By Peter Forrester | April 18, 2019
Greetings all! Let’s talk about one of the best-known group of stars (not a constellation) in the sky.
Yes, I mean the Big Dipper, which is referred to as an “asterism,” or a collection or shape of stars that are not recognized as a constellation (the 88 “official” ones were defined in 1928, at a meeting of the International Astronomical Union, which just celebrated its 100-year anniversary a few days ago).
Officially, by the way, a constellation is a region in the sky, not just a set of bright stars that form a shape but everything around them as well. For instance, when the Sun, Moon, or a planet is in the same direction as a constellation we say that it is “in” that constellation. The same is true of comets, asteroids, nebulas, quasars, pulsars, black holes, whole galaxies, or even dust. Every point in the sky is defined as belonging to one or more constellations (the “more” being along the borders).
But back to the asterism known as the Big Dipper. In this case, the 7 stars that make up the Dipper (called the Plough in the U.K.) are part of the constellation Ursa Major (the Great Bear).
I have not covered the Big Dipper until now because as of a few months ago, it was still very low to the horizon in the early evening, and thus not very visible if you have any kind of treeline.
Six of the seven stars in the Dipper are of 2nd degree magnitude, and the other one is 3rd degree. They are the brightest stars in Ursa Major.
Many of my readers will already know that the Big Dipper, as I said about Orion, can be used to find other stars and constellations easily. Tracing the side of the bowl upward, the line extends to Polaris, the North Star, which is part of the constellation called Ursa Minor. This is very useful for nighttime navigation without a compass, as Polaris is always located to the north. Sailors have been using the stars to find their direction for millennia, and the Big Dipper was what they used to locate Polaris, since Polaris is only 3rd degree and looks similar to every other star of the same color and brightness.
Using the same two stars that form a line to Polaris, tracing the line in the opposite direction brings you to another bright constellation, one of the Zodiac: Leo the Lion, whose shape resembles a rectangle that touches a backwards question mark. Thirdly, following the curve of the handle brings you to the constellations Bootes (resembling a kite, its brightest star called Arcturus) and Virgo (brightest star Spica). Hence the once-popular saying “Arc to Arcturus, and spike to Spica”). There are some other lesser known direction-finding lines using different stars in the Big Dipper, see the Big Dipper reference below if you want to know more about this.
Ursa Major is very close to the north celestial pole, and thus as explained previously, it is circumpolar. Therefore, meaning it never sets from the sky when seen from northern latitudes such as the United States or Europe, but appears to go in a circle every 24 hours around Polaris, the star located directly over the North Pole of Earth.
Incidentally, the Big Dipper has been considered as the tail of a bear, and various other images (this one makes no sense because bears don’t have long tails). The rest of the bear consists of dimmer stars to the right of and below the bowl when it is lying flat on its bottom (the bowl to the right of the handle – it is close to the horizon when it is in this direction).
In popular culture, the Big Dipper is found on the flag of the state of Alaska, and a few other flags and coats of arms. It may even be mentioned in the Bible (its reference in the book of Job is disputed by scholars).
I once made a scale-model view of the distance of the stars in the Big Dipper, a side-view, if you will. One of the stars was so much further than the rest that I had to mount it on a separate piece of cardboard from the rest. This was a school project, and unfortunately I no longer have it. But it is interesting to note that the shapes we see are only from our perspective, and most constellations would appear very different in other solar systems.
I’m going to leave the discussion there, because I’m feeling a bit under the weather, but I wish you all the best time while skywatching, and be sure to look up when you go out!
Previous in series: Circumpolar Constellations: Cassiopeia
Wikipedia. (2019, March 10). Asterism (astronomy). Retrieved from https://en.wikipedia.org/wiki/Asterism_(astronomy).
Wikipedia. (2019, April 2). Big Dipper. Retrieved from https://en.wikipedia.org/wiki/Big_Dipper.
Wikipedia. (2019, April 1). Constellation. Retrieved from https://en.wikipedia.org/wiki/Constellation.
Wikipedia. (2019, March 29). Ursa Major. Retrieved from https://en.wikipedia.org/wiki/Ursa_Major.