how long sidereal period
One sidereal day is the time it takes for a star in the sky to come back to the same place in the sky. Because, for all intents and purposes, the sky is âfixedâ, a sidereal day is when the earth rotates 360Â°. A sidereal day is 23 hours 56 minutes and 4.09 seconds long. A sidereal year is the time it takes for the sun to return to the same position with respect to the stars. Due to the precession of the equinoxes the sidereal year is about 20 minutes longer than the tropical year.