Sonnenjahr

Sonnenjahr, or solar year in German, denotes the time interval in which Earth completes an orbit around the Sun as observed from Earth. In astronomical usage it most commonly refers to the tropical year—the period between successive vernal equinoxes and thus the cycle of the seasons. The mean length of the tropical year is about 365.24219 days (365 days, 5 hours, 48 minutes, 45 seconds).

The term can also be used more broadly to denote the solar orbital period, but it should not be confused with other year definitions. The sidereal year (the time for Earth to return to the same position relative to the fixed stars) lasts about 365.25636 days, while the anomalistic year (the interval between successive perihelion passages) is about 365.25964 days. These lengths differ by small amounts due to orbital mechanics and precession.

In civil calendars, the Sonnenjahr is approximated by the mean calendar year. The Gregorian calendar uses an average year length of 365.2425 days, achieved through its leap-year rule of 97 leap years every 400 years. This keeps the calendar year closely aligned with the tropical year, maintaining the seasonal pattern over long periods.

The concept reflects long-term astronomical changes, notably the precession of the equinoxes, which slowly shifts the equinox points relative to the stars and affects the long-term alignment between calendar dates and seasons.