Earth's constant tilt

Last updated 11:15 13/10/2009

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Keith Ross of Taumarunui asks:

Why is the Earth's axis of rotation slightly off vertical compared with the Earth-Sun plane?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responds:

The sun and planets formed from a large cloud of dust and gas. At first, the dust gathered into tiny planets or planetesimals. Over time, the planetesimals gathered together and formed the bigger planets.

The closer a planetesimal was to the sun, the faster it orbited.

Planetesimals circling closer to the sun than a growing planet were most likely to hit the sunny side of the planet. They were also travelling more quickly and gave the sunny side a push in the forward direction.

Planetesimals from further out were going slower, mostly hit the dark side and pushed in the backwards direction.

If all the planetesimals were orbiting in the same plane, like marbles rolling around on a table, they would all hit the planet near the equator. The planet's spin axis would then be vertical to its orbit, like Jupiter's is now. But they weren't that well organised.

They hit the planet from different directions. A glancing impact near the pole of the planet would tilt the spin axis. So the tilts that most planets now have depends on how they were hit while they were growing.

Uranus must have had a really big off-centre impact, for its axis is tilted sideways. Earth did too. We think the debris from that impact made the Moon.

This explanation doesn't apply to the inner planets. For Mercury and Venus, the strong pull of the sun's gravity has straightened their spins to vertical. Fortunately for the Earth, the Moon's gravity keeps its tilt nearly constant.

Send questions to: Ask A Scientist, PO Box 31-035, Christchurch or email

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