The myth of Earth’s stable axis

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).