Fig7_3 Precession

Fig7_3 Precession

Fig. 7.3 . The Earth’s rotation axis traces out a circle on the sky once every 26,000 years, sweeping out a cone with an angular radius of about 23.5 degrees. The Greek astronomer Hipparchus (lived c. 146 BC) discovered this precession in the 2nd century BC. The north celestial pole, which marks the intersection of this rotation axis with the northern half of the celestial sphere, now lies near the bright star Polaris, but as the result of precession the rotational axis will point towards another bright north star, Vega, in roughly 13,000 years. This motion of the Earth’s rotational axis also causes a slow change in the celestial coordinates of any cosmic object.

Copyright 2010, Professor Kenneth R. Lang, Tufts University