Finally, over very long periods the positions of the stars themselves change, because of the stars' proper motions. The Earth's axis is also subject to other complex motions which cause the celestial poles to shift slightly over cycles of varying lengths (see nutation, polar motion and axial tilt). Because of a phenomenon known as the precession of the equinoxes, the poles trace out circles on the celestial sphere, with a period of about 25,700 years. Despite their apparently fixed positions, the celestial poles in the long term do not actually remain permanently fixed against the background of the stars. The celestial poles are also the poles of the celestial equatorial coordinate system, meaning they have declinations of +90 degrees and −90 degrees (for the north and south celestial poles, respectively). As Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other celestial points appear to rotate around them, completing one circuit per day (strictly, per sidereal day).
The north and south celestial poles appear permanently directly overhead to observers at Earth's North Pole and South Pole, respectively. The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The beginning of the four " astrological ages" of the historical period are marked with their zodiac symbols: the Age of Taurus from the Chalcolithic to the Early Bronze Age, the Age of Aries from the Middle Bronze Age to Classical Antiquity, the Age of Pisces from Late Antiquity to the present, and the Age of Aquarius beginning in the mid-3rd millennium. Diagram of the path of the celestial north pole around the ecliptic north pole.
0 kommentar(er)
