The Night Sky: September 2009

Contents

The Night Sky in September 2009

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

Moon’s Phases in Month
  Full "Corn" Moon on the 4thNew Moon on the 18th

Stars and Constellations

The Autumnal Equinox occurs on the 22nd of September at 5:18 pm, marking the official start of autumn in the Northern Hemisphere. On this date the durations of day and night are equal at 12 hours each, but thereafter the duration of darkness dominates until next March. Despite the formal arrival of autumn, the stars of summer are still well placed for viewing during the early evening hours. By about 9 pm, orange-colored Artcurus in the summer constellation Bootes is low, but still visible, in the west. 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 Scorpius is quite low in the southwest, and 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 city 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 after sunset. The constellation Cygnus contains many interesting object, and one in particular is the star Albireo, also known as beta Cygni, which lies about halfway between Vega and Altair. Albireo looks like an ordinary single star to the unaided eye, but a telescope reveals it to be a double star consisting of a bright orange star (similar to Arcturus) with a blue companion star (similar to Regulus). The color contrast is quite striking when seen through even a modest telescope. Albireo is listed in University of Illinois astronomer James Kaler's book, The Hundred Greatest Stars (New York: Copernicus Books, 2002) as lying at a distance of 380 light years from our solar system. The blue star has a luminosity 100 times that of the Sun, while the orange star is a giant with an output of 700 times that of the Sun. The distance between them looks miniscule, but in reality it is about 4000 astronomical units (one astronomical unit equals the average distance between Earth and Sun). The period of each star's orbit about a common center of gravity has been estimated as about 200,000 years.

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, consisting of 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.

 Naked-Eye Planets In the Evening and Morning Sky

As September opens, Mercury sets only about a half-hour after the Sun, and is therefore too close to the horizon to spot easily. Mercury reaches inferior conjunction with the Sun on the 20th, but shortly thereafter it swings into the morning sky. By the 30th, Mercury is rising about an hour and a half before sunrise, and looks like a yellow star hovering above the eastern horizon.

Saturn reaches conjunction with the Sun on the 17th, and is lost in its glare this month. It will reappear in the early morning sky next month. As Saturn descends into the evening twilight in the west, Jupiter ascends in the southeast. Jupiter resembles a brilliant cream-colored star, and it easily outshines everything in the night sky except the Moon and Venus, which doesn't rise until the early morning hours. A telescope or even a good pair of binoculars reveals the four large moons that Galileo discovered 400 years ago. Each of these four moons would be bright enough to see with the naked eye were it not for the overwhelming glare of parent Jupiter.

Mars, still anchored in the morning sky, is growing slowly but steadily brighter with each passing month. Mars has nearly the same brightness and color as the nearby star Aldebaran in Taurus. Mars rises around 1 am at the beginning of September and by 12:30 am at month's end. By the end of October, Mars will be rising before midnight, and by January 2010 it will be at its best as it reaches opposition with the Sun.

Venus is a spectacular sight in the pre-dawn sky. It sparkles like a yellow diamond in the east-northeast. Venus rises around 4 am in early September, and just before 5 am at month's end. On the morning of the 20th, Venus will be in conjunction with the star Regulus, the two separated by a full Moon width, making for an unusual and attractive sight

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

Times given apply for observers near to the latitude and longitude of Philadelphia, USA: 40 degrees North latitude, 75 degrees West longitude.