The Night Sky of September

Dr. Wayne Wooten
Professor of Astronomy

For September, the Moon will be last quarter on September 1st. The new moon will happen a week later (a week is the time it takes the moon to go through a quarter of its phase cycle), on September 8th. The slender crescent on the 9th marks the beginning of Rosh Hashanah, the first day of Jewish year 5771 AM.

On the 10th, the waxing crescent moon passes south of Mars and Spica, then passes just south of Venus a few hours later. First quarter moon is on the 15th. The full moon, the Harvest Moon, occurs on September 23rd, the day after the autumnal equinox; fall actually begins this year at 10:23 PM CDT on September 22nd. The moon will be 6 degrees north of Jupiter on the 23rd; Jupiter was at opposition itself on September 21st. Thus the moon is out of the evening sky during the first and last weeks of September, making them ideal for spotting deep sky objects.

To the west, we are losing Saturn into the sun’s glare, and Mars is also distant and fading fast. But Venus will be dominating in its greatest brilliance for the next two months. She is now retrograding between us and the Sun, and thus appears as a crescent in practically any telescope. Consider how much Venus changes for earthly viewers in September. At the start, Venus is 41% sunlit and shows a disk 29" of arc across, and sets at 9 PM; by the end of the month, her phase is only 20% sunlit, but her disk has grown to 44" across (this crescent will be easily noted even in binoculars by then) and she is much closer to the sun, setting about 8 PM. She will be at inferior conjunction, passing between us and the sun, before Halloween.

Jupiter dominates the eastern evening skies for September 2010. He reaches opposition on September 21st, just south of the Square of Pegasus in Pisces, and will be up all night, opposite the sun in the sky then. Amateurs and professionals all watched last spring when the southern "racing stripe", Jupiter’s South equatorial belt, mysteriously faded to white, and wonder when it will return to its normal prominent brown appearance.

The Big Dipper rides high in the NW at sunset, but falls lower each evening. Good scouts know to take its leading pointers north to Polaris, the famed Pole Star. For us, it sits 30 degrees (our latitude) high in the north, while the rotating earth beneath makes all the other celestial bodies spin around it from east to west. It is this time of year at an American Indian legend tells of the Bear and three hunters. The bowl is the bear, the three handle stars of the dipper the hunters. The first carries a bow, and has shot the bear in its flanks. The second optimistically carries a bowl on his shoulder for bear stew; look closely, and you can see the pot (Mizar. horse in Arabic, and Alcor its rider more traditionally). The last hunter carries firewood for the feast. The wound is minor, and the bear has not lost a step, but in the fall, as the bear goes into hiding along the NW horizon, the wound opens slightly, and blood oozes out to fall on the tree leaves and paint them red this time of year.

From the Dipper’s handle, we "arc" SE to bright orange Arcturus, the brightest star of Spring. Spike south to Spica, the hot blue star in Virgo. Note that Spica is now low in the SW, and by September’s end, will be lost in the Sun’s glare due to our annual revolution of the Sun making it appear to move one degree per day eastward. To the Greeks, Spica and Virgo were associated with Persephone, the daughter of Ceres, goddess of the harvest. In their version of "Judge Judy", the beautiful young daughter falls for the gruff, dark god of the underworld, Pluto. He elopes with her, much to the disapproval of mother Ceres, and they marry in his underworld kingdom of Hades…a honeymoon in hell…really, he does love her as well, and the marriage itself works well. But it is the reaction of Ceres that creates alarm.

Very despondent over the loss of her young daughter to a fate as bad as death, Ceres abandons the crops, which wither. Soon famine sets in, and humanity appeals to Jupiter to save us all. Calling all together, Jupiter hears that Ceres wants the marriage annulled, Persephone loves them both, and Pluto wants his mother in law to stop meddling. Solomon style, Jupiter decides to split her up, not literally, but in terms of time. In the compromise (aren’t all marriages so?), when you can see Spica rising in the east in March, it is time to plant your peas. For the next six months, she visits upstairs with a very happy mama, and the crops will prosper. But now, as Spica heads west (to the kingdom of death, in most ancient legends) for six months of conjugal bliss with Pluto, it is time to get your corn in the crib. This simple story, told in some form for as long as Noah’s flood, was one of the ways our ancestors 7,000 years ago knew the solar calendar and when to plant and harvest. As you watch Spica fade, thank this star for agriculture, and in a certain sense, even our own culture.

To the south, Antares rises about the same time in Scorpius. It appears reddish (its Greek name means rival of Ares or Mars to the Latins) because it is half as hot as our yellow Sun; it is bright because it is a bloated red supergiant, big enough to swallow up our solar system all the way out to Saturn’s orbit!

East of the Scorpion’s tail is the teapot shape of Sagittarius, which marks the heart of our Milky Way galaxy. Looking like a cloud of steam coming out of the teapot’s spout is the fine Lagoon Nebula, M-8, easily visible with the naked eye. This recent photo by EAAA member Eric King shows well this stellar nursery is ablaze with new stars and steamers of gas and dust blown about in their energetic births. In the same binocular field just north of the Lagoon is M-20, the Trifid Nebula. A little east is another telescopic treat for September, the fine globular cluster M-22, just to the upper left in the same binocular field as the star at the top of Sagittarius’ teapot.

The brightest star of the northern hemisphere, Vega dominates the NE sky. Binoculars reveal the small star just to the NE of Vega, epsilon Lyrae, as a nice double. Larger telescopes at 150X reveal each of this pair is another close double, hence its nickname, the "double double"…a fine sight under steady sky conditions.

To the northeast of Vega is Deneb, the brightest star of Cygnus the Swan To the south is Altair, the brightest star of Aquila the Eagle, the third member of the three bright stars that make the Summer Triangle so obvious in the NE these clear September evenings.

While the naked eye, dark adapted by several minutes away from any bright lights, is a wonderful instrument to stare up into deep space, far beyond our own Milky Way, binoculars are better for spotting specific deep sky objects. For a detailed map of northern hemisphere skies, about August 31st visit the www.skymaps.com website and download the map for September 2010; it will have a more extensive calendar, and list of best objects for the naked eyes, binoculars, and scopes on the back of the map.

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