obliquity

Did you ever ride a Tilt-A-Whirl, one of those cheap carnival rides that makes you dizzy and sick? Our planet would be like that if its inclination were out of control. Without tilt stability, a new study reveals, we wouldn't be sick, we'd be dead, or never alive in the first place. It's not enough to be in the Habitable Zone. Would-be inhabited planets need to avoid a new problem, called “tilt erosion.”