Sky Talk November 2008: Three Seasonal Asterisms at Once!

Each of the four seasons has an asterism or distinctive group of stars associated with it that can be used to find the constellations visible at their respective times of the year. Late evenings in November offer stargazers the opportunity of viewing three of them simultaneously—those of the Summer, Fall and Winter sky.

Many skywatchers enjoy special challenges, such as catching the earliest (thinnest) crescent after sunset when the Moon has passed through its New phase—or the annual Messier Marathon each Spring when observers attempt to view all 110 objects on this famous list of deep-sky wonders. Well, here’s another challenge to consider: seeing three of the four seasonal geometric star patterns at one time. This feat is only possible for a brief window in mid-November around 10:00 p.m. local time. It also requires an essentially unobstructed eastern and western horizon.

One of these three is the big Summer Triangle, visible low over the western horizon and formed by the brilliant blue-white stars Vega in the constellation Lyra, Deneb in Cygnus, and Altair in Aquila. Moving high to the left in the sky is the Fall Square. Actually rectangular in shape, it’s made up of four 2nd-magnitude stars marking the Great Square of Pegasus. And while not as obvious as the Summer Triangle, its big dim empty space is quite easy to recognize.

The third asterism is the Winter Circlet rising in the eastern sky. Use your Scientifics Star and Planet Locator (#30092-27) to see just how close some of the stars in the Circlet (notably Sirius) and the Triangle (notably Altair) are to their respective horizons! The Circlet (also sometimes called the “Heavenly G”) is unmistakable, being made up of bright luminaries from six different constellations. Starting with the highest star in the formation, we find golden Capella in Auriga. Proceeding counterclockwise are Castor and Pollux in Gemini, followed by Procyon in Canis Minor (all white in hue). The brightest star in the Circlet—and indeed in the entire sky—is dazzling Sirius in Canis Major, just climbing over the horizon and sparkling like a blue-white celestial diamond. Continuing around next to the radiant constellation Orion is bluish Rigel, and completing the circle is orange Aldebaran in Taurus. Smack dab in the middle of the Circlet is ruddy Betelgeuse (also in Orion), which is what some observers use to form the “G” mentioned earlier. Others see this star as the central blaze in some heavenly campfire. All three of these asterisms can be used to find your way around the skies of their respective seasons, and The Edmund Sky Guide has excellent sections on using each of them for this purpose. (For the record, the fourth asterism is the Spring Diamond, involving stars in the constellations Ursae Major, Bootes, Virgo and Leo.)

The annual Leonid Meteor Shower scheduled to peak in the early morning hours of November 17th will be washed out this year by a bright gibbous Moon. It rises around 8:00 p.m. local time, some three hours ahead of the radiant in the constellation Leo, and remains in the sky most of the night.

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of five books on stargazing.


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