Time based on the rotation of the Earth with respect to the background of fixed stars. Astronomers generally use sidereal time rather than solar time because it is better suited to observations beyond the solar system. &diamf3; A sidereal day is the time required for one complete rotation of the Earth on its axis with respect to a fixed star. It is an unvarying unit equal to 23 hours, 56 minutes, 4.09 seconds of solar time. &diamf3; A sidereal month is the average period of revolution of the Moon around the Earth with respect to a fixed star, equal to 27 days, 7 hours, 43 minutes of solar time. &diamf3; A sidereal year is the time required for one complete revolution of the Earth around the Sun with respect to a fixed star, equal to 365 days, 6 hours, 9 minutes, 9.54 seconds of solar time.