Autumnal Equinox II

StarDate: September 22, 2009

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If our planet had a little better posture, then we probably never would have invented the word "season." That's because if Earth sat perfectly upright -- the equivalent of having its shoulders straight and its eyes cast forward -- there would be no seasons.

Instead, though, Earth's axis tilts about 23 degrees. And because of that, the northern and southern hemispheres receive varying amounts of sunlight throughout the year.

At the summer solstice back in June, the north pole dipped toward the Sun, so the northern hemisphere received a lot of sunlight. In fact, at the pole itself, the Sun remained in the sky around the clock.

And at the December solstice, the south pole will nod toward the Sun, so the southern hemisphere will get most of the sunlight.

Halfway between the solstices, neither pole aims toward the Sun, so the entire planet sees roughly equal amounts of daylight and darkness. These halfway points are known as the equinoxes.

And we're at one of those points today -- the autumnal equinox, which marks the beginning of fall here in the northern hemisphere, and spring in the south.

Over the next three months, as the south pole begins to point sunward, the Sun will move farther south in the sky. As a result, we'll see less and less sunlight here in the north. And in December, we'll see the shortest day of the year -- the day that marks the start of winter -- one of our planet's four seasons.

Script by Damond Benningfield, Copyright 2009

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