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The dance of the planets still continues in the early evening sky, and will do so well into September. With a clear, unobstructed western horizon we can sit on the sidelines and enjoy the show.

For a while even tiny, first-planet-out-from-the-Sun, Mercury, has had a part in the dance. If you hurry it can still be seen very close to the western horizon about a half-hour to 45-minutes after sunset. But look soon, by Friday, Aug. 13, it will be gone from the skies until it returns as a morning object in early September.

Sometimes astronomy is not so much as what you see as what you don’t see. For example, there are numerous bright nebula or areas of gas and dust that are either lit by reflecting the light of a nearby star or by the light from a star embedded in the cloud causing the gas and dust to glow.

But there are some nebulae that don’t do either. In fact they scatter the light from distant background stars allowing no light at all to pass through them. They are called “dark nebulae.”

Go out to your favorite dark-sky place when it is good and dark and look up. Running almost overhead is the bright stream of stars called the Milky Way. It is our home galaxy.

Using just your eyes, or binoculars if you wish, slowly scan the length of the Milky Way from Cassiopeia in the north to Sagittarius in the south. 

Along the way you will notice several dark areas in the stream of bright stars where nothing appears. These areas for a long time were thought to contain no stars at all. 

Only when larger telescopes came into use was it discovered that these dark spots were actually vast clouds of dust and gas that was scattering and thus blocking the light of stars beyond them.

In the area of the constellation Cygnus alone there are dozens of these dark nebulae, the biggest of them runs down the length of Cygnus. It is called the “The Great Rift” and to just the eyes alone there appears to be no stars at all right in the middle of one of the brightest areas of the Milky Way.

As you shift your gaze further south other areas of “darkness” will appear. These areas are associated with the great clouds of interstellar dust that surround the center of our galaxy extending out into the various spiral arms. One glance at a photograph of the Andromeda Galaxy, or any other galaxy for that matter, will show these clouds of light-scattering dust and how they look.

In the southern hemisphere there is a famous dark nebulae. It is called the “Coal Sack” and is located in the Crux constellation. To the ancient societies of the Aborigines in Australia and the Polynesian residents of the south Pacific Ocean this dark area has been recognized for centuries. 

One 40,000 year-old Aboriginal bark drawing depicts the Coal Sack as a fish speared by two brothers. The brothers depicted by the two brightest stars of Crux.

To those in Polynesian and other southeast Asia populations the dark place was a Manta Ray.

So get out and look for the dark in the dark.

SKY WATCH: Third quarter Moon, yesterday, Aug. 3. Tonight a spectacular conjunction in the west after sunset of Mars, Saturn, and Venus. This grouping will continue for the next two weeks with the planets continuing to move closer together until Aug. 12 when they will be joined by a very slender crescent Moon.

NEXT TIME: The Great Perseid meteor shower and more astronomical blathering.