Lucky planet

Summer solstice: Did a rampaging planet create Earth's seasons? - CSMonitor.com

Of course I knew of the idea that it was a collision that formed the Moon, but I either didn't know (or had forgotten) that it may also have given us the Earth's very convenient axis tilt that gives us the seasons:

Without that tilt, we might've ended up with more seasons than we could deal with over the course of Earth's history, suggests Neil Comins, an astronomer at the University of Maine. He's written substantially on what alternate Earths might be like and how hospitable they would be for the emergence of life.

The collision thought to have generated the tilt also created the moon, which is responsible for stabilizing Earth's spin axis. Without a moon, "the Earth spinning on its axis is an unstable system in which the tilt goes from straight up and down to far closer to the plain of it orbit than the axis is today," he says.

At its most extreme, this could leave Earth orbiting the sun and spinning on its axis like a chicken roasting on a spit with head and tail alternately aimed at the fire, rather than cooking the bird broadside. Earthlings from one pole to the equator would be in darkness during the solstice, while the other half would be in toasty sunlight. The poles would swap lighting conditions at the next solstice.

Moreover, the moon's presence and its effect on the oceans act as a brake on Earth's rotation rate. By some estimates one turn of the Earth on its axis some 4.5 billion years ago would have taken 6.5 hours, versus 24 today. No moon means a far more speedy cosmic rotisserie.

I don't think the importance of the moon in keeping the axis tilt stable was something that I realised before. Very neat.