A point of reference that is independent from the observer's location is offered with the Celestial Pole. All stars seem to rotate arround it due to the earth's rotation. As the earth's axis is in circular motion itself, the Celestial Pole rotates around the Ecliptic Pole once in about 28850 years. In the equator system the celestial equator that is standing rectangular to the celestial pole is used as second point of reference. A star's position is defined by its height over the equator and angle to the vernal equinox. Since the vernal equinox is defined as a point on the celestial equator, it moves on the ecliptic as the celestial pole rotates.