A slice of the celestial pi.
The mean Tropical Year 365.24219 or 365.24217 days.
The solar calendars of the world organize the year into 365 days to approximate the Tropical Year. If it is important to keep the solar calendar in synch with the Tropical Year, then a leap day is added periodically.
The year is not a given, as there are various ways to measure and define the Earth’s orbit around the Sun relative to its rotation around its axis. If we use the ephemeris day, exactly 86,400 seconds, then the mean Tropical Year is 365.24219. If we use mean solar days, it works out to 365.24217.
Rethinking Time 