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:

Equinox: the moment when the Sun appears to pass directly over the earth's equator -- in this case southbound as we move from summer to fall. It happens twice a year; if you like warm weather, you may prefer the vernal equinox next March 20, when winter will give way to spring.

Opposition: In this case it has nothing to do with Barack Obama or Sarah Palin -- instead, it is the term astronomers use for the moment when one body appears to be directly opposite another in the sky as seen from earth. Jupiter was at opposition to the sun at about 5 p.m. ET on Sunday, and is still very close to it. If you were able to hover directly above or below the ecliptic -- the giant plane of the earth's orbit -- you would be able to draw a straight line from the sun to the earth to Jupiter.

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.

Harvest Moon: Native American tribes gave names to each of the dozen full moons of the year, and the list now is maintained by the Farmer's Almanac. The Harvest Moon is the one closest to the beginning of fall. It's time for the crops to come in.

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.

Moon Illusion: This last term is the one that perhaps does have a touch of magic to it. If you go out tonight just after sunset, and are lucky enough to have good weather and a clear view of the horizon, you'll see the moon, pumpkin-colored, slowly rising in the east -- and, boy, does it look large.

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.