Dawn or dusk there’s much to see

The Harvest Moon wears a colorful corona in a blanket of moving cumulus clouds last night. Details: 16mm lens at f/5.6, ISO 400 and 5-second time exposure. Photo: Bob King

Another view of last night’s lunar corona. This beautiful closeup was taken by Lyle Anderson of Duluth.

The sky partially cleared off last night and the Harvest Moon lit the ground so brightly the gravel road near my home glowed like fog from a distance. As the clouds came and went they painted a small disk of pale blue and reddish orange called a corona around the moon. At times the red was very pure and intense. Perhaps you saw this, too. Coronas happen when light is diffracted by countless microscopic water droplets or ice crystals within the clouds. The different waves of light either reinforce or cancel each other out like colliding waves from opposite directions on a pond or lake. Where light is reinforced we see a bright ring of color.

While you’re visiting with Arcturus again this week, see if you can make out the full outline of the constellation Bootes (bow-OH-teez) which is shaped like an ice cream cone. This map shows the sky around 8 o’clock local time. Arcturus will soon disappear from view below the western horizon but not for long. In late October it reappears in the morning sky at dawn. Maps created with Stellarium.

Looking around the sky, I noticed how low and level the Big Dipper has become in the northwestern sky. We can use the Dipper one last time to bid farewell to Arcturus, a brilliant orangish star so prominent in the early summer. Before it disappears from the evening sky, "arc to Arcturus" one more time by following the curve of the Big Dipper’s Handle. Arcturus, an orange giant star 37 light years from Earth, is the brightest star in Bootes the Herdsman and the third brightest star in the night sky. Only Canopus — visible from the southern states — and Sirius, now rising in the morning sky below Orion, are brighter. Because it’s so bright and at the same time so low, Arcturus is more strongly affected by air turbulence in the lower atmosphere than stars higher up. That means it twinkes a lot. Take a look and you’ll see what I mean.

On a night a week ago, star twinkling was so strong I watched in disbelief as some of the stars flashed well beyond their normal brightness and then moments later almost disappeared from view! Rarely have I seen such atmospheric schizophrenia under an otherwise clear sky. Twinkling not only affects a star’s brightness but also its color. Changing air densities and currents act like an optician’s box of lenses, refracting a star’s light from pale blue to yellow to red in no predictable pattern. Binoculars will help bring out the colors even better.

This map shows the dawn sky about 45 minutes – one hour before sunrise tomorrow morning October 6 (6:15 a.m. for Duluth). Venus will be very low in the northeast and easy to spot if you have an open view to the east. Mercury is 5 degrees or three fingers held horizontally at the end of your outstretched arm below Venus. It’s visible with the naked eye but binoculars will make it easy. Saturn will only be visible in binoculars. Higher up, look for a "cascade" of stars arcing towards Orion beginning with Castor (top) and including the planet Mars.

There’s a whole lot shakin’ in the morning sky right now for planets and bright stars. The winter constellations are well-placed just before dawn with Orion and his three Belt stars leading the charge high in the southeastern sky. Look for Mars directly below the Gemini twins of Castor and Pollux. Venus, Mercury and Saturn are much lower in the east below Regulus in Leo the Lion. To best enjoy the cascade of bright stars that includes Mars, go out 90 minutes to an hour before sunrise when the sky is still relatively dark. 45 minutes before sunrise, when Venus is better placed, try to use that planet to guide you to Mercury and Saturn. too.

If it’s cloudy by you tonight and tomorrow, not to worry. Arcturus will be around for at least another week, and the stars and planets of the morning sky will be in approximately the same place through the week as well.