Tropical Year

Because the equinox slides backwards under precession, the seasonal year is about 20 minutes shorter than the orbital one — 365.242 19 days rather than 365.256 36. Every calendar is a rational approximation to that awkward number. The Julian rule of 365.25 is 11 minutes too long and drifts a day every 128 years; the Gregorian fix of skipping leap years in century years not divisible by 400 gives 365.2425, accurate to one day in 3200 years, at the cost of dropping ten dates in October 1582.

The tropical year is also slowly shortening, by roughly half a second per century, and it varies slightly depending on which equinox or solstice you start from. Astronomers therefore avoid it for dynamics and reserve it for exactly what it is good at: keeping the calendar in step with the seasons.