Due to its rotation Earth is flattened at its poles. The gravitational attractions of the Sun, Moon and the other planets attempt to pull Earth’s equatorial bulge into the plane of the ecliptic. The main effect of this action is that Earth’s axis is forced to undergo a precession – a cone-shaped rotation, both north and south, the movement of which is similar to the motion of a spinning top. This wobbling motion does not affect the tilt angle of Earth but simply varies the direction in which Earth’s axis is pointing. Superimposed on precession is the much smaller oscillation nutation; analogous to the nodding of a spinning top, which causes the tilt of Earth’s axis to rock back and forth and has a period of ≈18.6 years.
Each end of Earth’s axis thus traces out a loop in the sky at an angular radius of ≈23.45°, moving at ≈50.3″ per year along each loop and completing a circuit in ≈25,700 years (≈1° or ≈1 day every 71 years).
The changing position of Earth’s axis makes the stars’ apparent centre of rotation appear to be moving anticlockwise around the loop. At present, Earth’s axis points north to ≈1° of Polaris, the Pole Star; and south to ≈1° of Sigma Octantis. Because precession and nutation cause celestial coordinates to change with time, the coordinates of celestial objects must be referred to a given date or epoch. The epoch in common use is 12 noon 1 January 2000 (written as J2000.0).
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