We have the seasons we do because of the tilt of our planet. It's 23.4 degrees out of vertical, which means the sun strikes it at different angles throughout the year.

The winter solstice occurs at the exact moment when the center of the sun crosses the Tropic of Capricorn -- the degree of latitude that runs just south of Rio De Janeiro. It's the farthest south the sun hits our planet directly.

Alan MacRobert , senior editor at Sky & Telescope Magazine, said this particular angle is due to the way, at the beginning of the solar system, that all of the space dust and debris eventually coalesced into Earth.

The nascent planet then survived a huge collision with another planet-like body that created our moon.

Of our closest neighbors, Mars and Saturn tilt at angles close to that of the Earth. Uranus, at 98 degrees, spins on its side. Mercury, at zero degrees, spins straight as a top.

MacRobert said Earth's angle has changed enough so that the icy poles as we know them on earth today may have moved.

"Fossil remains found at the poles show there was once a much different climate there," he said.

But without that tilt, he said, the world would be a far different place -- totally uniform.

"We'd have exactly 12 hours of sunlight and 12 hours of night, every day of the year," he said. "There would be no seasons. The only way you would be able to mark the seasons would be to follow the changes in the locations of the constellations in the night sky and almost no one would do that."

There would also be a much more uniform climate -- warmer in the tropics, colder at the poles, but much more temperate over much of the globe. Rivers would not flood in the spring because of melting snow. Prairies would not bake dry in the summer heat.

"It might be a more environmentally richer planet, because there wouldn't be the stress on things caused by the changes of the seasons," MacRobert said. "It might be ecologically poorer because stress and conflict brings about change and growth."

And for humans, living since time immemorial in years of spring, summer, fall and winter, life would lack some drama and mystery.

"I like the seasons myself," MacRobert said. "I think they make life much more interesting."

Contact Robert Miller

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