Weekend SkyWatcher’s Forecast – August 8-10, 2008


Greetings, fellow SkyWatchers! Are you ready for another weekend? As the seasons slowly begin to change for both hemispheres and the Moon grows more full, look for an optical phenomena known as a “nimbus” – or halo around the Moon. While it’s nothing more than a thin layer of ice crystals in the upper troposphere, it is a wonderfully inspiring sight and was once used as a means of weather forecasting. If you see a nimbus, try counting the number of stars visible inside the halo and see if it matches the number of days before bad weather arrives! In the meantime, follow me as we head out on our next weekend journey into the night…

Friday, August 8, 2008 – Our first order of business for the weekend will be to pick up a Lunar Club challenge we haven’t noted so far this year – Hipparchus. Located just slightly south of the central point of the Moon and very near the terminator, this is not truly a crater – but a hexagonal mountain-walled plain. Spanning about 150 kilometers in diameter with walls around 3320 meters high, it is bordered just inside its northern wall by crater Horrocks. This deep appearing “well” is 30 kilometers in diameter, and its rugged interior drops down an additional 2980 meters below the floor. To the south and just outside the edge of the plain is crater Halley. Slightly larger at 36 kilometers in diameter, this crater named for Sir Edmund Halley is a little shallower at 2510 meters – but it has a very smooth floor. To the east you’ll see a series of three small craters – the largest of which is Hind.

On this date in 2001, the Genesis Solar Particle Sample Return mission was launched on its way toward the Sun. On September 8, 2004, it returned with its sample of solar wind particles – unfortunately a parachute failed to deploy, causing the sample capsule to plunge unchecked into the Utah soil. Although some of the specimens were contaminated, many did survive the mishap. So what is “star stuff?” Mostly highly charged particles generated from a star’s upper atmosphere flowing out in a state of matter known as plasma.

Despite tonight’s Moon, let’s study one of the grandest of all solar winds as we seek out an area about three fingerwidths above the Sagittarius teapot’s spout as we have a look at the magnificent M8.

Visible to the unaided eye as a hazy spot in the Milky Way, fantastic in binoculars, and an area truly worth study in any size scope, this 5200 light-year diameter area of emission, reflection, and dark nebulae has a rich history. Its involved star cluster – NGC 6530 – was discovered by Flamsteed around 1680, and the nebula by Le Gentil in 1747. Cataloged by Lacaille as III.14 about 12 years before Messier listed it as number 8, its brightest region was recorded by John Herschel, and dark nebulae were discovered within it by Barnard.

Tremendous areas of starbirth are taking place in this region, while young, hot stars excite the gas in a region known as the “Hourglass” around the stars Herschel 36 and 9 Sagittarii. Look closely around cluster NGC 6530 for Barnard Dark Nebulae B 89 and B 296 at the nebula’s southern edge…and try again on a darker night. No matter how long you choose to swim in the “Lagoon” you will surely find more and more things to delight both the mind and the eye!

Saturday, August 9, 2008 – Today in 1976, the Luna 24 mission was launched on a return mission of its own – not to retrieve solar wind samples, but lunar soil! When we begin our observations tonight, we’ll start by having a look at another great study crater – Archimedes. You’ll find it located in the Imbrium plain north of the Apennine Mountains and west of Autolycus.

Under this lighting, the bright ring of this class V walled plain extends 83 kilometers in diameter. Even though it looks to be quite shallow, it still has impressive 2150 meter high walls. To its south is a feature not often recognized – the Montes Archimedes. Though this relatively short range is heavily eroded, it still shows across 140 kilometers of lunar topography. Look for a shallow rima that extends southeast across Palus Putredinus toward the Apennines. Mark your challenge notes!

Now let’s go have a look at a star buried in one of the spiral arms of our own galaxy – W Sagittarii…

Located less than a fingerwidth north of the tip of the teapot spout (Gamma), W Sagittarii (RA 18 05 01 Dec -29 34 48) is a Cepheid variable that’s worth keeping an eye on. While its brightness only varies by less than a magnitude, it does so in less than eight days! Normally holding close to magnitude 4, nearby field stars will help you correctly assess when minimum and maximum occur. While it’s difficult for a beginner to see such changes, watch it over a period of time. At maximum, it will be only slightly fainter than Gamma to the south. At minimum, it will be only slightly brighter than the stars to its northeast and southwest.

While you watch W go through its changes – think on this. Not only is W a Cepheid variable (a standard for the cosmic distance scale), but it is also one that periodically changes its shape. Not enough? Then think twice… Because W is also a Cepheid binary. Still not enough? Then you might like to know that recent research points toward the W Sagittarii system having a third member as well!

Sunday, August 10, 2008 – Today in 1966 Lunar Orbiter 1 was successfully launched on its mission to survey the Moon. In the days ahead, we’ll take a look at what this mission sent back! Tonight keep a very close watch on Selene as Antares is less than a degree away. Check for an occultation event!

Our lunar mission for tonight is to move south, past the crater rings of Ptolemaeus, Alphonsus, Arzachel and Purbach, until we end up at the spectacular crater Walter. Named for Dutch astronomer Bernhard Walter, this 132 by 140 kilometer wide lunar feature offers up amazing details at high power. It is perhaps most fascinating to take the time to study the differing levels, which drop to a maximum of 4130 meters below the surface. Multiple interior strikes abound, but the most fascinating of all is the wall crater Nonius. Spanning 70 kilometers, Nonius would also appear to have a double strike of its own – one that’s 2990 meters deep!

Eta Sgr
Eta Sgr
Although it will be tough to locate with the unaided eye thanks to the Moon, let’s take a closer look at one of the most unsung stars in this region of sky – Eta Sagittarii (RA 18 17 37 Dec -36 45 42). This M-class giant star will display a wonderful color contrast in binoculars or scopes, showing up as slightly more orange than stars in the surrounding field. Located 149 light-years away, this irregular variable is a source of infrared radiation and is a little larger than our own Sun – yet is 585 times brighter. At around three billion years old, Eta has either expended its helium core or just began to use it to fuse carbon and oxygen – creating an unstable star capable of changing its luminosity by about 4%. But have a closer look…for Eta is also a binary system with an 8th magnitude companion.

Keep an eye out for the beginnings of the Perseid meteor shower and a futher report! Wishing you clear skies and a great weekend…

This week’s awesome images are: Nimbus – Credit: Shevill Mathers, Hipparchus: Credit: Tammy, M8 – Credit: NOAO/AURA/NSF, Archimedes – Credit: Wes Higgins, Walter – Credit: West Higgins and Eta Sagittarii – Credit: Palomar Observatory courtesy of Caltech. Thank you for sharing!

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