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A year is the orbital period of the Earth moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by changes in weather, the hours of daylight, and consequently vegetation and fertility. In temperate and subpolar regions, generally four seasons are recognized: ''spring'', ''summer'', ''autumn'' and ''winter''. In seasonal tropical and subtropical regions, the ''wet'' (''rainy'' or ''monsoon'') season and the ''dry'' season are generally recognised. A calendar year is an approximation of the Earth's orbital period in a given calendar. The Gregorian calendar (and Julian calendar) has a calendar year being either a common year of 365 days, or a leap year of 366 days. The average year length across the complete leap cycle of the Gregorian (modern) calendar is 365.2425 days. ISO 80000-3, in an informative (cf. normative) annex, proposes the symbol, ''a'', (for Latin ''annus'') to represent a year of either 365 or 366 days. In English, the abbreviations, ''y'' and ''yr'', are used. In astronomy, the Julian year is a unit of time, defined as exactly 365.25 days each of exactly SI seconds, totalling seconds.〔International Astronomical Union "(SI units )" accessed February 18, 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〕 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'' or ''Venusian year'' is the time in which Mars or, respectively, Venus 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== West Saxon ''ġēar'' (), Anglian ''ġēr'' continues Proto-Germanic '' *jǣran'' ('' *jē₁ *yeh₁rom'' "year, season". Cognates outside of Germanic are Avestan ''yārǝ'' "year", Greek "year, season, period of time" (whence "hour"), Old Church Slavonic ''jarŭ'' and Latin ''hornus'' "of this year". Latin ''annus'' (a 2nd declension masculine noun; ''annum'' is the accusative singular; ''annī'' is genitive singular and nominative plural; ''annō'' the dative and ablative singular) is from a PIE noun ', which also yielded Gothic ''aþn'' "year" (only the dative plural ''aþnam'' is attested). Both '' *yeh₁-ro-'' and '' *h₂et-no-'' are based on verbal roots expressing movement, '' *h₁ey-'' and '' *h₂et-'' respectively, both meaning "to go" generally (compare Vedic Sanskrit ''éti'' "goes", ''atasi'' "thou goest, wanderest"). The Greek word for "year", , is cognate with Latin ''vetus'' "old", from PIE '' *wetos-'' "year", also preserved in this meaning in Sanskrit ' "yearling (calf)" and ' "year". Derived from Latin ''annus'' are a number of English words, such as annual, annuity, anniversary, etc.; ''per annum'' means "each year", ''anno Domini'' means "in the year of the Lord". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Year」の詳細全文を読む スポンサード リンク
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