Last modified: Wednesday, September 1, 2004
A season for auroras
BLOOMINGTON, Ind. -- If you're in the right place at the right time on a clear September night, you may see a spectacular display called an aurora ("northern lights"). These silent, shimmering ribbons and curtains of light are unlike anything else you will ever see.
Numerous photographs of auroras seen in Alaska are available at http://www.geo.mtu.edu/weather/aurora/images/aurora/jan.curtis/.
Auroras can appear whenever the sun is active, but especially from February to April and August to October. This is when the parts of the sun with the most sunspots are tilted farthest toward Earth. 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. It may also cause occasional power outages and disruptions of radio communications.
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/pmap/. 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 effect is similar to the light from a color television set. In the picture tube, a beam of electrons striking the screen makes it glow in different colors, according to the type of chemicals coating the glass. In an auroral display in the sky, the color depends on the kind of molecule struck by the particles from the sun. High-altitude oxygen is the source of rare, 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, while neutral nitrogen glows red. The nitrogens create the purplish-red lower borders and edges of an aurora.
Brilliant white Venus will rise almost four hours before the sun this month and be halfway up the eastern sky by sunrise, dominating everything around it.
Saturn will begin September slightly above Venus and about 50 times fainter. The two planets will separate quickly as the days pass. Saturn will rise earlier each morning and appear higher in the sky at daybreak, while Venus drifts downward toward Regulus, the brightest star in the constellation Leo the Lion.
Mercury will have a very good morning appearance, reaching its greatest distance above the eastern horizon on Sept. 10 about an hour before sunrise. Regulus will be extremely close to Mercury that morning, helping you pick out the little planet in the morning twilight. Trees or buildings may block the sight, so choose a location with a clear view of the eastern horizon. Find Regulus first and then look for the pinpoint of white light barely below it. Both will be far to the lower left (north) of Venus.
Jupiter and Mars will be out of sight during September as they pass behind the sun.
The sun will reach the September equinox on Sept. 22 at 12:30 p.m. EDT (16:30 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. 28 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 at third quarter on Sept. 6, new on Sept. 14, at first quarter on Sept. 21 and full (the Harvest Moon) on Sept. 28.