The mean interval between vernal equinoxes.However, the interval from a particular vernal equinox to the next may vary from this mean by several minutes. The tropical year differs from the solar year by one part in 26,000, since this is the period of the Earth's precession about its rotational axis combined with precession of the perihelion of the Earth's orbit.

In 1994-98, the tropical year was 365.242190 days long. An expression based on the orbital elements of Laskar (1986) for calculating the length of the tropical year is given by

where

and JD is the Julian date (Doggett 1992).