Sidereal day length and axial tilt for the 8 largest planets in our Solar System . Wiki: Sidereal time is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coordinates in the night sky. Briefly, sidereal time is a time scale that is based on Earth's rate of rotation measured relative to the fixed stars. . Viewed from the same location, a star seen at one position in the sky will be seen at the same