Winter’s Road Map to the Constellations

by Rod Kennedy

One of the challenges to new sky-watchers is finding their way around the sky.  The old mantra given by more experienced observers is: “learn to recognize the constellations.”  This is sound, practical advice.  However, the problem lies in our urban (or sub-urban) light polluted skies.  Many constellations are made up of stars too faint to cut through this artificial haze. In addition only a handful of constellations have more than one bright star.  Fortunately January offers us a chance to find 6 well known constellations simply by finding their bright stars.

We begin by locating the most famous constellation in the winter sky, Orion the Hunter.  Orion can be found midway up the sky above the southern horizon.  Orion is easy to pick out because he is made up of seven bright stars that form the rough shape of an hour glass.  The bright red star in his shoulder is Betelgeuse.  Moving diagonally across and down the constellation we find the bright blue-white star Rigel.  Rigel is the beginning point for our navigation.

From Rigel we move down and toward the East to the white star Sirius in Canis Major.  Sirius is the brightest star in the evening sky.  This is partly due to its temperature.  Sirius is a very hot white star but is not as hot as Rigel which appears blue white.  This tells us that Sirius is much closer to us than Rigel.  In fact Sirius is nearly 800 times closer than Rigel.  Moving up and to the East we find the star Procyon in Canis Minor.  Procyon is a yellow-white star, meaning that it is slightly hotter than our own sun.  Procyon is also about twice as large as our sun in radius.

Higher in the sky we find Gemini the Twins.  The two brightest stars in Gemini are Castor and Pollux.  They are close enough together to appear as twins themselves.  Castor is higher in the sky than Pollux and appears white.  Pollux appears orange, meaning it is far cooler than Castor.  Still higher in the sky and slightly toward the Northwest we find Capella in Auriga the Charioteer.  Capella is a G type star like our sun so it appears yellow although it is twelve times larger than our sun.  Capella is about 45 degrees north of the Celestial Equator, which means it is one of the brightest stars in the Northern part of the sky.  The ancient Olmec people of Central America aligned  a building in their principal city of Monte Alban to mark the spot where Capella first rises above the Northeastern horizon.

Moving South from Capella we find Aldebaran, in Taurus the Bull.  Aldebaran is a red giant star, but it looks red-orange in the night sky.  Aldebaran is in the midst of a wide cluster of stars known as the Hyades, even though Aldebaran is actually much closer to Earth than the cluster itself.  Aldebaran is also slightly south of a much more famous cluster of stars, the Pleiades.  The Pleiades are easy to see under moderately dark skies but light pollution almost completely blots them out.

This tour of bright stars forms a large asterism known as the Winter Hexagon.  An asterism is a group of stars that form an easily seen shape of stars that is part of a larger constellation or several constellations.  The winter hexagon is one of the brightest and largest asterisms in the night sky.  Under light polluted skies this asterism gives us an easy road map for finding constellations that may be otherwise difficult to identify.

January 2011 Star Chart

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