December 18 – Search The GLOBE

Today’s factismal: The “Big Dippper” is not a constellation.

If you ask anyone other than a Trekkie what a constellation is, they’ll tell you that it is a grouping of stars in the sky.  And if you ask them to name a constellation, odds are that they’ll tell you about the Big Dipper. While they would be mostly right about what a constellation is, they would be mostly wrong about the Big Dipper being one. Instead, the Big Dipper is what astronomers refer to as an “asterism”. So what’s the difference and why does it matter?

Since time immemorial, people have looked up into the night sky and grouped together the stars that they see. They typically pulled names from their mythology to assign to the groups of stars, and the groups changed from people to people. For example, the Chinese know of a constellation called the Supreme Palace Right Wall, the Vedic astrologers called part of it Purvaphalguni and part of it Chitra, and it makes up part of the Leo and Virgo constellations of the ancient Greeks. Given all of the confusion over naming the groups of stars, in 1922 the IAU defined a set of constellations with specific names and groups of stars. Naturally, since most of the IAU members at the time were European and American, they followed the Greek names for the most part.

The asterism of the Big Dipper (Image courtesy NASA, taken by MMM MMM on the ISS)

The asterism of the Big Dipper
(Image courtesy NASA, taken by Donald R. Pettit on the ISS)

As a result, there is a constellation known as Ursa Major (“The Big Bear”) that includes both faint, hard to see stars and bright, easily recognizable stars. The most easily seen set of stars in Ursa Major form a shape like a plow or a dipper, which is why they are sometimes referred to as “the Plow” or “the Big Dipper”.  But that asterism is only part of the whole; where the Big Dipper includes just seven stars, there are at least twenty in Ursa Major.  (As an amusing sidelight, the Big Dipper is actually the tail of the bear; you can make up your own joke about what end most people see.)


The sad thing about the Big Dipper and many other asterisms is that they are becoming the only part of constellations that most people can see today. Thanks to light pollution, the fainter stars are being washed out of the night sky and only the brighter stars, which form most of the asterisms, are left visible. This is more than a loss of beauty; light pollution also causes problems for sea turtles (who rely on the light of the Moon for navigation), whales, and many other creatures. If you’d like to learn more about light pollution and maybe even help measure it, why not join GLOBE at Night? This long-running citizen science project has been measuring light pollution for nearly a decade now and has made over 100,000 measurements of the night sky.  To learn more, scope out:

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