mean tropical year (rounded to five decimal places) for the epoch 2000. 365.2424 days: a vernal equinox year (rounded to four decimal places) for the epoch 2000. 365.2425 days: the average length of a year in the Gregorian calendar. 365.25 days: the average length of a year in the Julian calendar. 365.2564 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 365.2425 days 52.1775 weeks 8765.82 hours 525949.2 minutes 31556952 seconds (mean solar not SI). A common year is 365 days 8760 hours 525600 minutes 31536000 seconds. A leap year is 366 days 8784 hours 527040 minutes 31622400 seconds. The 400-year cycle of the Gregorian calendar has 146097 days and hence exactly 20871 weeks. See also 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 SP81110 and ISO 80000-3:200611 suggest the symbol a is taken from the Latin word annus.12 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.121314 Symbol a NIST SP81115 and ISO 80000-3:200616 suggest the symbol a (in the International System of Units although a is also the symbol for the 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.12 13 14 The Unified Code for Units of Measure17 disambiguates the varying symbologies of ISO 1000 ISO 2955 and ANSI X3.50 1 by using ar for are (unit) and: at at 365.24219 days for the mean tropical year aj aj 365.25 days for the mean Julian year ag ag 365.2425 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 2000:1819 a 31556925.445 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.19 SI prefix multipliers ka (for kiloannum) is a unit of time equal to one thousand (103) years. 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 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 includes "ago" explicitly.). In astronomical applications the year used is the 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 billion on the short scale one milliard on the long scale). It is commonly used in scientific disciplines such as 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 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 quadrillion on the short scale one billiard on the long scale). The half-life of the nuclide cadmium-113 is about 8 Pa. This symbol coincides with that for the 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.20 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".1321 They are not SI units using y to abbreviate English year but following ambiguous international recommendations use either the standard English first letters as prefixes (tmand b) and/or the familiar metric multiplier prefixes (k m and g). These abbreviations include: SI-prefixed equivalent order of magnitude kyr "ka" * Thousands forms myr "Ma" * Millions forms byr "Ga" * Billions forms tya or kya "ka ago" Main articles: 1 E10 s 1 E11 s and 1 E12 s 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 "Ma ago" Main articles: 1 E13 s 1 E14 s and 1 E15 s Pliocene 5.3 to 2.6 mya The last geomagnetic reversal was 0.78 mya22 The (Eemian Stage) Ice Age started 0.13 mya Holocene started 0.01 mya bya or gya "Ga ago" Main articles: 1 E16 s 1 E17 s and 1 E18 s oldest Eukaryotes 2 bya age of the Earth 4.5 bya Big Bang 13.7 bya 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.1314 This ad hoc distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America.23 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 25700 years and cannot be determined precisely as the 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.24 See also Time portal 2011 Astronomical year numbering ISO 8601: standard for representation of dates and times Jran List of calendars List of years Man-hour Orders of magnitude (time) Unit of time References Notes International Astronomical Union "SI units" accessed 18 February 2010. (See Table 5 and section 5.15.) Reprinted from George A. Wilkins & IAU Commission 5 "The IAU Style Manual (1989)" (PDF file) in IAU Transactions Vol. XXB 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)." International Earth Rotation and Reference System Service. (2010).IERS EOP PC Useful constants. Astronomical Almanac for the Year 2011. Washington and Taunton: U.S. Government Printing Office and the U.K. Hydrographic Office. 2009. p. M18 (Glossary). http://asa.usno.navy.mil/SecM/Glossary.html#y.  Astronomical Almanac for the Year 2011. Washington and Taunton: U.S. Government Printing Office and the U.K. Hydrographic Office. 2009. pp. A1 C2.  Calendar Description and Coordination Maya World Studies Center Astronomical Almanac for the Year 2010. Washington and Taunton: U.S. Government Printing Office and the U.K. Hydrographic Office. 2008. p. B3.  Solar mass is 21030 kg decreasing at 5109 kg/s or 81014 solar mass per year. The period of an orbiting body is proportional to where M is the mass of the primary. 300 W of radiation produces 9.5109 J orbital energy decrease per year; this varies as 1/R and period varies as R1.5 Ambler Thompson Barry N. Taylor (2008) (PDF). Special Publication 811: Guide for the Use of the International System of Units (SI). National Institute of Standards and Technology (NIST). http://physics.nist.gov/Document/sp811.pdf.  "ISO 80000-3:2006 Quantities and units Part 3: Space and time". Geneva Switzerland: International Organization for Standardization. 2006. http://www.iso.org/iso/isocatalogue/cataloguetc/cataloguedetail.htmcsnumber31888.  a b c Russ Rowlett. "Units: A". How Many A Dictionary of Units of Measurement. University of North Carolina. http://www.unc.edu/rowlett/units/dictA.html. Retrieved January 9 2009.  a b c d "AGU Editorial Style Guide for Authors". American Geophysical Union. 21 September 2007. Archived from the original on 2008-07-14. http://web.archive.org/web/20080714134306/http://www.agu.org/pubs/styleguideintro.html. Retrieved 2009-01-09.  a b c North American Commission on Stratigraphic Nomenclature (November 2005). "North American Stratigraphic Code". The American Association of Petroleum Geologists Bulletin 89 (11): 15471591. http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13.  Ambler Thompson Barry N. Taylor (2008). "Special Publication 811 Guide for the Use of the International System of Units (SI)". National Institute of Standards and Technology (NIST). para 8.1. http://physics.nist.gov/Document/sp811.pdf.  "ISO 80000-3:2006 Quantities and units". Geneva: International Organization for Standardization. 2006. Part 3: Space and time. http://www.iso.org/iso/isocatalogue/cataloguetc/cataloguedetail.htmcsnumber31888.  Gunther Schadow Clement J. McDonald. "Unified Code for Units of Measure". http://aurora.rg.iupui.edu/ucum.  Norman E. Holden Mauro L. Bonardi1 Paul De Bivre Paul R. Renne and Igor M. Villa (2011). "IUPAC-IUGS common definition and convention on the use of the year as a derived unit of time (IUPAC Recommendations 2011)". Pure and Applied Chemistry 83 (5): 11591162. doi:10.1351/PAC-REC-09-01-22.  a b Celeste Biever (April 27 2011). "Push to define year sparks time war". New Scientist. http://www.newscientist.com/article/dn20423-push-to-define-year-sparks-time-war.html. Retrieved April 28 2011.  F. A. Danevich et al. (2003). " activity of natural tungsten isotopes". Phys. Rev. C 67: 014310. arXiv:nucl-ex/0211013. doi:10.1103/PhysRevC.67.014310.  North American Commission on Stratigraphic Nomenclature. "Article 13 (c)". North American Stratigraphic Code. http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13. "(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.".  Bradford M. Clement (8 April 2004). "Dependence of the duration of geomagnetic polarity reversals on site latitude". Nature 428 (6983): 63740. doi:10.1038/nature02459. PMID 15071591.  "Time Units". Geological Society of America. http://www.geosociety.org/TimeUnits/. Retrieved 17 February 2010.  "Science Bowl Questions Astronomy Set 2". Science Bowl Practice Questions. Oak Ridge Associated Universities. 2009. http://www.orau.gov/SCIENCEBOWL/teams/files/astrset2.pdf. Retrieved December 9 2009.  Further reading Fraser Julius Thomas (1987). Time the Familiar Stranger (illustrated ed.). Amherst: University of Massachusetts Press. ISBN 0870235761. OCLC 15790499.  Whitrow Gerald James (2003). What is Time. Oxford: Oxford University Press. ISBN 0198607814. 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