The myth of Earth’s stable axis

DEC 01, 2011

David Waltham

Johanna Miller’s otherwise excellent article on the Martian icecap (PHYSICS TODAY, June 2011, page 12 ) was slightly marred by the opening paragraph, which repeated the myth that Earth, unlike Mars, has a stable axis because of our large moon. I’m afraid things are not that simple. The Moon does indeed have a stabilizing influence since its presence increases Earth’s precession rate thus avoiding chaotic resonant interactions with the rest of the solar system. ¹ However, increased tidal drag resulting from the Moon’s presence slows our rotation; that slowing in turn reduces Earth’s equatorial bulge and leads to slower precession and, eventually, to an unstable axis. ² Long-term axial stability is best achieved by rapid spin (to give a large equatorial bulge) and no moon (to reduce tidal drag).

References

1. J. Laskar, F. Joutel, P. Robutel, Nature 361, 615 (1993). https://doi.org/10.1038/361615a0
2. W. R. Ward, Icarus 50, 444 (1982). https://doi.org/10.1016/0019-1035(82)90134-8

More about the Authors

David Waltham. (d.waltham@rhul.ac.uk) Royal Holloway London.