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I was told that the axle of the Earth changes over time (very slowly), is that truth? If so, how will the Earth be affected and what impacts will that bring to the living kinds on the planet?

- Raymond Tsang (14.2.2001)

Yes. The motion you mentioned is called precession. The pointing direction of the Earth's rotation axle is wobbling through space. It takes about 25,800 years to complete one cycle.

The oblate shape of the Earth gives it an equatorial diameter 43 km longer than its polar diameter. This means that there is an excess of the Earth's material around its equator. The Sun, the Moon and, to a much lesser extent, other celestial bodies exert gravitational pull on the equatorial bulge and to swing the polar axis of the Earth in a circle, resulting the celestial north and south poles describe enormous circles against the stars.

Luckily, this motion will not cause any harm to the living things on Earth. Actually, noticeable effects are mainly astronomical: the pole star changes, and the position of the equinoxes and the solstices shifted westward.


CHAN Ki-hung
Asst. Curator, Hong Kong Space Museum