Year length

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

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)
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 )

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:

the Julian and Gregorian calendars the normal year with 365 and the leap year with 366 days,
and derived from this the Julian year with 365.25 days and the Gregorian with 365.2425 days.
the Islamic lunar year with 12 lunations = 354 or 355 days
and other specifications, see calendar invoice .

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.

literature

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