but 1600 and 2000 are leap years. Difference between Tropical year and Sidereal year is 20 minutes 24 seconds or approximately 20.5 minutes/year, because of precession . Crude estimate of the precession time: 365.2422 days/20.5 minutes/year * 24 hours/1 day * 60 minutes/1 hour = 25,700 years.   Nutation 9.2 seconds of