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Last modified: Thursday, August 30, 2007


Jupiter gleams low in the south at sunset

Aug. 30, 2007


Photo by: NASA


Print-Quality Photo

BLOOMINGTON, Ind. -- If you're outdoors on a clear September night, you may be lucky enough to see an aurora (sometimes called "northern lights"). These silent ribbons and curtains of light have a colorful beauty that can be fascinating.

Auroras can appear whenever the sun is active, but they are especially likely from August to October. Eruptions from the sun's surface hurl enormous amounts of material into space. When some of these solar particles head in our direction, they cause auroral activity that is sometimes visible even from middle latitudes.

Such a display happened on Aug. 7, 2006, for example, when auroras were seen as far south as Colorado. For details and photographs, see

You can watch for auroras when they are most likely to happen by checking Web sites such as and Sightings of auroras are reported at

In the Northern Hemisphere, auroras are sometimes seen as far south as Florida and California. They happen when particles ejected from the sun collide with Earth's magnetic field, sending charged particles down into Earth's upper atmosphere. Molecules of air glow when they are struck by these particles raining down along the lines of the magnetic field. The result is a beautiful aurora.

The color of an aurora depends on the kind of air molecule struck by the particles from the sun. High-altitude oxygen produces all-red auroras. Oxygen at lower altitudes glows a brilliant yellow-green, the brightest and most common auroral color. Ionized nitrogen molecules give off blue light, and neutral nitrogen glows red. The two colors of nitrogen create the purplish-red lower borders and edges of an aurora.


Anyone looking fairly low in the southern sky after dusk during September will notice Jupiter, the solar system's largest planet. The brilliant white object will be easy to find within the constellation Scorpius if trees or buildings don't block it from sight. Nearby and even lower will be the constellation's brightest star, Antares (literally "rival of Mars," so named because of its similar red-orange color). Don't wait long to view Jupiter; it will set earlier each evening as the month goes by.

Mercury will be very low in the west-southwest all month in the Northern Hemisphere, out of sight for anyone who doesn't have a clear view of the western horizon. Binoculars may be needed to pick out Mercury in the bright sky of twilight. In the Southern Hemisphere, however, Mercury will be high above the western horizon and easy to spot near the bright white star Spica. On Sept. 21 a half hour after sunset, Mercury and Spica will be very close.

Mars will rise in the east in late evening this month in the constellation Taurus the Bull, causing the Bull to appear to have two glaring red eyes instead of the usual one. Mars will be near the bright orange star Aldebaran in Taurus, slightly outshining its ruddy companion. The red planet will be highest in the south at dawn.

Venus, the brightest planet, will climb quickly into the eastern sky before morning twilight. This "morning star" will rise at 5:30 a.m. local time on Sept. 1 and by 4 a.m. at month's end. Find a place with a clear view of the eastern horizon to see it.

In frustrated pursuit of Venus will be lumbering Saturn, emerging from behind the sun in the east at the start of the month. The yellow planet will be in the constellation Leo the Lion near Leo's bright white star Regulus. Those with telescopes will be able to tell that Saturn's magnificent rings are now tilting toward us less than they have since 1998.

Solar eclipse

Observers in South America south of Ecuador will see a partial eclipse of the sun on Sept. 11. A map showing where the eclipse will be visible can be found at


The sun will reach the September equinox on Sept. 23 at 5:51 a.m. EDT (9:51 Universal Time), marking the start of fall in the Northern Hemisphere and spring in the Southern Hemisphere. For the next six months in the Northern Hemisphere, the nights will be longer than the days.

Moon phases

The moon will be at third quarter on Sept. 3, new on Sept. 11, at first quarter on Sept. 19 and full on Sept. 26.