Constellation of the Month: Scutum, the Shield

While most of the constellations in the sky represent mythological, scientific or technological entities, two are related to actual people alive around the time of Johannes Hevelius. These two oddballs are Berenice's Hair (Coma Berenices), named by Tycho Brache, and Sobieski's Shield (Scutum Sobiescianum), one of Hevelius's many creations. The latter was shortened to "Scutum" when the 88 official constellations were codified. It is the fifth smallest constellation, only 109 square degrees.

Click the image for a larger map Scutum is the small constellation (the boundaries form a vertical rectangle) near the middle of our sky map, looking about 60 degrees up in the southern sky in mid-August. With such a small constellation, it is not surprising that there are few deep-sky objects in Scutum. Nonetheless, it has the Scutum Star Cloud, which you can see with your naked eye from a dark site.

The eldest son of the Castellan of Crakow, Jan Sobieski (1629-1696) grew to be a great military leader in the Polish Army. Sobieski was elected king in May 1674, becoming King John III. He later returned to the military and led an army of 30,000 Poles that helped drive invading Turks out of Austria, a Polish ally, in 1683. The defeat was the beginning of the end for the Islamic Ottoman Empire's invasion, securing Christianity as the main religion of Europe. In 1690, Hevelius commemorated this victory by putting the Janina Coat of Arms of King John III in the sky.

Scutum is high in our southeast as it gets dark. Tucked between Serpens Caput, Aquila and Sagittarius, this little constellation is entirely in the Milky Way. Not a uniform band of brightness around the sky, the Milky Way has bright areas, knots of stars and dark rifts where few stars are found. One of the bright knots, the Scutum Star Cloud, occupies the northeastern part of the Shield.

One deep-sky object in Scutum is M11, the "Wild Duck" cluster. The name comes from the telescopic views of the brightest stars of this cluster that form a "V," like a flock of ducks migrating through the skies. This open cluster is almost half the size of the full moon, 14 minutes-of-arc across. It has over 2,900 stars, bringing its total brightness up to magnitude 6.3, barely visible to the naked eye in a dark site.

With so many stars, M11 is a telescopically rich cluster, with over 500 stars brighter than magnitude 14. It was first discovered by Gottfried Kirch of the Berlin Observatory in 1681. Charles Messier added it to his catalog as number 11 on May 30, 1764. Some 6,000 light-years away, these stars all formed from the same gas cloud 250 million years ago and travel with the original speed and direction of the cloud, so they remain clustered together in northeast Scutum.

The Planets for August 2007

Venus finishes its appearance in the evening sky for this year as it heads back down toward the Sun. At the beginning of the month, it can be found very low in the western sky as it starts to get dark. Saturn is 10 degrees to the right of Venus, but the sky will still be so bright that you probably will not be able to find it. Mercury will be below this pair, but also cannot be glimpsed due to the Sun's glow. On Aug. 1, Venus is a huge crescent, only 9 percent illuminated but 50.5 seconds-of-arc across. Venus will be magnitude -4.3 as it hurtles along in its orbit between the Earth and Sun. Venus will be in Sextans during the early part of the month.

Ophiuchus is still home to Jupiter this month. The King of the Planets is 40.2 seconds-of-arc across and shines at magnitude -2.3. Jupiter is in the southern sky at sunset, and sets by 1 a.m.

About the same time, Mars comes up in the east-northeast. Mars will be drifting eastward in Taurus all month, passing just north of the Hyades star cluster on Aug. 21. At mid-month, Mars is magnitude 0.42 and 7.5 seconds-of-arc across.

Tuesday morning, Aug. 28, will give us an opportunity to see a total lunar eclipse. This kind of eclipse occurs when the Moon travels through the Earth's shadow. This can occur only around full moon, but usually does not because the Moon can be up to five degrees above or below the center of the shadow. But this month it travels just south of the center of the shadow, giving us a long eclipse.

The eclipse begins at 1:54 a.m., when the Moon enters the penumbral or partial part of the shadow. At first you will not see anything happen, but as the Moon moves farther into the shadow you will see a shadowing on its northeast quadrant. By 2:51 a.m., the northeast corner of the Moon will no longer see the Sun at all, entering the umbral phase. The trailing southwestern edge will fully enter the umbral shadow at 3:52 a.m.

At this point the entire Moon will be illuminated only by the sunrises and sunsets all around the world. On the Earth, sunlight travels a long way through our atmosphere near the sunrise or sunset areas. The blue light is scattered out, creating a blue sky, but leaving the red light that we see at sunrise or sunset. The atmosphere also bends the red light so it can illuminate the eclipsed Moon when the Sun's light cannot reach it directly, giving a reddish hue.

The Moon reaches maximum eclipse at 4:37 a.m. It continues through the shadow until 5:22 a.m., when the eastern edge pops into sunlight again. By 6:23 a.m., the entire Moon will see at least a little sunlight, but our sky is starting to get very bright as the sun rises at 6:39 a.m. Finally, the Moon, still partially eclipsed, drops below the horizon at 6:48 a.m. Lunar eclipses are fun to watch, so don't miss this one and "keep watching the sky"

An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.

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