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SKY REPORTER: August 2011

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Sky Data

Mercury's Limb from Messenger

Messenger Mission image of Mercury taken June 28, 2011.

Credit: NASA/Jet Propulsion Laboratory/Caltech


Monday August 1st civil twilight begins at 5:21 a.m. and sunrise is at 5:52. The following evening the sun sets at 8:11 p.m. and civil twilight concludes at 8:42.

On Wednesday August 31st, civil twilight begins at 5:53 with sunrise at 6:21 a.m. That evening sunset occurs at 7:30 p.m. and civil twilight ends at 7:58.

During August the average overnight temperatures drop from 69 degrees Fahrenheit on the 1st to 65 degrees on the 31st.

Primary Phases of the Moon are:

First Quarter August 6
Full Moon August 13
Last Quarter August 21
New Moon August 28

Sky Cast

At the start of August, Mars rises at 2:39 a.m. It is then in the eastern part of Taurus, near that constellation's borders with Orion and Gemini. All this month Venus is too close to the Sun's direction for casual viewing. Mercury also is close to the Sun's glare during the first three weeks of August. It then rapidly gains altitude in the eastern pre-dawn sky and becomes increasingly visible during the last few days of the month. Saturn is a considerable presence in our evening sky throughout August, setting at 10:57 p.m. on the first. Jupiter is in Aries rising at 11:52 p.m.at the beginning of this month.

The evenings of Wednesday August 3rd and Thursday the 4th, a waxing crescent moon is in the vicinity of Saturn, which currently has a visual magnitude of 0.92. That is comparable to bright stars such as Antares and Spica, the wide-ringed planet's current stellar neighbor in the constellation Virgo.

On the night of Sunday August 7th the waxing gibbous moon reaches the vicinity of Antares, the huge red supergiant and brightest star in Scorpius.

One of the usual treats of August skies is the annual appearance of the Perseid meteor shower. It comes every year just before mid-month, when more than a few residents of urban areas are out of town and able to enjoy the show at relatively dark sky observing sites. This year the gibbous moon, for several nights before and after Full Moon of Saturday August 13, significantly interferes with views of the Perseids, which are near their anticipated numerical peaks during early morning hours of the 12th and 13th. However, for a week or so before and after those dates, some Perseid meteors may be seen on clear nights, although not in any significant numbers.

On Monday the 15th of August Mars rises at 2:24 a.m. in Gemini. Saturn sets at 10:04 p.m., followed by the rising of Jupiter at 11 p.m.

During the nights of Friday August 19th and Saturday the 20th, the waning gibbous moon is less than five degrees of arc from Jupiter, which shines with magnitude minus 2.62 in the constellation Aries.

Early on the morning of Thursday August 25th the waning crescent moon might be viewed about three degrees from Mars. The "red" planet currently has a magnitude of 1.40.

Wednesday August 31st Mars, continuing its prograde (eastward) motion through Gemini, rises at 2:10 a.m. Mercury follows, rising at 4:55 a.m. This planet with the smallest orbit reaches an altitude about 11 degrees above the horizon at a point halfway between east and east-northeast at 6:00 a.m. in brightening twilight, about 20 minutes before sunrise. Venus has only a 4.5 degree separation from the Sun in the western sky on this date and sets before the end of evening civil twilight. Saturn sets at 9:05 p.m. and Jupiter rises at 9:58. Note there is about a two hour advance in rising and setting times for stars and these planets, caused by our changing perspective of the heavens relative to the Sun, caused by Earth's orbital motion.

Vesta South Pole in 3-D

Three-dimensional image of south polar region of asteroid Vesta taken July 9, 2011. (View with red left eye and green or blue right eye glasses.)

Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA


Both the Messenger Mission to Mercury and the Dawn mission to asteroid Vesta have provided valuable data since their recent arrivals at those destinations. At first glance (or second or third) images of Mercury's surface look much like those of the moon. However, the most significant difference is that Mercury lacks the dark lowland areas that were named maria by Galileo when he first examined them with his small homemade telescope. Those dark lunar "seas", that seen with unaided eyes form the "man in the moon," resulted from lava flooding huge impact craters such as the Imbrium basin. Forms of volcanism evident on Mercury are currently being studied by the Messenger team.

Even under the best of circumstances neither Mercury nor Vesta is an easy catch for sky watchers. However they provide interesting challenges. Always elusive Mercury may be seen on clear early mornings during the last week of this month. Mercury may be located using binoculars and a solid knowledge of its horizon direction and altitude on a particular date and time. Third and fourth magnitude stars in Capricornus provide jumping off points for binocular star-hopping to the asteroid Vesta, which is above the horizon from evening until a few hours before dawn throughout August.

At about 1:30 a.m. on August first Vesta is best placed for viewing at its highest elevation above the horizon when it transits the celestial meridian at an altitude of 22 degrees of arc directly south. That night Vesta shines at visual magnitude 5.4 and it is two degrees west of 4th magnitude star Zeta in the constellation Capricornus.

By the 15th of this month Vesta will have moved to a position about one degree north-west of magnitude 4.5 star 24 Capricorni, which in turn is midway along a line between Zeta and 4th magnitude Omega Capricorni. Vesta transits this night at about 12:20 a.m.

On the last evening of August look for Vesta close to 4th magnitude star Psi Capricorni. Vesta has faded slightly during August to magnitude 5.6 and this night it transits at about 11 p.m.

Sky Lore

Having identified Spica due to its apparent proximity with the moon on the nights of August 3rd and 4th, keep an eye on this star whenever you have an opportunity. It then will continue to serve as one of your sky markers. Spica is 260 light years from Earth and actually consists of a pair of very close, nearly identical component stars. This pair orbits their common center of mass over a period of just four Earth days. The main components of Spica are closer to each other than the minimum distance between Venus and Earth. Each star is a blue-white member of the main sequence, the type of star that shines solely due to thermonuclear conversion that transforms hydrogen into helium and energy within their cores.

Although Scorpius and its brightest star Antares are at best low in our southern sky, the set of these vivid points of light are very pleasant to trace on clear dark nights. Antares is one of the largest stars known. Unlike the relatively simple internal structures of main sequence stars such as the components of Spica, Antares' energy producing zones consists of concentric shells, each engaged in various forms of nuclear fusion. Antares has greatly swelled in size compared to its former existance as a main sequence star. Currently Antares is at least 400 times the Sun's diameter. Its vast, distended surface shines at the relatively low temperature (for a star) of about 3,500 on the Absolute Kelvin scale. This compares to about 5,800 Kelvins for the Sun and 20,000 for each of the components of Spica.

For several months we've been calling attention to the bright corner stars of the Summer Triangle. They are a fine way to get started orienting your sky views, especially if you are observing from a dark sky vacation site, where it's easy to get lost in a sea of hundreds of star lights, few of which we can see ever from town.

Located in the constellation Lyra, the Harp, Vega is the brightest star in its part of the sky. Only Arcturus, now considerably further west during August evenings, is comparable to Vega in brightness. Lyra has alternatively been described as a Plunging Eagle, the celestial complement to nearby Aquila, called the Soaring Eagle. Vega's brightness is almost exactly magnitude 0.0, just slightly less bright than Arcturus, and about two and a half times more vivid than average stars of the first magnitude such as Spica.

Vega, the most vivid object in Lyra, is a beautiful blue-white, middle aged star. Like all stars in their youth and middle age, including the Sun, Vega derives its energy from the crushing effects of gravity that causes fusion of hydrogen into helium at their cores.

The Summer Triangle asterism is defined at each corner by Vega, Altair in Aquila, and Deneb in the constellation Cygnus, the Swan.

Despite its name and prominence during July and August, stars of the Summer Triangle linger in our western sky during evenings until at least Thanksgiving. On the other hand early risers in late August, at predawn hours when most commercial lights are off, may delight in views of Orion and its brilliant stellar neighbors. Those vistas are usually associated with winter evenings, when those stars are visible at hours when far more of us are awake. Seeing brilliant Orion during mornings at the end of August reminds us summer is drawing to a close.

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