Question

I'm really confused about something I heard on PBS's Nova "Origins", and also found on your website. According to both, the moon stabilizes the tilt angle of Earth's rotation. So what happened to conservation of angular momentum? As far as I know, conservation of angular momentum should keep the Earth's rotational axis pointing in the same direction, unless Earth is acted upon by something else. This is the law that makes gyroscopes work. Why would the Earth, with all of its billions of tons of rotating mass, be exempt from this law without the moon?

Angular momentum in a system can be conserved even if the direction of rotation changes. This is what happens to a top that precesses as it spins under the force of gravity. The gravity of all the other members of the solar system can act to change the axial tilt for any planet over long periods of time. Thus is it possible for a planet to change its spin direction without violating any physical laws. Whether it will change, and by how, much, is a complicated problem, however. In 1920 Serbian scientist Milutin Milankovich first showed how such changes in the Earth's axial tilt could have caused the ice ages. Today large computer simulations are used to study these changes. The presence of the Moon does, indeed, limit the size of such excursions for the Earth. On Mars, in contrast, the changes in tilt are at least a factor of two larger. Whether this stabilizing effect from the Moon has been critical for the course of evolution of higher life on Earth, however, is disputable.

David Morrison
NAI Senior Scientist
October 7, 2004