The Night Sky: August 2009

Contents

The Night Sky in August 2009

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

Moon’s Phases in Month
  Full "Sturgeon" Moon on the 5thNew Moon on the 20th

Stars and Constellations

When it gets dark on August evenings, you should easily be able to locate the Big Dipper, a part of the constellation Ursa Major, which is now beginning to dip into the northwest.  The two front stars in the Dipper point to Polaris, the North Star, while the "arc" of the Dipper’s handle leads to yellow-orange Arcturus, which stands high in the west.  The second star from the end of the handle of the Dipper is Mizar, or zeta Ursa Majoris. As a test of your vision, see if you can spot Mizar’s faint companion star, Alcor, at less than half a moon’s diameter away.  It is not known for certain if Mizar and Alcor are a truly gravitationally bound pair, but they are both at a distance of approximately 80 light years, the same as most of the other stars in the Ursa Major cluster. If the sky is dark enough, you may be able to identify the Little Dipper, which is part of Ursa Minor, extending outward from Polaris toward the upper left.

The large, but faint summer constellation Draco winds across the sky high above the Little Dipper.  Draco represents  the mythological dragon that guarded the golden apples in the Garden of the Hesperides.  Draco is especially notable to astronomers because one of its stars, Thuban (meaning "serpent"), was the polestar some 4700 years ago, at about the time the Pyramids of Egypt were being built.  Polaris is of course the polestar today, but gravitational forces from both the Sun and Moon acting on the Earth cause a slow wobble or precession of Earth’s polar axis, similar to what happens to a spinning top.  The cycle of precession takes approximately 26,000 years, and its effects are to cause a change in the position of the North Celestial Pole over that period of time. Consequently, by 7500 AD, the polestar in the Northern Hemisphere will have migrated from Polaris to Alderamin in the constellation Cepheus. And in 22,400 AD, the polestar will again be Thuban.  By 27,200 AD, the polestar will have come full circle back to Polaris.

Try to get one last glimpse of the spring star Spica as it sets in the southwest.  To the far left of Spica is orange-red Antares in Scorpius, the Scorpion, which is visible low in the south-southwest.  The body of the Scorpion snakes down toward the horizon and then upward into a curved stinger.  Look closely, and you will to see the "cat’s eyes," a pair of stars located at the end of the tail.  Shaula is the left and brighter of the two stars, while Lesath is the fainter one on the right.  To the upper left of the cat’s eyes is the "teapot" of Sagittarius, an easy grouping to identify.  The teapot marks the general direction of the core of our Milky Way galaxy. If you live away from city lights, you can make out the Milky Way as a hazy band stretching across the sky from southwest to northeast.  It passes through the Summer Triangle, which at this time stands high in the east-northeast.  The Triangle consists of Vega in the constellation Lyra, Deneb in Cygnus, and Altair in Aquila.  All three stars are whitish in color, indicating that they are all somewhat hotter than our Sun.

By late evening, you should be able to distinguish the "W" shape of the constellation Cassiopeia as it rises low in the northeast.  Also, the Great Square of Pegasus, which consists of four stars in the form of a rectangle lying on its edge, can be seen rising in the east.  These are two celestial signs that autumn is not far away.

 Naked-Eye Planets In the Evening and Morning Sky

Mercury sets about an hour after the Sun for most of August, and reaches greatest evening elongation with the Sun on the 24th.   It looks like a yellow star hovering very low above the western horizon soon after sunset.  Unless you have an unobstructed view of the western horizon, you will probably not be able to see it.  At the very beginning of August, Mercury passes just above the star Regulus, a sight which can be enhanced through  binoculars.   Saturn looks like a white-yellow star low in the southwest shortly after sunset during August.  Saturn sets at about 10 pm as August begins and a little after 8 pm by month’s end.  A telescope would normally reveal the magnificent rings, but they are currently oriented nearly edge-on to our line of sight and thus are not easily detected.  Saturn’s performance as an evening planet for 2009 ends this month; it reaches conjunction with the Sun in September.

While Saturn fades into the evening twilight during August, Jupiter is at its finest, reaching opposition with the Sun on the 14th.  At this time, Jupiter rises when the Sun sets, around 8 pm EDT, which is a little over an hour before Saturn sets.  Once it rises in the southeast, looking like a brilliant cream-colored star, Jupiter dominates the night sky until Venus rises in the early morning hours.  In mid-July, a dark spot apparently from a comet or asteroid impact appeared in telescopic images of Jupiter.  For more information, go to http://jupiter.samba.org/jupiter-impact.html.

Mars, situated in the morning sky, is growing slowly but steadily brighter with each passing month.  Mars is about the same brightness and color as the star Aldebaran, which lies in the same constellation, Taurus, which Mars inhabits for most of August.  Mars rises by around 1:30 am at midmonth.    Venus is the dominant luminary in the pre-dawn sky.  Once it rises it cannot be missed, sparkling like a yellow diamond in the east-northeast.  Venus rises around 3 am in early August, and just before 4 am at month’s end.  This is roughly two hours before morning twilight begins, and so Venus will appear especially bright against the dark night sky.

Earth passes through the debris of Comet Swift-Tuttle on August 11-12, producing a meteor shower.  Meteors should become visible in late evening, with the best show occurring after midnight.  Look toward the northeast, but meteors can be seen in any part of the sky.

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.