The Night Sky: August 2010

Contents

The Night Sky in August 2010

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

Moon’s Phases in Month
  New Moon on the 9th               Full "Sturgeon" Moon on the 24th

Stars and Constellations

As night falls on clear August evenings, the first star to emerge is orange-yellow Arcturus, which stands high in the west about an hour or so after sunset. Arcturus, is an orange giant star lying 37 light years away from our solar system. This identification can be verified once it gets truly dark by first locating the Big Dipper, a part of the constellation Ursa Major, which is now dipping into the northwest. The two front stars in the bowl of the Big Dipper, Dubhe and Merak, point to Polaris, the North Star, while the arc of the Big Dipper’s handle leads to Arcturus. If the sky is dark enough, you may be able to see the Little Dipper, which is part of Ursa Minor, extending outward from Polaris toward the upper left.

  One large, but faint summer constellation is Draco, the Dragon, which winds across the sky high above the Little Dipper. The Dragon’s head lies in the vicinity of Vega and its tail is near Dubhe. Draco is circumpolar for most US residents, meaning that its stars never set, but simply revolve around Polaris during the night. Draco is perhaps most notable because one of its stars, alpha Draconis, or Thuban, was the polestar some 4000 years ago, as was described in this column last August. The brightest star in Draco, gamma Draconis, or Eltamin, is an orange giant star similar to Arcturus but lying about four times farther away. Eltamin’s claim to astronomical fame is that it was used by English astronomer James Bradley in 1728 to measure a phenomenon of light known as aberration. The phenomenon has an analog in ordinary experience – as you walk through a light rainfall, the raindrops appear to be coming at you, and so you need to tilt your umbrella forward slightly. Similarly, starlight incident on planet Earth as it revolves about the Sun at a speed of 30 kilometers per second appears to be coming from a slightly different direction than it really is, and consequently, over the course of an entire year, the star itself appears to trace out a tiny elliptical path in the sky. This effect is a direct result of the finite velocity of light, and also serves as a demonstration of the Earth’s orbit about the Sun.

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 be able to see the “cat’s eyes,” Shaula and Lesath, a pair of stars located at the end of the tail which seem to be winking at you. To the upper left of the cat’s eyes is the “teapot” of Sagittarius, which 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. In fact it passes through the region of the summer triangle, which at this time stands high in the east-northeast. The triangle is comprised of Vega in the constellation Lyra, Deneb in Cygnus, and Altair in Aquila. All three stars of the summer triangle are whitish in color, indicating that they are all somewhat hotter than our Sun.

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 be able to see the “cat’s eyes,” Shaula and Lesath, a pair of stars located at the end of the tail which seem to be winking at you. To the upper left of the cat’s eyes is the “teapot” of Sagittarius, which 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. In fact it passes through the region of the summer triangle, which at this time stands high in the east-northeast. The triangle is comprised of Vega in the constellation Lyra, Deneb in Cygnus, and Altair in Aquila. All three stars of the summer triangle are whitish in color, indicating that they are all somewhat hotter than our Sun.

Late in the evening 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. You should also be able to distinguish the “W” shape of Cassiopeia as it rises low in the northeast. When you see these two celestial signs, autumn is surely just around the corner!

 Naked-Eye Planets In the Evening and Morning Sky

All five of the naked-eye planets are up in the evening sky this August. Mercury begins the month as an evening “star,” located low in the west at dusk. Your best bet to find it is to look about a half-hour after sunset, but be aware that you will need a western horizon unobstructed by trees or houses, and even so it will be difficult to spot Mercury against the bright evening twilight. Mercury sets about an hour after the Sun during the first week of August, and vanishes into the twilight afterwards.

In contrast to Mercury, Venus is a spectacular evening “star,” scintillating like a yellow diamond in the west at dusk. Venus actually brightens over the month of August, as the distance between it and Earth decreases. At the same time, Venus sets earlier each night: two hours after sunset, or about 10 pm EDT, at the beginning of August, but an hour and a half after the Sun by month’s end. Venus races through Virgo during August, passing Saturn on the 7th, Mars on the 18th, and the star Spica on the 31st.

Mars and Saturn begin the month of August together in the western part of Virgo, where Saturn shines like a yellow star, distinctly brighter than orange Mars, and comparable with blue-white Spica further to the east. During the course of the month, Mars continues to track eastward through Virgo, leaving Saturn behind. But Venus is moving faster than either of them, and by the end of the first week of August, the three planets are close enough to form a beautiful, though unequal, trio. During August, Mars and Saturn set, respectively, around 10 pm and 9:30 pm at midmonth. Mars will remain an evening planet (albeit a faint one) through autumn, while Saturn will vanish into the twilight by late September and reappear in the morning sky in October.

Jupiter stands out like a brilliant cream-colored star in the dim constellation Pisces, outshining everything in the night sky except the Moon and Venus. Jupiter currently lies diametrically opposite in the sky to its fellow giant planet Saturn (and also Mars and Venus), so that Jupiter rises in the east at about the same time as Saturn sets in the west, which is around 9:30 pm at midmonth. Jupiter is truly a magnificent sight in even a small 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.

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 generally in the northeast direction, but meteors can be seen in any part of the sky.

Some content for this article has been obtained from the 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. 

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/