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 ]

Calculation

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.

Symbol

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

References

  1. "SI Units" . www.iau.org . 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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