Year
A unit of time based on Earth's orbit around the Sun.
A year measures the time Earth takes to complete one orbit around the Sun. More precise scientific definitions include the tropical year (roughly 365 days, 5 hours, 48 minutes, and 45 seconds) and the sidereal year (about 20 minutes longer). The Gregorian calendar approximates the tropical year by adding leap years. The word "year" also describes other cycles of similar length, like the lunar year (about 354 days based on twelve lunar phases), and periods tied to the calendar or astronomy, such as the seasonal, fiscal, or academic year.
Earth's axial tilt causes the seasons over the course of a year, marked by changes in weather, daylight hours, and effects on vegetation and soil. Temperate and subpolar regions generally recognize four seasons: spring, summer, autumn, and winter. In tropical and subtropical areas, some regions lack defined seasons, but seasonal tropics track annual wet and dry seasons. The term "year" can also apply to the orbital period of any astronomical object around its primary, like a Martian year of roughly 1.88 Earth years. It may refer to any long cycle, such as the Great Year.
A calendar year typically approximates the number of days in Earth's orbit, as counted in a given calendar. The Gregorian calendar, the modern world standard, has either a common year of 365 days or a leap year of 366 days, similar to the Julian and Solar Hijri calendars. Over its 400-year leap cycle, the Gregorian calendar's average year length is 365.2425 days (97 leap years). Lunisolar calendars, traditional in Asia, have twelve lunar months but add a thirteenth leap month every three years or so. The lunar Hijri calendar (often called the Islamic calendar in the West) has twelve lunar months and disregards the solar year.
In English, "year" is commonly abbreviated as "y" or "yr." The symbol "a" (from Latin *annus*) appears in scientific literature, though its exact duration can vary.
The English word "year" (from West Saxon *ġēar*, Anglian *ġēr*) continues Proto-Germanic *jǣran*. Cognates include German *Jahr*, Old High German *jār*, Old Norse *ár*, and Gothic *jer*, from the Proto-Indo-European noun *yeh₁r-om* meaning "year, season." Other cognates include Avestan *yārǝ*, Greek *hṓra* (source of "hour"), Old Church Slavonic *jarŭ*, and Latin *hornus* ("of this year"). Latin *annus* comes from PIE *h₂et-no-, also yielding Gothic *aþn*. Many English words derive from Latin *annus*, such as annual, annuity, and anniversary; *per annum* means "each year," and *annō Dominī* means "in the year of the Lord." The Greek word *ἔτος* ("year") is cognate with Latin *vetus* ("old"), from PIE *wetos-*, also seen in Sanskrit *vat-sa-ras* and *vat-sa-* ("yearling"), and Latin *vitulus* ("bull calf"), English *wether* ("ram"). Some languages count years by referencing seasons, like "summers" or "winters." Chinese *年* ("year") originally depicted a person carrying wheat, meaning "harvest." Slavic uses *godŭ* ("time period; year") and *lěto* ("summer; year").
Astronomical years do not have an integer number of days or lunar months, so any calendar following an astronomical year needs intercalation, such as leap years. In the Julian calendar, the average year length is 365.25 days, with a leap year every fourth year adding a leap day to February. In astronomy, the Julian year is defined as exactly 365.25 days of 86,400 seconds each, totaling 31,557,600 seconds. The Revised Julian calendar, proposed in 1923 and used by some Eastern Orthodox Churches, has 218 leap years every 900 years, giving an average year of 365.2422222 days—close to the mean tropical year of 365.24219 days. In the year 2800 CE, the Gregorian and Revised Julian calendars will differ by one day. The Gregorian calendar aims to keep the northward equinox on or before March 21, following the tropical year. With 97 leap years in 400 years, its mean year is 365.2425 days, with a relative error below one part per million.
- Definition
- Unit of time based on Earth's orbit of the Sun
Lore & Background
The term 'year' derives from Proto-Germanic *jǣran and Proto-Indo-European *yeh₁r-om, with cognates in German, Old Norse, Greek, and Latin. Latin annus, from PIE *h₂et-no-, yields English words such as annual, annuity, and anniversary. The Greek word for year, ἔτος, is cognate with Latin vetus 'old', from PIE *wetos- 'year'. In some languages, years are counted by referencing seasons, as in 'summers' or 'winters', or harvests, as in Chinese 年, originally a pictograph of a person carrying wheat.
Reader's Guide
The year is defined by Earth’s orbit around the Sun, with the tropical year (about 365 days, 5 hours, 48 minutes, 45 seconds) and the slightly longer sidereal year being precise scientific measures. The modern Gregorian calendar approximates the tropical year through a leap-year system, averaging 365.2425 days over 400 years. The Earth’s axial tilt causes the seasons—spring, summer, autumn, and winter in temperate regions, and wet and dry seasons in the seasonal tropics—which in turn affect weather, daylight, vegetation, and soil fertility. The term ‘year’ also describes the orbital period of any astronomical body around its primary, such as the Martian year of roughly 1.88 Earth years. Beyond astronomy, the year is adapted into fiscal, academic, and seasonal cycles, and into calendar systems like the lunar year (about 354 days) or lunisolar calendars that intercalate a thirteenth month. The Julian calendar used a mean year of 365.25 days with a leap day every fourth February, while the Revised Julian calendar, with 218 leap years per 900 years, achieves a mean year length very close to the tropical year. The Gregorian calendar specifically follows the northward equinox year, ensuring the equinox falls near March 21. The word ‘year’ derives from Proto-Indo-European roots meaning “to go” or “move,” and cognates appear across many languages, including Latin *annus*, Greek *hōra*, and Sanskrit *vatsa*. In some cultures, years are counted by seasons, such as “summers” or “harvests,” as seen in the Chinese character for year, which originally depicted a person carrying wheat.
Did You Know?
- The word 'year' derives from Proto-Indo-European *yeh₁r-om, meaning 'year, season'.
- A Martian year is roughly 1.88 Earth years.
Mythic Origins and Dynastic Standardization
The story of Chinese measurement begins in legend. The Liji credits the Yellow Emperor with devising the first units, while the Xiao Erya and Kongzi Jiayu trace length standards to the human body itself. The Records of the Grand Historian notes that these body-based measures created inconsistency across regions, prompting Yu the Great to impose a unified length system. Archaeological evidence supports early formalization: rulers bearing decimal units have been recovered from Shang dynasty tombs, confirming that base-10 numeration was already in use by that era. The Zhou dynasty saw the king grant measurement authority to various nobles, which fragmented standards from state to state. It was not until Qin Shi Huang unified China that a single set of measurement units was imposed nationwide. The Han dynasty inherited these Qin standards and recorded them systematically in the Book of Han, establishing a documentary tradition that later dynasties would follow when proclaiming their own official measures and cataloguing their predecessors' systems.
The Market System and Metric Integration
A distinctive feature of Chinese measurement is the coexistence of decimal numerals, in use since the Shang, with several units built on a hexadecimal (base-16) structure. The modern market system (shìzhì) emerged from a deliberate policy of blending tradition with metrication. In 1915, the Beiyang government issued a law adopting the metric system as standard while also codifying a Chinese-style set based on Qing dynasty definitions. The Nationalist government followed in 1929 with the Weights and Measures Act, which made metric the official standard and confined the newer market-use units—defined as rounded fractions of metric values—to private trade, effective 1 January 1930. The People's Republic continued this dual system via a 1959 State Council decree, redefining one catty as exactly 500 grams and splitting it into ten new taels rather than the old sixteen, while exempting Chinese prescription drugs from conversion to avoid dosing errors. A 1984 decree set a 1990 deadline for full metrication, though farmland measures were granted an extension pending further study.
Ancient Units and Their Internal Logic
The traditional Chinese measurement vocabulary spans length, mass, volume, and time, each with its own internal structure. For length, the chi, bu, and li form a hierarchy whose ratios shifted across dynasties: one bu could equal five or six chi, and one li could comprise 300 or 360 bu. Mass units cluster around the jin (catty), with smaller and larger derived units filling the scale. Volume is perhaps the most distinctive: the Book of Han defines the fundamental unit as the yuè, the space occupied by 1,200 grains of proso millet. One hundred yuè make a gě, and ten gě make a shēng (roughly 0.2 liters). Larger dry measures included the dǒu, hú, and shi or dàn. These grain-based volume units also served as salary denominations for imperial bureaucrats, linking measurement directly to governance. By the Qing dynasty, the shēng had converged closely with the modern metric liter. Small units such as lí and fēn appear across length, area, and mass, but their magnitudes differ depending on the quantity they measure.
Regional Divergence in the Modern Era
Today, the legacy of the market system plays out differently across Chinese-speaking territories. On the mainland, the catty is fixed at exactly 500 grams, and the naming convention distinguishes traditional units with the character shì (market) from metric equivalents marked with gōng (common or public) to prevent confusion. Taiwan, having had its traditional units standardized to Japanese values during the colonial period, now uses a metric-based system; its ping, for instance, equals approximately 3.306 square meters, derived from the square ken. The Hong Kong Special Administrative Region retains its traditional units in active legal use, with the catty defined at precisely 604.78982 grams under local statute. A 1976 Metrication Ordinance permitted gradual adoption of SI units, and as of 2012 all three systems—metric, Imperial, and Chinese—remain legal for trade. Macau's 1992 law allowed Chinese, Imperial, and US customary units as secondary to SI for a transitional period. The islands of Kinmen and Matsu, under Republic of China administration, still retain the 1930 market units.
Frequently Asked Questions
Who is Year?
Year is a fundamental unit of time defined by the period Earth takes to complete one full orbit around the Sun. It sits at the heart of how we structure seasons, agriculture, and civil calendars across the globe.
What are Year's powers/role?
Year serves as the backbone of our timekeeping system, anchoring the Gregorian calendar and giving structure to months, seasons, and long-term planning. In scientific contexts it also appears in two refined forms—the tropical year and the sidereal year—each tuned to a slightly different orbital reference.
Why is Year important?
Year is the natural rhythm around which human societies organize planting, harvests, education, and governance, making it far more than an abstract number. It also connects to related periods like the lunar year and the seasonal year, showing how one orbital cycle can be interpreted through many cultural and scientific lenses.
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