Lunar Mission

IMAGINE WHAT WOULD HAPPEN IF NEXT YEAR'S WINter were 100 degrees colder than normal or the summer 150 degrees warmer. Such swings would wipe out animals and plants; in fact, if Earth's climate were that irregular, life might never have arisen. But the seasons are relatively stable because Earth's axis of rotation stays at a constant 23 degrees tilt from the vertical. In July, for example, the sunward-leaning northern hemisphere experiences summer while the southern hemisphere, angled away from the sun, has winter. Six months later, the opposite is true.

Now it appears that the regularity of Earth's seasons is a lucky accident, a side effect of the moon. New computer simulations reported in Science and ; Nature have shown that the tilt of a planet like Mars, which has no large moon, will wobble dramatically over periods of a few million years. The gravitational influence of Earth's moon, by contrast, stabilizes our planet. So if life never arose on the Red Planet, one reason may be lack of a suitable satellite.