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The Night Sky of September

Dr. Wayne Wooten
Professor of Astronomy
Escambia Amateur Astronomers' Association

For September, the Moon will be full on September 4th. Two days before, the waxing gibbous moon passes 3 degrees north of brilliant Jupiter in the SE on September 2nd. This full moon is the Harvest Moon, the Full Moon closest to the Autumnal Equinox, which occurs on September 22nd at 12:22 PM CDT. The next two weeks find the moon waning in the morning sky. Last quarter moon sits high in the sky and half-lit at sunrise on September 12th.

The waning crescent moon passes a degree north of Mars on September 13th, then 3 degrees south of brilliant Venus on September 16th. The new moon is on September 18th, so the last two weeks find the moon waxing in the evening sky, with it reaching first quarter on September 26th, and passing Jupiter again on September 29th, when it is 3 degrees north of Jupiter.

Note this defines the sidereal month of 27.3 days, for the moon to orbit us once, and come against the same background of stars, or in this case, a slow moving outer planet. By contrast, the synodic month of 29.5 earth days is based on the moon's phases. It takes an extra two days due to the earth also revolving around the Sun in the course of the month, and is based on alignments of earth, sun, and moon.

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.

Jupiter in the south east in Capricornus dominates the evening skies for September 2009. He was at opposition in August, and also had a small comet or asteroid hit his north polar region in early August, creating a black impact similar to those seen in July 1994 when 21 pieces of Comet Shoemaker-Levy 9 hit the giant planet.

Also, the four moons make for an interesting physics demonstration, as Io the innermost moves around Jupiter in only 2 days, while outermost Callisto takes over two weeks to make the trip. It was 400 years ago this month that Galileo was amazed to spot the four tiny "Medician stars" in a row around Jupiter's equator, strong proof of the Copernican theory, for these orbited Jupiter, not us.

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 associate 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 means to "plant your peas".

For the next six months, she visits upstairs with as 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 since prehistory, 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! The scorpion's tail also houses two naked eye star clusters, M-6 and M-7, one of the nicest vistas for binocular users out there.

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. 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. The fine Ring Nebula, M-57, lies midway between the bottom two stars of the parallelogram of Lyra and appears as a smoke ring at 50X in smaller scopes. Only giants reveal the tiny white dwarf star in this center that powers this cosmic neon light display.

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.

Read past issues of the Sky at Night