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Starwatch September 2011

August 19, 2011


Planet Mercury surface.

In this NASA/APL image, it's easy to see why Planet Mercury intrigues scientists.

By Deane Morrison

The fading summer leaves us more hours of darkness to enjoy the stars, which is nice for finding some of the less obvious treasures of the night sky.

High in the south after sunset, the Summer Triangle still dominates. From bright Deneb, in Cygnus, west to brilliant Vega, in Lyra, and south to Altair, in Aquila, it comprises three of the sky's brightest stars and comes out shortly after nightfall. 

Break out the binoculars to spot the aptly named Coathanger, a grouping of stars three-eighths of the way along an imaginary line from Altair to Vega. Moving eastward from the Coathanger, look for the thin form of Sagitta, the arrow, and leaping Delphinus, the dolphin. And before you leave this section of sky, be sure to turn the binocs on the Milky Way, which flows through or close to these constellations.

In the predawn sky, Mars glides out of Gemini and into Cancer. Look on the 15th, when the Red Planet forms a straight line with the Gemini twins Castor and Pollux above the eastern horizon. On the 23rd, a waning moon appears near Mars. But the planet's best moment of the month is its last: On the 30th, Mars hovers at the edge of the beautiful Beehive star cluster, the jewel of otherwise dim Cancer.

Mercury pops into the morning sky early in September. Look for it low in the east-northeast, especially on the 9th, when it pairs up with Regulus, the brightest star in Leo.

Jupiter rises in the east about two and a half hours after sunset on the 1st, but by the 30th, we'll only have to wait about 90 minutes after sundown to see its brilliant golden form. If you're up in the predawn hours, you'll find it high in the south, between the Great Square of Pegasus to the west and Orion to the east.

Wherever Jupiter is, look 16 or 17 degrees below it and a few degrees west to find an intriguing star called Mira. Five years ago, astronomers found that this star, which began life like our sun, is shedding a tail of gas and dust as it hurtles through space. Now 13 light-years long, the tail has formed over the last 30,000 years and may seed the formation of new stars, planets and even life.

Because it is billions of years older than the sun, Mira is a case study in how our sun is likely to evolve. It has grown into a large star called a variable red giant, the "variable" part referring to its pulsating brightness as it periodically swells and shrinks. Its brightness is expected to peak this month, so don't miss this chance to see it.

September's harvest moon shines on through the night of the 11th-12th. This moon has long been a boon to farmers working late to bring in their crops because near the time of the fall equinox, the full or close-to-full moon rises as little as 22 minutes later each night.

At other times of year the interval is much longer. Around the spring equinox this March, for example, the fullish moon rose 81 minutes later from night to night. Things go the opposite way in the Southern Hemisphere, however, so farmers there enjoyed a harvest moon then.

Speaking of the equinox, fall arrives at 4:05 a.m. on the 23rd. At that moment the Earth's axis tilts neither away from nor toward the sun, and an observer from space would see our planet lighted from pole to pole.

The University of Minnesota offers public viewings of the night sky at its Duluth and Twin Cities campuses. For more information and viewing schedules, see:

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

Tags: College of Science and Engineering

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