A steadier axis

The evidence

With the Moon, Earth’s tilt stays within about ±1.3° of 23.3°. Laskar’s team found that without it the tilt could wander chaotically from 0 to about 85°. Later simulations by Lissauer’s team find the tilt of a moonless Earth typically swinging over a span only 20 to 25° wide, for hundreds of millions of years.

Key studies

• Stabilization of the Earth's obliquity by the Moon Laskar J., Joutel F., Robutel P.·Nature·1993 With the Moon the tilt stays within about ±1.3°; without it, it could wander chaotically from 0 to about 85°.
• Obliquity variations of a moonless Earth Lissauer J. J., Barnes J. W., Chambers J. E.·Icarus·2012 Without the Moon the tilt would typically swing over a span 20 to 25° wide for hundreds of millions of years, far less than feared.