Well, it's finally here. No, I don't mean the end of the year, or the profoundly significant holiday season, or even the annual ritual of crass commercialism. What I mean is we are at the time of the year - in this next week - when that sun finally stops getting lower in the sky. The winter solstice is here; there is no need to worry that the sun might continue to sweep the sky ever lower in a slow, daily, downward descent into the horizon, never to rise again, plunging our land into everlasting darkness.

It is a gift we get every December holiday season, this stopping of the sun, or in its Latinized form, the "solstice." And most readers of this column will recall that it is the tilt our planet has with respect to the sun that allows these solstices to occur. At the moment we in the Northern Hemisphere are tilted far over and away from the sun so that the days are shortened and it is colder.

What about other planets? Does tilt play any important role there with their surface conditions? Not like it does here.
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Mercury is barely tilted at all. But it is so close to the sun that it has more important things to worry about, like days that are 800 degrees Fahrenheit and nights that plunge to -280.

Venus is slightly tilted with respect to the sun, but rotates so slowly and has such a inconceivably thick atmosphere, that tilt plays almost no role there, either.

Let's skip out to the giant planets. There, distance and spin rate and ultra thick atmospheres play such an overwhelmingly huge role that the tilts of those planets don't make much of a difference. And the Pluto-like bodies way, way out there get such a small amount of solar energy that, tilt or not, they are not comfortable places to live.

The only place outside of Earth where tilt might play a role worth considering is Mars, which is tilted much like Earth and experiences seasons like we do. The problem? There is no Moon to stabilize the planet, and over long periods of time Mars wobbles around like a flailing, failing top.

That leaves us. Our ideal tilt, along with our distance from the sun, our wobble-stabilizing Moon, our optimal atmosphere, our perfect combination of continents and oceans, and many other factors, allow us to enjoy these dark days of winter knowing we could really ask for nothing better. This list of design features allows us to experience the greatest amount of living space on a rocky body in the middle of space.

Observe for yourself the return of the sun in the next month or so as we slowly tilt back over. And remember, despite the cold and darkness now, spring is on its way.