 The sidereal year is the length of time that the Sun takes to return to the same position with respect to the stars of the celestial sphere.Precession causes the stars to change their longitude slightly each year, so the sidereal year is longer than the tropical year.Using observations of the equinoxes and solstices, Hipparchus found that the length of the tropical year was 365+1/4−1/300 days, or 365.24667 days (Evans 1998, p. 209).