Last modified: Wednesday, August 31, 2005
Venus and Jupiter meet as Mars brightens rapidly
BLOOMINGTON, Ind. -- Mars will blaze bright orange in the east. Venus and Jupiter will rendezvous low in the west. And if you're lucky, you'll see a spectacular aurora. September is a fine month for skywatching.
If you're outdoors on a clear September night, you may see a colorful display in the sky called an aurora ("northern lights"). These silent, shimmering ribbons and curtains of light have an eerie beauty that can be spellbinding.
Auroras can appear whenever the sun is active, but especially from August to October. Eruptions from the sun's surface hurl enormous amounts of material into space. When some of this solar matter heads in our direction, it causes auroral activity that is sometimes visible even from middle latitudes.
Such a display happened on Aug. 24 when auroras were seen as far south as Colorado and Utah. For details and photographs, see http://www.spaceweather.com/aurora/gallery_01aug05.htm.
You can watch for auroras when they are most likely to happen by checking Web sites such as http://www.spaceweather.com and http://www.sec.noaa.gov/. Sightings of auroras are reported at http://www.spacew.com/www/aurora.html.
In the Northern Hemisphere, auroras are sometimes seen as far south as Florida and California. They are caused when charged particles from the sun collide with Earth's magnetic field, sending 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 molecule struck by the particles from the sun. High-altitude oxygen is the source of all-red auroras. Oxygen at lower altitudes glows a brilliant yellow-green, the brightest and most common auroral color. Ionized nitrogen molecules produce blue light, and neutral nitrogen glows red. The nitrogens create the purplish-red lower borders and edges of an aurora.
Venus and Jupiter, the two brightest planets, will gleam low in the west-southwest 30 to 45 minutes after sunset during September. Venus will be brighter, but both are so brilliant that they'll be easy to see if they're not hidden by trees or buildings. After being closest on Sept. 1, they will drift apart. Don't wait long to look for them, for they will set as twilight ends, and the sky darkens quickly at this time of year. On Sept. 6, they will be joined by the crescent moon. To find the time of sunset each day at your location, go to http://skyandtelescope.com/obloc.asp and enter the necessary information.
Mars will rise fiery orange in the east before 11 p.m. local time at the start of the month, climbing very high in the south before the start of morning twilight. For those with telescopes, this is the time to turn them on Mars. The higher the red planet is, the better the view of it through a telescope will be. Each week Mars will rise earlier and get larger and brighter as Earth catches up with it, reaching a climax on Oct. 31.
Saturn will be low in the east in early dawn, below the bright stars Pollux and Castor of the constellation Gemini. Binoculars will show the Beehive star cluster nearby on Saturn's lower left (north).
Mercury will be visible very low in the east for the first week of the month, rising about an hour before the sun far below Saturn. Mercury will then quickly drop into the predawn glare.
The sun will reach the September equinox on Sept. 22 at 6:23 p.m. EDT (22:23 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. Some of the traditions associated with the September equinox are described at http://www.equinox-and-solstice.com/html/autumnal_equinox.html.
The Harvest Moon is the full moon closest to the September equinox. For several nights in a row around Sept. 17-18 in the Northern Hemisphere, the nearly-full moon will rise soon after dusk and seem to linger near the horizon for a while, colored yellow-orange by dust particles and other debris in the lower atmosphere. As the moon rises higher into the sky, it will turn its usual brilliant white.
The moon will be new on Sept. 3, at first quarter on Sept. 11, full (the Harvest Moon) on the night of Sept. 17-18 and at third quarter on Sept. 25.