Autumnal Equinox

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StarDate: September 21, 2009

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The long, hot summer is about to come to an end. Fall arrives tomorrow afternoon, as the Sun crosses the equator from north to south -- the autumnal equinox. Day and night are roughly equal lengths on the equinoxes, and they're the only days when the Sun rises due east and sets due west across the whole planet.

To be precise, the equinox marks the changing of the astronomical seasons -- seasons that begin at important points in the Sun's north-south trek through the sky.

Summer, for example, began with the solstice in June, when the Sun appeared farthest north for the year. And winter begins with the next solstice, in December, when the Sun is farthest south.

There are other ways to mark the seasons, of course. In many older calendars, a solstice or equinox was the middle of a season, not the beginning. Under those calendars, the changing of the seasons was celebrated at the cross-quarter days -- days like Halloween.

And in today's world, many families follow the school calendar, even though there can be a big difference from school to school.

But most of the world follows the astronomical seasons. They provide exact starting times, so everybody knows precisely when the seasons change. And they're based on physical events that can't be changed at a whim.

So the astronomical seasons are sort of like standard time zones -- they keep all of us synced up to the same calendar.

More about the equinox tomorrow.

Script by Damond Benningfield, Copyright 2009

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