Autumnal Equinox: First Day of Fall Greeted by Full Moon

Autumnal Equinox coincides with Hunter's Moon, Jupiter at opposition.

September 22, 2010, 12:54 PM

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.

Autumnal Equinox: Harvest Moon Greets First Day of Fall

But as astronomer Tony Phillips put it, "not all close encounters are the same." Phillips writes about the heavens for a website called Science@NASA. Jupiter's orbit is slightly elliptical -- it can be as much as 5.4 times as far from the sun as we are, or as little as 4.95. Right now it's at that near point, known as perihelion.

"This makes a difference of 75 million km [about 45 million miles]!" Phillips said in an e-mail. On Sunday, at closest approach, Jupiter was 368 million miles from Earth, and for several weeks it will appear to be the brightest object in the sky after the sun, moon and Venus -- three times brighter than Sirius, the brightest star, which is best seen in the evening sky in winter.

Just for the record, the next full moon is referred to by the almanac as the Beaver Moon, though it's also called the Hunter's Moon or Blood Moon. Time to set your traps if you need furs for the winter.

It is, in fact, no larger in the sky than when it's overhead, but our minds fool us, perhaps because we have a reference point -- something on the horizon -- that we lack when it is high among the stars.

"For instance," said Phillips, "when you see the moon in close proximity to a tree, your brain will miscalculate the distance to the moon, mentally bringing it closer (like the tree) and thus making it bigger. It seems so real, but this beautiful illusion is all in our minds."

It's a quiet, pleasant show that the heavens put on tonight. If the weather favors you, you will literally get to see the stars align.

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