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Starwatch - September 2010

August 11, 2010


The darker, clearer skies of September mean good viewing conditions for Pisces, Aquarius and other watery autumn constellations as they flow into the evening sky from the east. In addition, Jupiter, the sun and the moon all lay claim to special attention this year. 

First up is a close call with Jupiter. On the 21st, Earth passes the giant planet in the race around the sun, an event we call opposition because it places a planet directly opposite the sun in the sky.

On that day Earth swings within 367.5 million miles of Jupiter. Although that’s one of the closest approaches our two planets ever make, it’s still nearly four times the distance from Earth to the sun. At opposition Jupiter will shine brilliantly in the southern sky, an unmistakable beacon just beneath the circle of stars called the Circlet of Pisces, which itself lies below the Great Square of Pegasus. A bright moon will compete with Jupiter’s glory for several days around opposition, but the planet will be at near-maximum brightness most of the month.

This September we also get an excellent chance to find Uranus, thanks to its proximity to Jupiter. When no moon is up, use good, steady binoculars or a small telescope to search for a tiny spot within a degree above Jupiter. If your eyes are sharp, you may be able to make out the planet’s bluish tinge.

The seventh planet from the sun, Uranus follows an orbit that takes it as far as 1.8 billion miles from its parent star—20 times as far as Earth’s distance from the sun. Nothing strange about that, but Uranus has one bizarre distinction: Its axis of rotation is tilted nearly to the plane of its orbit, leaving the planet to rotate on its side like a gigantic rolling ball. A leading theory for how this came about holds that early in its history, Uranus collided with a planet-sized body that knocked it over. It’s a good thing Earth’s tilt is nowhere near so extreme; if it were, the seasonal changes we experience would be drastic indeed.

Next in the spotlight comes the sun, via the autumnal equinox. It happens at 10:09 p.m. on the 22nd, when the sun crosses the equator on its way into the southern sky. At that moment an observer in space would see Earth lighted from pole to pole, and day and night are equal.

The shift in daylight at the equinoxes may have implications for travelers. Since the March equinox, travelers going north have encountered progressively longer periods of daylight. After the September equinox, however, the farther north you go, the shorter the days. In the Southern Hemisphere, the pattern is reversed.

Rounding out our triad of special events, the harvest moon arrives officially at 4:17 a.m. on the 23rd. The harvest moon is traditionally the full moon closest to the fall equinox, and this year the two events miss each other by a whisker—barely six hours. If you want to see this iconic moon at its best, get out early that morning, since the moon will set shortly after 7 a.m. (exact times vary with location).

Venus, now a downwardly mobile planet, spends the month sinking into the sunset. But don’t write it off just yet; on the 11th Venus and a young moon make a pretty pair, the last such spectacle before the planet disappears on its next sweep between Earth and the sun.

As darkness falls, the first three stars to come out will be the Summer Triangle, consisting of Deneb at the northeast point, Vega at the northwest and Altair at the south. All are the brightest stars in their constellations: Deneb in Cygnus, the swan, which contains the Northern Cross; Vega in Lyra, the lyre; and Altair in Aquila, the eagle.
You can find a somewhat dim but very pleasing pattern of stars by sighting with binoculars along an imaginary line from Altair to Vega. About a third of the way to Vega you’ll see 10 stars in the shape of an upside-down coathanger. The Coathanger is an asterism, a group of stars that aren’t a constellation. The Summer Triangle is itself an asterism; other famous ones include the Big Dipper in Ursa Major, the Little Dipper in Ursa Minor and the Great Square of Pegasus.

The University of Minnesota offers public viewings of the night sky at its Morris, Duluth and Twin Cities campuses. For more information and viewing schedules see:
Morris, UMN 16-inch telescope schedule: cda.mrs.umn.edu/~kearnsk/Telescope/PubObs.htm

Duluth, Marshall W. Alworth Planetarium: www.d.umn.edu/planet

Twin Cities, Department of Astronomy (during fall and spring semesters): www.astro.umn.edu/outreach/pubnight

Find U of M astronomers and links to the world of astronomy at http://www.astro.umn.edu

Contact: Deane Morrison, University Relations, (612) 624-2346, morri029@umn.edu

Tags: College of Science and Engineering

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Related Links

Map of Jupiter

Coathanger Map