By Darrell Heath
Astronomy Columnist
True or False: The Big Dipper is the most instantly recognized constellation in our Northern Hemisphere sky.
False, the Big Dipper is not a constellation.
That was a bit of a trick question but, yeah, the Big Dipper really isn’t one of the 88 constellations recognized by the International Astronomical Union (the official body for naming celestial objects). We call the unofficial star patterns like The Big Dipper, Little Dipper, Northern Cross, Summer Triangle, etc. “asterisms”. Asterisms are patterns made up of several stars from one or more of the other official constellations. The Big Dipper is an asterism made up of the seven brightest stars in the constellation of Ursa Major, The Great Bear, and forms the animal’s hindquarters and long tail
Of the 88 official constellations we associate 48 of them with the Ancient Greeks but many of these star patterns probably go back even further in time. There are some archaeoastonomers (astronomers who study how ancient people interpreted the sky) who think that Ursa Major may go back 50,000 years ago when a bear cult is thought to have existed among Ice Age people. It’s notable that many disparate cultures have seen a bear when looking at these stars. Aside from the Greeks and Romans the Lakota and many other First Nations people interpret these stars as a bear. As have the Sami from the European Arctic. To be fair though there are many more cultures that see something totally different, including a plow, wagon, coffin, skunk, camel, shark, canoe, bushel, sickle, even a hog’s jaw.
The mythology behind the constellation is varied but one of the most popular versions has the Roman god Jupiter lusting after a nymph named Callisto. Jupiter fathers her child, a boy named Arcas. Eventually Jupiter’s wife, Juno, learns about her husband’s indiscretion (one of many it seems) and gets her revenge by turning Callisto into a bear. Unable to reverse Juno’s magic, and in order to keep the two together, Jupiter turns their son into a bear as well, grabs them both by the tail and hurls them into the sky, the big bear becoming Ursa Major and the little bear Ursa Minor. According to one witty astronomer this explains why these bears have such unusually long tails. Juno gets the last word in by making sure that the two bears never rise and set like the other constellations and, so, never get to dip their feet into Earth’s oceans. This bit is the ancient’s way of explaining why we see the bears year round in our northern sky. Most of the constellations come and go with the seasons and we see a given set of them rise nightly in the east and set in the west just like the Sun does during the day. But five constellations in our northern hemisphere sky stay with us all year long: Cassiopeia, Cepheus, Draco, Ursa Major, and Ursa Minor. All of the stars in these northern sky constellations rotate nightly in a counterclockwise circle; the hub of the circle is Polaris, the North Star, the tip of Ursa Minor’s tail and the end of The Little Dipper’s handle. It also marks the location of the North Celestial Pole.
Polaris has been used for centuries by wayward travelers to find north and the other cardinal compass points. But that won’t last forever. Because the Earth slowly wobbles upon its axis over time our North Pole will not always be aimed at Polaris, 12,000 years from now it will shift to the star Vega in the constellation of Lyra, the Harp.
Finding Polaris is easy, just go to the two stars that make up the front end of the bowl of The Big Dipper, Merak and Dubhe, and draw a line between the two. Now, extend that line outward from the bowl’s opening about five times the Merak/Dubhe distance and you will come to Polaris. Many people believe that Polaris is the brightest star in our night sky. Wrong, not even close. Polaris isn’t even among the top 40 brightest stars; it comes in at number 48, just bright enough to be seen from most suburban areas.
Because The Big Dipper circles around the North Star every 24 hours people have also used the dipper’s orientation in the sky as a celestial hour hand to tell the time of night. But we can do more with The Big Dipper; we can use it as a jumping off point to locate other stars and constellations.
At 9:00PM this time of the year The Big Dipper is high up in the sky and oriented with the bowl facing downwards while the curved handle is aiming towards the East. If you extend the handle out in an arc one more time its original length you will come to the second brightest star in our night sky, Arcturus, within the constellation of Bootes (pronounced “Boe-OH-teez”). Bootes is often depicted as a herdsman but he is also said to represent the man who invented the plow, which is also another interpretation of The Big Dipper.
Boy Scouts learn to use the dipper’s handle as a guide to Arcturus by the mnemonic “Arc to Arcturus”, followed by “drive a spike to Spica”. Leaving Arcturus let’s extend another line, equal in length to the dipper’s handle, to the southeast and we come to another fairly bright star known as Spica, the brightest star in the otherwise dim and confusing constellation of Virgo. Virgo is said to be the daughter of Demeter, the goddess of the harvest and the star Spica is a stalk of wheat she is holding in her hand.
By using a good star map and the Great Bear as your guide you too can be navigating the ocean of night in no time.
Darrell Heath works at the University of Arkansas at Little Rock and is a producer and host for the UALR Television show “The Night Sky.”
