Year

• 1. Introduction

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  A year is one trip of the Earth around the Sun, and yet no two definitions of it quite agree. The tropical year runs about 365 solar days, 5 hours, 48 minutes, and 45 seconds. The sidereal year runs roughly 20 minutes longer than that. The calendar hanging on your wall ignores both of those exact figures and rounds to a whole number of days. To paper over the difference, it adds a leap day every so often. So how many kinds of year are there, and why do scientists, accountants, students, and astronomers each measure the same stretch of time differently? The answers reach from the wheat-bundle picture buried inside a Chinese character to a Galactic year of roughly 230 million Earth years.

• 2. Marking The Seasons

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  The Earth's axial tilt is the reason a year feels like anything at all. As the planet circles the Sun, that tilt sends the seasons rolling past, with shifts in weather, in the hours of daylight, and in turn in vegetation and soil fertility. In temperate and subpolar regions, four seasons are usually named: spring, summer, autumn, and winter. Tropical and subtropical regions do not all share that pattern. Several geographical sectors there have no defined seasons at all. In the seasonal tropics, people instead track an annual wet season and dry season. This habit of counting by season left fingerprints on language itself. Some tongues count years as summers, winters, or harvests. The Chinese character for year, originally written as a person carrying a bundle of wheat, denoted the harvest. Slavic uses a word meaning summer to mean year, a reminder that a year was first something you watched arrive over the fields.

• 3. A Word That Means To Move

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  English year traces back through West Saxon and Anglian forms to Proto-Germanic, and its cousins crowd the family tree: German Jahr, Old High German jar, Old Norse ar, and Gothic jer. They all descend from a Proto-Indo-European noun meaning year or season. From that same root branch Avestan yare, Greek hora meaning year, season, or period of time, and the source of the English word hour. Latin took a different path to the same idea. Its word annus comes from a separate Proto-Indo-European noun, one that also gave Gothic its word for year. Linguists suspect both ancient words for year ultimately come from verbs meaning to go or to move, fitting for something defined by the Earth's travel. From annus English inherited annual, annuity, and anniversary, plus the phrases per annum, meaning each year, and Anno Domini, meaning in the year of the Lord.

• 4. The Calendar Compromise

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  Astronomical years refuse to contain a whole number of days or lunar months, so every workable calendar needs a trick called intercalation. The Julian calendar set its mean year at 365.25 days, inserting a leap day into February every fourth year. The Gregorian calendar refined that aim, trying to keep the northward equinox on or shortly before March 21. Because only 97 of every 400 years are leap years there, its mean year runs 365.2425 days. The Revised Julian calendar, proposed in 1923 and used in some Eastern Orthodox Churches, goes further still with 218 leap years every 900 years, giving a mean of 365.2422222 days. That precision has a payoff with a date attached: in the year 2800 CE, the Gregorian and Revised Julian calendars will first differ by a single calendar day. Other systems keep the Moon in view. Lunisolar calendars run twelve lunar months and splice in a thirteenth leap month every three years or so, a method the Hebrew and various Hindu calendars still keep for liturgical reasons.

• 5. Counting From An Epoch

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  A calendar era pins a number to each year by choosing a reference event, called the epoch, as its starting point. The Gregorian era, the world's most widely used civil calendar, takes its epoch from a 6th century estimate of the birth of Jesus of Nazareth. Two notations share that scale: Anno Domini, abbreviated AD, and Common Era, abbreviated CE, the latter preferred by many of other faiths and none. Year numbers here use inclusive counting, which is why there is no year zero, only AD and CE running forward and BC and BCE running back. Astronomers found that gap inconvenient and built their own scheme. In astronomical year numbering, the number 0 stands for 1 BC/BCE, and -1 stands for 2 BC/BCE. Other systems anchor elsewhere entirely. Ancient Rome counted Ab Urbe Condita, from the foundation of the city. The Hebrew calendar counts Anno Mundi, the year of the world. The Islamic Hijri year counts from the Hijrah and, running on twelve lunar months, falls short of a solar year.

• 6. Years For Money And School

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  A fiscal year is a twelve-month window for drawing up annual financial statements, and the law rarely insists it match the calendar. The proof is in how widely the start date scatters. In Canada and India the fiscal year opens on April 1. In the United Kingdom it begins April 1 for corporation tax but April 6 for personal taxation and state benefits. Australia starts on July 1, and the United States federal government starts on October 1. The academic year wanders just as much across the globe. In much of the Northern Hemisphere, school runs from August or September to May, June, or July, with a summer break wedged between. In Israel it opens around October or November, aligned with the second month of the Hebrew calendar. India normally runs June 1 to May 31, Nepal starts July 15, and Australia's year roughly tracks the calendar because the southern hemisphere takes its summer from December to February. Even the count of teaching days diverges: about 180 in the United States, 190 for state-school pupils in Canada, New Zealand, and the United Kingdom, and 200 in Australia.

• 7. The Astronomer's Many Years

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  365.256363004 days is how long the sidereal year takes, measured against the fixed stars, the Latin sidera. The tropical year falls about 20 minutes shorter because of the Earth's axial precession, since it tracks the Sun's ecliptic longitude climbing a full 360 degrees through the seasons. The anomalistic year, timed between perihelion passages, averages 365.259636 days, the gap reflecting that the Earth's orbit is an ellipse with a nearest point and a farthest point. Eclipses keep their own clock. The draconic year, at 346.620075883 days, measures the Sun's return to the same lunar node, the place where the Moon's orbit crosses the ecliptic, and it splits into two eclipse seasons. Stars supply still more definitions. The Sothic year is the heliacal year of Sirius, currently very close to the Julian year of 365.25 days. The Besselian year is named for the 19th-century German astronomer and mathematician Friedrich Bessel, and begins when the fictitious mean Sun reaches an ecliptic longitude of 280 degrees, near January 1. The Julian year itself, fixed at exactly 365.25 days of 86400 SI seconds, is what astronomers use to define the distance of a light-year.

• 8. When The Year Itself Drifts

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  The exact length of an astronomical year changes over time, pushed by forces no calendar can freeze. Precession drags the equinoxes and solstices westward against the stars, while gravity from the other planets nudges Earth's apsides the other way. Because the Earth speeds up and slows down depending on its place in orbit, the interval between equinox passages can shrink or grow from year to year. Tidal drag adds a slower creep. As the Moon and Sun pull on the Earth, they lengthen both the day and the month, and since civil life measures the year in mean solar days, those days are now around 86400.002 SI seconds and getting longer. Scientists chasing precision tried to cut the year free from the day entirely. In 2011 a task group of the IUPAC and the IUGS recommended defining the geological annus as 31556925.445 seconds, a multiple of the SI second rather than any orbit. As of April 2025 the IUPAC Green Book gives 31556925.9747 seconds. Stretch that unit upward and the scale turns staggering: the formation of the Earth at roughly 4.54 Ga, the age of the universe near 13.8 Ga, and one full lap of the Solar System around the Galactic Center, a single Galactic year of roughly 230 million Earth years.