Autumn is nearly here.

This year, the season officially begins on Sept. 22 at 8:45 a.m. PDT (11:45 a.m. EDT). Many people think that the beginning of autumn occurs when the sun rises due east and sets due west, and when the length of our day and night are equal. And, as long as we don't concern ourselves with fine details, those are both reasonably true statements.

So why, then, do we often cite the exact time for the beginning of autumn?

That's because astronomers know t ...

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