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Winter begins in the northern hemisphere this afternoon, as the Sun appears farthest south in our sky for the year — a point known as the December solstice.
The seasons change because our planet is tilted with respect to its orbit around the Sun. Right now, we’re tilting “back” as viewed from north of the equator, so the southern hemisphere gets the most sunlight. Six months from now, the northern hemisphere will tilt sunward, giving us the longer, hotter days of summer.
If the axis weren’t tilted, we’d have no seasons at all. Any given spot on Earth would see the same amount of sunlight every day of the year.
Astronomers aren’t sure just why Earth is tilted, but the leading idea says it’s because our planet got a big “whack” early in its history. According to this theory, a planet as big as Mars slammed into Earth when the solar system was just a few million years old. The impact knocked our planet askew. It also vaporized the outer layers of both bodies, spraying them into space. Much of this material coalesced to form the Moon.
Other planets may have been knocked around, too. Uranus, the solar system’s third-largest planet, lies on its side. And Venus spins backwards. Again, there’s no definitive explanation for these orientations, but it’s possible that both were caused by giant collisions.
Incidentally, the December solstice occurs at 5:03 p.m. Central Standard Time — the start of winter in the northern hemisphere.
Script by Damond Benningfield, Copyright 2014