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Last modified: Tuesday, September 2, 2003

STAR TRAK/September 2003

Mars still dominates in September

Mars will continue to dominate the southern sky during September, after passing closest to Earth on Aug. 27. The flaming orange planet will still be an impressive sight. While you're admiring Mars, if you're in the right place at the right time, you may also see a spectacular display called an aurora ("northern lights").

Auroras can occur 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, along with occasional power outages and disruptions of radio communications.

By checking Web sites such as http://www.spaceweather.com and http://www.sec.noaa.gov/pmap/ or signing up for solar activity alerts by e-mail at http://www.skypub.com/news/astroalert/astroalert.html, you can watch for auroras when they are most likely to happen. Sightings of auroras are reported at http://www.spacew.com/www/aurora.html.

Auroras in the Northern Hemisphere can sometimes be seen as far south as Georgia and California. They are caused when particles from the sun collide with Earth's magnetic field, sending charged particles down into Earth's upper atmosphere. Auroral light is molecules of air glowing when they are struck by these charged particles raining down along Earth's magnetic field lines.

The color of an aurora depends on the type of atom or molecule struck by the charged particles, since each kind of gas glows with its own color. High-altitude oxygen, about 200 miles up, is the source of rare, all-red auroras. Oxygen at lower altitudes, about 60 miles up, produces 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 ripple edges of an aurora.

Planets

Mars will fade slightly from its peak of brightness, but it will still be a brilliant yellow-orange. It will appear higher in the southeast each evening after sunset, reaching its greatest elevation in the south at a more convenient time for viewing, especially with telescopes. Take advantage of this opportunity while you can. In a few weeks, Mars will begin to shrink rapidly as we move away from it.

Saturn will rise in the east-northeast around 2 a.m. local daylight time at the beginning of September and by midnight at month's end. It will be high in the southeast by dawn, in the constellation Gemini the Twins. This part of the sky has many brilliant stars, but bright yellow Saturn will outshine all of them except Sirius.

Jupiter will be five times brighter than Saturn when it rises in the east during morning twilight among the stars of the constellation Leo the Lion. Leo's brightest star, Regulus, will rise a half hour before Jupiter as if heralding the arrival of the giant planet.

After midmonth, Mercury will closely follow Regulus and Jupiter into the morning sky. This will be a very good appearance for viewers in the Northern Hemisphere, with Mercury reaching its greatest distance above the eastern horizon on Sept. 26. Look for the little point of bright white to the lower left (north) of brilliant white Jupiter. Jupiter will be obvious, but binoculars will help in picking out Mercury in the morning twilight. The slender crescent moon will join Regulus, Jupiter and Mercury on the morning of Sept. 24, making a pretty sight very low in the east. The best view will be between an hour and 45 minutes before sunrise, and it won't last long in the brightening sky.

Venus will be hidden in the solar glare during September.

Equinox

The sun will reach the September equinox on Sept. 23 at 6:47 a.m. EDT (10:47 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.

Harvest Moon

The Harvest Moon is the full moon closest to the September equinox. For several nights in a row around Sept. 10 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 it rises higher into the sky, it will turn its usual brilliant white.

Moon phases

The moon will be at first quarter on Sept. 3, full (the Harvest Moon) on Sept. 10, at third quarter on Sept. 18 and new on Sept. 26.