The Night Sky: September 2008

The Night Sky in September, 2008

By Harry J. Augensen
Professor of Physics & Astronomy, Widener University

Moons Phases in September
  Full "Harvest" Moon on the 15thNew Moon on the 29th

Stars and Constellations
Despite the formal arrival of autumn in September, the stars of summer (and even a few of spring) are still well placed for viewing during the early evening hours.  By about 9 pm, orange-colored Artcurus in the constellation Boötes, the Herdsman, is low, but still visible, in the west.  Somewhat above Boötes is Corona Borealis, the Northern Crown. The Big Dipper, a part of the constellation Ursa Major, is low on the northwestern horizon, and so you will not be able to see it if trees or buildings obstruct your view of this part of the sky.  The handle of the Dipper "arcs" to Arcturus. Orange-red Antares in the Scorpion is quite low in the southwest.  You should also be able to find the asterism known as the "teapot" of Sagittarius, just east of Antares.  The constellation Sagittarius contains the nucleus or core of our own Milky Way Galaxy, which may be seen (if you are away from urban lights) as a hazy band stretching across the sky.  The summer triangle of Vega in Lyra, Deneb in Cygnus, and Altair in Aquila is nearly overhead for a couple of hours after sunset.

Three relatively faint, diminutive constellations can also be found on late summer evenings if the sky is dark and clear enough:  Delphinus (Dolphin), Sagitta (Arrow) , and Corona Australis (Southern Crown).  The Dolphin and Arrow lie above and to the left of the Eagle, while the Southern Crown lies just below Sagittarius.  According to Greek mythology, the poet and musician Arion had won prizes in a musical contest in Sicily. While sailing home to Corinth, the ship's murderous crew seized his prizes and threw him overboard. A kindly dolphin rescued Arion, however, and carried him ashore.  Years later, when the dolphin died, it was placed among the stars as the constellation Delphinus, in recognition of this noble deed.  Sagitta was the arrow of Apollo which he used to kill the Cyclops, the producer of Zeus's thunderbolts.  Later Apollo changed the arrow into a constellation to commemorate his battle with the Cyclops.  Corona Australis, the southern counterpart to Corona Borealis, is said to represent a laurel wreath worn by Sagittarius, the Archer.

As the stars of summer gradually fade into the evening twilight, the first stars of autumn are rising in the eastern sky.  Making its appearance in the southeast after about 10 pm EDT is the white star Fomalhaut, which lies in the constellation of Pisces Austrinus (the Southern Fish).  Rising due east at about this time is the Great Square of Pegasus, four whitish stars in the form of a rectangle (not quite square) lying on its edge.  Low in the northeast is the famous "W" shape of the constellation Cassiopeia, the Queen of ancient Ethiopia.  The "W" opens up toward Polaris, the North Star. A faint, but important constellation lying just above Cassiopeia is Cepheus, the King of Ethiopia and Cassiopeia's husband.  One of Cepheus's seemingly insignificant member stars is mu Cephei, also known as "Herschels Garnet Star."  This name refers to the stars intensely red color, a result of its relatively cool temperature.  Mu is listed in University of Illinois astronomer James Kaler's book, The Hundred Greatest Stars (New York: Copernicus Books, 2002) as one of the largest and most luminous stars in the Galaxy.  It lies at a distance of about 2000 light years from our solar system.

Planets
Mercury can be glimpsed very low in the evening twilight sky for the first half of the month. It looks like a moderately bright star.  Mercury keeps close company (apparently) with two other terrestrial planets, Venus and Mars, for much of the month.  By months end, Mercury disappears into the evening twilight.  Venus continues its slow ascent into the evening sky, resembling a very bright star low in the west. It sets only about an hour after the Sun for much of September. It will be another month or two before Venus rises to true magnificence in the evening sky.  For now, the interest lies in the clustering of Venus with fellow terrestrial planets Mercury and Mars, low in the western sky shortly after sunset.  Mars is by far the faintest of the three, and will be essentially vanishing from the evening sky by the end of this month.

Jupiter remains well positioned for viewing during September.  It resembles a brilliant cream-colored star hovering well above the southern horizon during the evening hours.  Jupiter continues to reside within the constellation Sagittarius, near the famous "teapot" asterism. In mid-September, Jupiter reaches its highest point above the southern horizon (i.e., transits the meridian) around 8 pm and sets at about 1 pm, EDT. Saturn reaches conjunction with the Sun on the 4th, and hence is inaccessible for viewing early in the month. By midmonth, however, Saturn rises an hour before the Sun and by the 30th about 2 hours before it.  Look for Saturn in the east at dawn it resembles a bright yellow star in Leo.

Earth reaches the September Equinox in its orbit on September 22 at 11:44 am EDT, marking the beginning of autumn in the Northern Hemisphere.  This date was previously referred to as the Autumnal  Equinox, but a neutral designation is now used to acknowledge the fact that spring also begins on this date in the Southern Hemisphere.

For more information on astronomy and weather, visit the Widener University Public Viewing Website at http://www.widener.edu/stargazing/, then click on Web Links & Resources. A set of free sky maps can be obtained at http://www.skymaps.com/

Some content for this article has been obtained from US Naval Observatory Data Services