year


year
(a or y or yr)
   1. a unit of time, defined to be the period of time required for the Earth to make one revolution around the Sun. To be more precise, the year we use in ordinary life (described in the next entry) is designed to approximate the interval between two arrivals of the Sun at the Tropic of Capricorn, marking the summer solstice in the Southern Hemisphere and the winter solstice in the Northern Hemisphere. Astronomers call this unit the tropical year.
   There are 365.242 199 days in a tropical year, or, to be even more precise, 31 556 925.9747 seconds. Since the symbol yr is specific to English, the symbol for the year often used in scientific writing or other international contexts is a, taken from the Latin word, annus. Thus 1 Ma stands for a million years and 1 Ga for a billion years.
   2. a traditional unit of time usually equal to 365 or 366 days. We need a whole number of days for the calendar year used in ordinary life. Ancient astronomers knew that the year1 is approximately 365 days long, and we now know the correct figure is approximately 365.242 days. If we use 365 as the number of days in every calendar year, the extra 0.242 day adds up quickly and causes large errors in predicting the seasons. To solve this problem, the Roman emperor Julius Caesar decreed in 46 BC that the calendar year should have 365 days generally, but that every fourth year should have an extra, or 366th, day. The longer year is called a leap year. In this Julian calendar, four years equal exactly 1461 days, so the average Julian year is exactly 365.25 days.
   This was a big step toward accuracy in the calendar, but the Julian year is too long by 0.008 day, or a little over 11 minutes. By the time of the Renaissance, these 11-minute errors had accumulated to a total error of about 10 days (since the Council of Nicaea in 325 AD, which set the rules for deciding when Easter should be celebrated). The spring equinox was occurring near March 11 instead of March 21. In 1582, Pope Gregory XIII decreed that 10 days should be dropped from the calendar: the day after 1582 October 4 was October 15. To reduce future errors, the pope further decreed that years divisible by 100 are not leap years unless they are also divisible by 400. Thus 2000 and 2400 are leap years, but 2100, 2200, and 2300 are not. It took many years, but the Gregorian calendar has now been accepted as the civil calendar in all countries of the world.
   With the Gregorian adjustment, there are exactly 146 097 days in every 400 years, and the average Gregorian year is exactly 365.2425 days. The Gregorian year is still too long, but by less than half a minute. It will take thousands of years for this error to accumulate to 1 day, so the calendar year and the tropical year are in good enough agreement to last us a long time.

Dictionary of units of measurement. 2015.

Look at other dictionaries:

  • year — [jɪə, jɜː ǁ jɪr] written abbreviation yr noun [countable] 1. also calendar year the period of time beginning on January 1 and ending on December 31: • The Small Business Administration arranged 55,000 small business loans last year. 2 …   Financial and business terms

  • Year — Year, n. [OE. yer, yeer, [yogh]er, AS. ge[ a]r; akin to OFries. i?r, g?r, D. jaar, OHG. j[=a]r, G. jahr, Icel. [=a]r, Dan. aar, Sw. [*a]r, Goth. j?r, Gr. ? a season of the year, springtime, a part of the day, an hour, ? a year, Zend y[=a]re year …   The Collaborative International Dictionary of English

  • year — /year/, n. 1. a period of 365 or 366 days, in the Gregorian calendar, divided into 12 calendar months, now reckoned as beginning Jan. 1 and ending Dec. 31 (calendar year or civil year). Cf. common year, leap year. 2. a period of approximately the …   Universalium

  • year — W1S1 [jıə, jə: US jır] n ▬▬▬▬▬▬▬ 1¦(12 months)¦ 2¦(january to december)¦ 3 years 4 all (the) year round 5 year by year 6 year after year/year in, year out 7¦(period of life/history)¦ 8 the school/academic year 9¦(s …   Dictionary of contemporary English

  • year — or sidereal year [yir] n. [ME yere < OE gear, akin to Ger jahr < IE * yēro , year, summer (> Gr hōros, time, year, OSlav jara, spring) < base * ei , to go (> L ire, to go): basic sense “that which passes”] 1. a) a period of 365… …   English World dictionary

  • Year Up — is a nonprofit organization based in Boston, Massachusetts. Founded in 2000 by Harvard Business School graduate Gerald Chertavian, Year Up has sites in Boston and Cambridge, New York City, Providence and Washington, D.C. and is expanding to other …   Wikipedia

  • year — [ jır ] noun *** 1. ) count a period of 365 days, or 366 in a leap year, divided into 12 months: He lived in Paris for a few years. a ) used about a particular period of time, beginning on January 1 and ending on December 31, or between the first …   Usage of the words and phrases in modern English

  • YEAR — (Heb. שָׁנָה, shanah), the period during which the earth makes one complete revolution around the sun. This period corresponds roughly to 12 revolutions of the moon around the earth. The determination of the length of a year and its 12 parts for… …   Encyclopedia of Judaism

  • year — (n.) O.E. gear (W.Saxon), ger (Anglian) year, from P.Gmc. *jæram year (Cf. O.S., O.H.G. jar, O.N. ar, Dan. aar, O.Fris. ger, Du. jaar, Ger. Jahr, Goth. jer year ), from PIE *yer o , from r …   Etymology dictionary

  • year — ► NOUN 1) the time taken by the earth to make one revolution around the sun. 2) (also calendar year) the period of 365 days (or 366 days in leap years) starting from the first of January, used for reckoning time in ordinary affairs. 3) a period… …   English terms dictionary

  • year|ly — «YIHR lee», adjective, adverb. –adj. 1. once a year; in every year: »He takes a yearly trip to the mountains from his home in the city. 2. lasting a year: »The earth makes a yearly revolution around the sun. 3. for a year: »He is paid a yearly… …   Useful english dictionary


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