Sidereal and Tropical Years

The adjacent animation illustrates how the difference between the sidereal and tropical year results from the precession of the Earth's rotation axis. The sidereal year is the true year, referenced to the background stars. The tropical year is defined to be the time for two successive returns of the Sun to the vernal equinox. It is the year of the seasons and the year that the calendar is adjusted to as closely as possible.

The two years differ because of precession of the equinoxes. Since the celestial equator precesses because of precession of the rotation axis, the position of the vernal equinox shifts by about 50 arc seconds (50") over a year in the opposite direction of the Sun's motion on the ecliptic. Thus, the Sun returns to the vernal equinox about 20 minutes earlier each year than it would if there were no precession (the fraction

50"/360o = 50"/1,296,000"

of 365.25636 days is 20 minutes). Hence the tropical year, to which the seasons are tied, is about 20 minutes shorter than the sidereal or true year. Usage: Use "Play" to start and "Stop" to halt. "Home" returns to the beginning. "Step" moves forward through the animation one frame at a time; "Back" moves backward one frame.