Autumnal Equinox: First Day of Fall Greeted by Full Moon

Sept. 22, 2010 — -- Tonight is a night of celestial coincidences. The autumnal equinox -- the beginning of fall -- comes at 11:09 p.m. ET, and for the first time in 19 years, it comes on the same night as a full moon, the one known as the Harvest Moon.

And for good measure, if you are blessed with clear skies tonight, you will see an unusually bright star right near the moon. It's actually not a star at all; it's the planet Jupiter at "opposition," coming closer to the earth than at any time since 1963.

There's no real magic to all this, just the bodies of the solar system doing what they do as they follow the rules of orbital mechanics. But there is a pleasant effect on us earthlings, if we pause to enjoy the combination.

A quick review of the key terms at play here:

Oppositions of Jupiter are routine. Jupiter orbits the sun once every 11.9 earth years, so we on earth overtake the planet in our closer orbit once every 13 months or so. At oposition, Jupiter hovers highest in the sky right around midnight. And this is our closest encounter with it each year.