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Hal Kibbey

Last modified: Wednesday, September 4, 2002

STAR TRAK for September 2002

Watch for auroras in September

Auroras ("northern lights") 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.

The sun has been unusually active during the last couple of months, with repeated eruptions from the surface hurling enormous amounts of material into space. Some of this solar matter has headed in our direction, causing auroral activity that is sometimes visible even from middle latitudes.

By checking Web sites such as and or signing up for solar activity alerts by e-mail at, you can watch for auroras when they are most likely to happen. Sightings of auroras are reported at

In addition to providing a daily view of the sun, reports on violent events on the solar surface called coronal mass ejections and indicates whether one of these blasts of solar particles is heading toward Earth. If so, a display of the northern lights could be triggered in the following couple of nights, along with power outages and disruptions of radio communications.

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.


Venus is nearing the end of its reign as the "evening star," though it will reach its greatest brightness during the last days of September. Appearing low in the west-southwest as the sky darkens for viewers in the Northern Hemisphere, Venus will easily pierce the murky sky near the horizon if it is not blocked by buildings or trees. It will set less than two hours after the sun at the beginning of the month and only an hour after the sun by month's end. The situation will be the opposite for those watching in the Southern Hemisphere, as Venus will appear high in the western sky even before sunset. Venus will cross into the morning sky during October.

As seen from the Northern Hemisphere, Mercury will be barely above the western horizon a half hour after sunset during September, far to the lower right (north) of Venus. Without a telescope, Mercury will probably be too difficult to find in the bright evening twilight during the short time before it sets. For observers in the Southern Hemisphere, Mercury will be farther above the horizon as the evening sky darkens but fading rapidly as the days pass. By the final week of the month, Mercury will be lost in the solar glare as it crosses between Earth and the sun.

Saturn will rise in the east-northeast within an hour after midnight local time at the beginning of September and two hours earlier by month's end, remaining visible for the rest of the night in the constellation Orion the Hunter. The yellow planet will be highest in the south just before morning twilight, the best time for viewing its famous rings with a telescope.

Brilliant white Jupiter will rise three hours after Saturn in the constellation Cancer the Crab. For the first two weeks of September, binoculars will reveal a pretty sight as Jupiter passes just below the Beehive Star Cluster.

Mars will be out of sight behind the sun for the next few months.


The sun will reach the September equinox on Sept. 22 at 11:55 p.m. CDT (Sept. 23 at 5:55 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, and this year they almost coincide. For several nights in a row around Sept. 21 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, before it rises higher into the sky and turns its usual brilliant white.

Moon phases

The moon will be new on Sept. 6, at first quarter on Sept. 13, full (the Harvest Moon) on Sept. 21 and at third quarter on Sept. 29.