Jimmy Westlake: The big and little dippers
April 8, 2013
The celestial Bears, Ursa Major and Ursa Minor, are coming out of their winter hibernation and can be seen parading around the north celestial pole this month. Better known in the U.S. as the Big Dipper and the Little Dipper, these star patterns are known and loved by all.
The seven bright stars that form the Big Dipper shine prominently above the northeastern horizon as darkness falls in late March. It looks as if the Big Dipper is balancing precariously on its bent handle. The seven stars of the Big Dipper are named, in order from top to bottom, Dubhe, Merak, Phecda, Megrez, Alioth, Mizar and Alkaid.
Dubhe and Merak, the two stars at the top of the Big Dipper's bowl, are nicknamed the "Pointer Stars" because a line drawn through the two stars and extended downward will lead you to the North Star, Polaris. Polaris is about as bright as the stars that form the Big Dipper and is important because it lies very close to the north pole of the sky, called the north celestial pole. As the Earth rotates on its axis, the celestial sphere appears to pivot around the north celestial pole with Polaris positioned near the center of the bull's eye. It remains nearly motionless in our sky all night and can always be relied on to point out the way north.
Polaris also happens to be the last star in the handle of the star pattern we call the Little Dipper. Like the Big Dipper, the Little Dipper is composed of seven stars, although only two of the seven are prominent, Polaris and Kochab. Moving down the handle from Polaris, the names of the remaining stars are Yildun, Epsilon, Zeta, Eta, Pherkad and Kochab. Together, Kochab and Pherkad are nicknamed the "Guardians of the Pole" because they circle Polaris as if protecting it from harm.
The four stars that make up the Little Dipper's bowl are very conveniently and quite by chance classified by their brightness as second-, third-, fourth- and fifth-magnitude stars. On the magnitude scale used by astronomers, a sixth-magnitude star would be a star at the limit of visibility to the human eye. One can judge the quality of the night sky and/or the degree of light pollution by noting how many of the Little Dipper's bowl stars can be seen. If the sky is dark and clear, all four stars in the bowl should be visible. Unfortunately, from many urban and suburban areas the two fainter stars are rendered invisible by artificial lighting that needlessly points upward and brightens the night sky.
In the early spring, the two dippers are positioned so that the Big Dipper seems to be pouring its contents into the Little Dipper below it. Perhaps the overflowing bowl of the Little Dipper is responsible for the April showers that rain down on us and bring the May flowers that we all enjoy.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out his astrophotography website at http://www.jwestlake.com.