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Starwatch: Meteor shower set to peak mid-August

In the first few days of August, the moon wanes itself out of the evening sky and leaves us with a rare spread of bright planets and stars.   

Look low in the south after nightfall to see four beacons. From east to west we have reddish Mars, Saturn, reddish Antares, the heart of Scorpius and Jupiter. Saturn appears above the pleasing Teapot of Sagittarius and just west of the hooklike line of stars known as the Teaspoon.

If your eyes are good, you may be able to distinguish the subtly different colors of all these objects. Saturn and Jupiter often appear pale yellow, while Jupiter may also have tones of white, brown, red and orange.

Mars, of course, shines a soft ruddy color, as does Antares, whose brightness and tone earned it the nickname “rival of Mars.” These days, however, it gives Mars no competition.

With binoculars or a small telescope — and a star chart if necessary — try finding the star cluster above and left of the scorpion’s stinger. Known as Messier 7 or the Ptolemy cluster, it’s an open cluster like the Pleiades, where the stars were all born from the same cloud of interstellar gas and are loosely bound by gravity.

The annual Perseid meteor shower is predicted to peak the night of Aug. 12-13. It should be good because no moon will be around to interfere. Under ideal conditions, we could see 40-50 meteors an hour.

The meteors radiate from a point — called the radiant — near the helmet of Perseus, which will be low in the northeast at nightfall. If the radiant is still low and you're lucky, you may see what's called an earth-grazer — a meteor that bounces along Earth’s upper atmosphere like a stone skipping over water and shoots overhead rather than down.  

Meteors represent the fiery demise of debris left behind by comets. The Perseids arise when pebbles the size of sand grains hit Earth’s upper atmosphere going about 37 miles per second. As they streak through the atmosphere, the pebbles get very hot and heat the air around them. The glowing air creates the trails we see. Some may look as though they could hit somebody, but most are close to 60 miles above us.

The Perseid debris was left behind by Comet Swift-Tuttle. With a nucleus 16 miles in diameter, this comet is bigger than the object believed to have wiped out the dinosaurs. Swift-Tuttle comes around every 133 years and last visited in 1992.  

August’s full moon arrives the morning of Aug. 26. However, the moon sets in the southwest close to sunrise, before the instant of perfect fullness. Therefore, you may want to catch this moon on the evening of Aug. 25 or before the start of morning twilight Aug. 26.

The University of Minnesota offers public viewings of the night sky at the Marshall W. Alworth Planetarium on its Duluth campus. Visit www.d.umn.edu/planet for information and schedules.

Minnesota Starwatch is a newsletter describing the night sky in the Midwest. Updated monthly, it is produced by Deane Morrison, University of Minnesota Institute for Astrophysics. Find U of M astronomers and links to the world of astronomy at astro.umn.edu.

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