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About the cover


Scutum, the Shield

Plus the planets for September.


Scutum is a small, rectangular constellation just north of Sagittarius, where the center of the Milky Way resides. Like Sagittarius, Scutum also has the Milky Way passing through it. Scutum is 84th in size among the 88 official constellations, with only a few deep-sky objects in it. On September evenings it can be found about halfway up in the southern sky.

star map

Click the image for a larger map.


Scutum, the Shield, is a rectangular constellation just above the teapot-shaped constellation Sagittarius. This Shield is decorated with the brightest part of the Milky Way, the Scutum Star Cloud. This bright area is unobscured by interstellar dust that dims other parts of the Milky Way.

Scutum is not one of the ancient mythological constellations, but rather was invented in 1684 by Polish astronomer Johannes Hevelius and published posthumously in his 1690 catalog Prodromus Astronomiae. Its original name was Scutum Sobiescianum (the Shield of Sobieski), referring to Polish King Jan III Sobieski, who led forces of the Polish-Lithuanian Commonwealth along with those of the Holy Roman Empire to defeat the Ottoman Empire near Vienna in 1863. This broke the Ottoman siege of Vienna and put the Habsburg dynasty solidly in control of the Holy Roman Empire and Central Europe. When Scutum became an official constellation, the name was shortened.

In north-central Scutum is a bright part of the Milky Way called the Scutum Star Cloud. This area is one of the brightest sections of the Milky Way, where the light coming to us from the stars beyond is unimpeded by the dust common in other areas of our galaxy. The Scutum Star Cloud has no official designation, but is rather obvious in our sky. It is not a cluster, nebula or other astronomical object, just a bright region along the Milky Way.

Many other areas of the Milky Way are dimmed by dust. We are all familiar with dust here on Earth — particles of the local soil small enough to be picked up by the wind and far too often deposited in our houses. But what constitutes dust out in the Milky Way (and other galaxies)?

Dust in space has nothing to do with soil at all. It is solid matter that is composed of various molecules sticking together. The dust is not the hard, round, grains that you might imagine, but forms rope-like strands that are bound together by molecular bonds. These molecules are made up of silicates, carbon, ice and/or iron compounds.

When you look at dust clouds in space, the most important thing that determines how the cloud looks is whether there is a nearby star in front of it. If there is such a star, its light falls on the dust cloud, illuminating it. Because of the size of the dust particles, they are more likely to reflect blue light, while red light still easily passes through. This means these illuminated clouds appear blue in photographs. In fact, almost all of the blue nebulae are dust clouds reflecting the blue light of a nearby star.

But what about the dust clouds lacking a nearby star? These dust clouds are not bright, but dark. Stars far behind the cloud have the blue part of their light scattered away, but some of the red light gets through with only slight scattering. This makes those background stars appear much redder and dimmer than they actually are.

These un-illuminated clouds often appear as dark areas in the Milky Way, with only the brightest stars shining through. Examples of these dark clouds include the area behind the Horsehead Nebula, the continental edges around the North American Nebula and the dark areas west of the Scutum Star Cloud. There are many more that are not associated with a bright nebula. Small dark nebulae where star formation may be occurring are called Bok Globules, after Dutch-American astronomer Bart Bok who first studied these objects in the 1940s. Perhaps our own Sun was born on a Bok Globule some 5 billion years ago.



The Planets for September


Making a very poor appearance in our evening sky this month is Mercury. Due to the tilt of its orbit, Mercury is traveling southeastward across the already southward-leaning ecliptic that is in the western sky at this time of year. This means that for northern hemisphere observers, Mercury will not get very high above the western horizon. It will first appear in the west during the second week of the month. It will then travel slowly southward only five degrees above the horizon as it gets dark. It will not even reach its farthest distance from the Sun until next month. At midmonth, the Messenger of the Gods' disc will be 5.2 seconds-of-arc across and 87% illuminated. It shines at magnitude -0.3, setting just before 8 p.m. Mercury starts the month in Leo, moving into Virgo where it ends the month in the south-central part of that constellation. It passes 0.8 degrees north of Spica on Sept. 24.

Watch the Skies
(times MDT)


Sept. 5, 5:36 a.m. — New Moon
                  7 a.m. — Venus 1.8 degrees north of Spica

Sept. 8, evening — Venus, Spica and Moon together in                                        western sky

Sept. 12, 11:08 a.m. — First Quarter Moon

Sept. 19, 5:13 a.m. — Full Moon
                     6 p.m. — Venus 4 degrees south of Saturn

Sept. 22, 2:44 p.m. — September Equinox

Sept. 24, 1 p.m. — Mercury 0.8 degrees north of Spica

Sept. 26, 9:55 p.m. — Last Quarter Moon

Venus will pass Spica earlier in the month, on Sept. 5, when it will be 1.8 degrees north of that first-magnitude star. Venus starts the month in Virgo, moving into central Libra where it ends the month. The Goddess of Love is about 16 degrees up in the west-southwest as it gets dark and sets by 9 p.m. At midmonth it is magnitude -4.1 with a disc that is 16.4 seconds-of-arc across and 69% illuminated, becoming thinner every day.

Saturn is magnitude +0.7 at midmonth, shining out from western Libra, where it is moving slowly eastward. The disc is 15.7 seconds-of-arc across with the Rings spanning 35.6 seconds-of-arc. They are tilted down 18.7 degrees with the northern face showing. Saturn is getting lower each day, only 20 degrees up in the west-southwest as it gets dark and setting around 9:20 p.m. Venus will pass 4.0 degrees south of Saturn on Sept. 19.

Jupiter rises around 1:30 a.m. in the east-northeast. Moving slowly eastward in central Gemini, Jupiter's disc is 36.0 seconds-of-arc across and it is 59 degrees up in the east-southeast as it gets light. The King of the Gods glows at magnitude -2.1.

Mars rises around 3:30 a.m. and is 34 degrees up in the east as it gets light. The Red Planet moves eastward from west-central Cancer into western Leo during September. It is magnitude +1.6 with a disc that is 4.2 seconds-of-arc across at midmonth.

It is equinox time again; this time the Sun crosses the equator traveling north to south. In the northern hemisphere, this marks the beginning of autumn on Sept. 22 at 2:44 p.m. Nights will keep getting longer, so you will have more and more time to "keep watching the sky"!




An amateur astronomer for more than 40 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.


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