Year - Año

Poster showing the change of year between 2018 and 2019 , both belonging to the Gregorian calendar .

One year , Earth year , sidereal year sidereal or Julian year (symbol: a) [ 1 ] refers to the orbital period of the planet Earth , ie the time it takes that planet to complete one lap around the sun . The term year is also used to refer to the orbital period of any planet and, by extension, applies to other orbital cycles. [ 2 ]


The simplicity of the definition hides the complexity of its computation.

Various years are considered according to the reference chosen to determine its duration:

The year is sidereal or sidereal year when:

  • The time that elapses between two consecutive steps of the Earth through the same point of its orbit. Generally used by astronomers , it is the most accurate measurement of a year.
  • Reference: the stars.
  • Duration: 366.255936 sidereal days , or 365.256 363 004 average solar days , 365 days 6 hours 9 minutes 9.76 seconds, that is, minus half (1/2) solar day.

The tropic year, solar year, or tropical year when:

  • The time elapsed between two successive steps of the Sun through the mean equinox . In other words, spring to spring time for example. Reference: vernal equinox. The Mesopotamian and Egyptian civilizations achieved values ​​very close to the real one for the tropic year, especially considering the scarcity of means of the time.
  • Duration: 365,242 190 402 average solar days, 365 days 5 hours 48 minutes 45.22 seconds.

Calendar year when:

  • The number of full days considered as a year for civil or religious purposes. To fit in with astronomical cycles, which import fractions of a day, this reckoning of time varies each year.

Note: Civilly the tropical year and the calendar year are used, always with exceptions: Islamic civilization , for example, is governed by a lunar calendar .

Other definitions

In astronomy and astrophysics , in addition to the sidereal year are of interest:

  • The anomalistic year : time elapsed between two successive passes of the Earth by the perihelion of its orbit (365.2596 days)
  • The cosmic or galactic year : time spent in an orbit of the Sun around the center of the Milky Way (about 220 million years)
  • The Platonic year : time in which the Earth's axis describes a complete circle on the celestial sphere due to precession (about 25,800 years)
  • The eclipse year : time between two successive passages of the Sun through a node in the Moon's orbit , at which time an eclipse can occur .

Historical evolution of the solar year

Egyptian year
He estimated that a year lasted 365 days. The Egyptian calendar suffered very notable time lags and it was tried to reform during the Congress of Canope , where it was concluded that the year lasted 365.25 days.
Julian solar year
Based on the Cánope Congress , he estimated that a year lasted 365.25 solar days, or 365 days and 6 hours. He also considered that 366 solar days would be counted every four years. That fourth year was called a leap year because the ancient Romans counted the date February 24 twice.
Gregorian solar year
It is the calendar year. It is a correction in the Julian solar year count, because in 1582 some astronomers discovered a gradual time lag in the Julian calendar . After a long investigation, they concluded that a year lasted approximately 365.2425 solar days, that is, 365 days, 5 hours, 49 minutes and 12 seconds. It was also determined that secular years could be leap years , only if they were divisible by 400.


The symbol established for the year in ISO 80000 is a (from Latin annus ). From it can be derived, with the use of multiples, Ma (megayears, millions of years) and ka (kiloyears, thousands of years).

See also


  1. "SI Units" . . International Astronomical Union (IAU) . Retrieved April 4, 2017 .
  2. ^ Elisa de Castro (March 1, 2020). Sidereal year . Archived from the original on December 25, 2014 . Retrieved December 25, 2014 .

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