The Night Sky: September 2010

Contents

The Night Sky in September 2010

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

Moon’s Phases in Month
  New Moon on the 8th               Full "Harvest" Moon on the 23rd

Stars and Constellations

The Sun reaches the Autumnal Equinox on September 22 at 11:09 pm, signaling the official end of summer and the beginning of autumn in the Northern Hemisphere. The formal arrival of autumn notwithstanding, the majority of summer stars and even a few spring stars are still well placed for viewing during the early evening hours. By about 9 pm, orange-colored Artcurus in the summer constellation of Boötes (the Herdsman) is low, but still visible, in the west. The Big Dipper, which is really just a portion 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.

  The famous summer triangle of Vega in Lyra, Deneb in Cygnus, and Altair in Aquila is nearly overhead for a couple of hours after sunset. 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. If you are located away from city lights, you may be able to make out the Milky Way as a hazy band stretching across the sky, passing through Sagittarius, Aquila, Lyra, Cygnus, and Cassiopeia. The Milky Way is the galaxy of about 400 billion stars in which our solar system resides, and its nucleus lies in the direction of Sagittarius, but at a distance of 27,000 light years, which is far beyond the mere several hundred light years of the stars which define the constellation Sagittarius. There must be some exotic activity taking place in the core, because as early as 1931 radio engineer Karl Jansky of Bell Labs detected strong radio emissions coming from that region. Astronomers now have evidence that a supermassive black hole of several million solar masses lies at the very center of the Milky Way’s core.

 As the stars of summer gradually fade into the evening twilight, the first stars of autumn are appearing in the eastern sky. Making its appearance in the southeast after about 10 pm 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. Note that the direction in which the “W” opens up is toward Polaris, the North Star.

 Naked-Eye Planets In the Evening and Morning Sky

Since early spring, Venus has glimmered like a yellow diamond in the western sky shortly after sunset, and the spectacular evening “star” reaches its peak brightness late this month. Unfortunately, since June Venus has been setting earlier each night, and it now hovers pretty low above the western horizon at dusk. Unless your view in that direction is relatively unobstructed, Venus’s brilliance may go unnoticed. On the 1st, Venus, accompanied by the much fainter true star Spica to its right, sets about an hour and a half after sunset, or 9 pm EDT. By month’s end, Venus sets less than an hour after the Sun, but at this time a telescope will reveal an impressive crescent phase like the Moon’s. Venus will vanish entirely from the evening sky during October, only to reappear prominently in the early morning sky in November.

Mars spends September in the vicinity of Venus in Virgo. Both planets stand low in the west after sunset, but while Venus needs no optical aid to see, it will help to use binoculars to locate the much fainter Mars, which will resemble an orange star. During September, Mars sets roughly one and a half hours after sunset, or at 9 pm on the 1st and 8 pm on the 30th. Saturn, like Venus and Mars, is located in Virgo, but in the eastern portion of that constellation nearer to the Sun’s position and therefore difficult to pick out in the Sun’s glare. Saturn will vanish into the twilight by the end of September.

Jupiter reaches opposition with the Sun on the 21st, and is therefore visible all night long. Now at its closest to Earth, Jupiter sparkles like a brilliant cream-colored star in the dim constellation Pisces; it outshines everything in the night sky except for the Moon and Venus. Jupiter is a magnificent sight in a telescope, revealing cloud bands and (at the right moment) the Great Red Spot in its atmosphere and four Galilean moons surrounding the planet’s globe.

Mercury is too close to the Sun to be viewed in early September, but by midmonth Mercury begins to appear as a “morning star” low in the east at dawn. At its best, on the 19th, Mercury rises only about 90 minutes before the Sun. To spot it, one should look about a half-hour before sunrise, but it may be a challenge to spot Mercury against the bright twilight.

Information on lunar phases and rise/set times of Sun and planets is obtained from the US Naval Observatory Data Services at http://www.usno.navy.mil/astronomy/. Times given apply for observers near to the latitude and longitude of Philadelphia, USA: 40 degrees North latitude, 75 degrees West longitude. 

For more information on astronomy and weather, visit  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/