The astronomical year is opposed to the civil year (or calendar year ), which to measure correctly was one of the main tasks that the astronomical experts of the ancient civilized peoples tried to solve. The difficulty lies in the fact that the solar year is not a multiple of whole days . The year of the Egyptians originally had 365 days and was therefore six hours too short. Therefore, since the middle of the 3rd century BC they added A leap day in every fourth year . Caesar adopted this method in the Julian calendar that was named to him . However, this made the year eleven minutes too long, and this mistake had already added up to ten days in the 16th century. The Gregorian calendar (according to Pope Gregory XIII ) from 1582, which is still valid today, compensates for this error by eliminating the leap days in every full century ( secular year ) - except in those whose year can be divided by 400. The Gregorian year is therefore 365 days, 5 hours, 49 minutes and 12 seconds, which is 26 seconds longer than the tropical year . However, this difference only increases to a full day in more than 3,000 years.
The year is the period in which all seasons change once. Originally this period was calculated as the lunar year and later as the solar year. The lunar year is the sum of twelve lunar orbits, calculated from full moon to full moon, but only 354 days.
For this reason, the astronomically calculated year was replaced by the civil year (calendar year), as the astronomical year is also determined in three different ways.
Sidereal year (solar year)
The sidereal year is the time between two successive passes of the sun through the same point on the ecliptic , i.e. the exact period of the earth's orbit around the sun.
The tropical year as the basis of our time calculation is the time between two successive passes of the sun through the spring equinox . Since the spring equinox moves westward on the ecliptic by 50.2 arc seconds every year, i.e. towards the sun, the sun reaches it before it completes its exact orbit. Therefore, the tropical year has:
- 365 days, 5 hours, 48 minutes and 45.261 seconds or 365.242 days
The anomalistic year is the time between two passes of the earth through the point of the earth's orbit closest to the sun . Since this point on the ecliptic advances an average of 11.5 arc seconds annually, the anomalistic year is slightly larger than the sidereal year, namely:
- 365 days, 6 hours, 13 minutes and 52.539 seconds or 365.2596 days
The astronomers also expect the Great ( Platonic ) year, d. H. with the orbital period of the celestial pole (pole of the extended earth axis) around the pole of the ecliptic with about 26,000 years.