Year length

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The term length of the year is of astronomers and earth scientists used

  • for the duration of the year with regard to several coordinate systems,
  • as well as an indication of their long-term change.

Both aspects related to the solar year (the duration of the circulation of the earth around the sun) and the period of Earth's rotation with respect to (more precisely Ephemeridentage each with 86,400 the sun Ephemeridensekunden ). Astronomically, at least three values ​​must be distinguished, with regard to the ecliptic, the starry sky or the point of the earth's orbit closest to the sun :

Astronomical years

  1. Tropical year : the period during which the sun's mean ecliptical longitude increases by 360 °. For epoch 2000.0 it is exactly 365.24219052 days = 365 d 5 h 48 min 45.261 s and is slightly shorter than according to the definition valid until 1955 (from one spring equinox to the next)
  2. Sidereal year : the period of rotation of the earth in inertial space , i.e. de facto with reference to the fixed stars . It is currently 365.2563604167 days = 365 d 6 h 9 min 9.54 s. The 20.4 minute difference comes from the fact that the earth's axis slowly changes direction (see precession )
  3. Anomalistic year : the time between two perihelion passages of the earth. Because the perihelion is slowly shifting, this year with 365.259635864 days is about 4.7 minutes longer than the sidereal year.

All three of these years are changing very slowly :

  • First, because they are calculated in days and the earth's rotation slows down by about 0.0017 seconds per century ,
  • second, through secular changes in the planetary orbits, and
  • thirdly, due to the sun's gradual loss of mass as a result of nuclear fusion (hydrogen to helium) inside.
  • It is still unclear whether other small effects ( gravitational constant , contraction of celestial bodies, interplanetary medium, etc.) exist.

Calendar calculation and historical development

From the astronomical values, the various cultures have defined the lengths of the year rounded to days:

How the knowledge of the "true" year length developed historically is shown in summary:

  • 365 days in the younger stone age
  • with the Babylonians 365.15 to 365.27 days
  • at Ptolemy's (200 BC) 365d 5h 55m
  • with the Arabs (9th century) 365d 5h 46m 24s
  • from Kepler (1620) to the Paris Academy (around 1780) 365d 5h 48m 45s.


  • Jean Meeus , D. Savoie: The history of the tropical year . In: Journal of Brit. Astron. Assoc. Volume 102/1, London 1992
  • JL Simon et al .: Numerical expressions for precession formulas and mean elements for the Moon and the planets . Astronomy and Astrophysics, vol. 282, pp. 663-683 (1994) bibcode : 1994A & A ... 282..663S
  • IAU: Explanatory Supplement of the Internat. Astron. Union as amended
  • Jeffrey Bennett, Megan Donahue et al .: Astronomy. The cosmic perspective (Ed. Harald Lesch ), 5th edition, chapter time measurement. Pearson Studium Verlag, Munich 2010, ISBN 978-3-8273-7360-1