Astronomy: The Dipper and the Bear


Tom Burns - Stargazing



Last week, we talked about the most famous star in the Big Dipper. Of course, most people know the Dipper when they see it. When I ask people what constellation they learned first, they almost always mention the Big Dipper.

What most people don’t know is that the Dipper is not a constellation at all. The Big Dipper is part of a larger constellation called Ursa Major, the Big Bear. Mere collections of stars within a constellation are technically called asterisms.

The handle of the dipper is the bear’s tail, and the rough square that forms the dipper is its rather fat posterior. Fainter stars form its head and the three legs that stretch below.

In March, it rises quite high in the sky, almost to the zenith, and the seven dipper stars are visible to the unaided eye.

Stories about this constellation are truly ancient, and references can be found as far back as Homer’s great epic, The Odyssey:

“… the Great Bear

By others called the Wain, which wheeling round,

Looks ever toward Orion and alone

Dips not into the waters of the deep.”

Poor Ulysses. Of all the Greeks who survived the Trojan War, only he was forced to wander the unknown sea for years before he could return home.

Ursa Major expresses that lonely life because from our latitudes it must forever circle the pole star, never setting below the horizon. Most of the other constellations rise and set into the horizon, which for a country like Greece with its long coastlines, means that they set into the water, the “great deep.”

Like the bear in his lonely wanderings, Ulysses never seems to be able to return to the comforting abyss of home, hearth and family.

The Greeks and Romans who read those lines would have felt that loneliness immediately, since they would have recognized the myths associated with Ursa Major. The best-known story illustrates what must have been the most important rule for the ancients — don’t fool around with the gods.

Calisto didn’t listen. She attracted the amorous attentions of Zeus, the head honcho on Mount Olympus. His wife, Hera, was jealous, so she changed Calisto into a fat bear, constantly pursued by hunters.

One of those hunters was Arcas, brave and powerful son of Calisto. He pursued the great bear with unflagging ardor. He never knew that the great bear was his mysteriously missing mother.

Finally, Arcas found the bear, and now the stage was set for one of the most dramatic and poignant moments in Greek mythology. Calisto spied her son. Her joy was so unbounded that she rushed to embrace her son. Arcas, in fear of his life, raised his mighty bow and was about to kill the woman who had given him birth.

At that moment, Zeus intervened. He turned Arcas into a bear smaller than his mother, and his bow fell from his clumsy paws. Zeus then grabbed the greater and lesser bears by their tails, stretching them to their extraordinary lengths as he dragged them into the sky as Ursa Major and Ursa Minor.

Hera got the last laugh by conspiring with the god of the sea to put Calisto and her son in the north — in a place where they must ceaselessly circle the North Pole in a never-ending quest for the sweet oblivion of sleep.

The “waters of the deep” could also refer to our galaxy, the Milky Way, stretching to the west through the setting constellations Perseus and Auriga and to the east through the rising glory of Cassiopeia and Cygnus.

Because Ursa Major is so far from the densest part of the Milky Way, we can see through it to other galaxies, such as M81 and M82, a spectacular pair near the bear’s head. Those galaxies are as much as a million times farther away than the familiar stars of our galactic neighborhood, which are visible all around as you stand in the warm fragrance of a spring night.

But as you stand there surrounded by all that beauty, think too of the cold winter that has just passed and of the lonely wanderings of Ulysses and Calisto and the unending quest of the Great Bear, who will never merge into the gloriously rising splendor of the summer Milky Way.

Planets

A lot of folks have been asking me about two bright “stars” that they have been seeing lately.

It’s hard to miss extraordinarily bright Venus high in the western sky during evening twilight. Venus gets its brilliance partly from its proximity. We live on the third planet from the sun. Venus is the second.

Mostly, Venus is simply very reflective. Its layer of white sulfuric-acid clouds makes for a very shiny mirror of the sun’s light.

Jupiter is not so bright or so close as Venus, but it’s still pretty obvious in deep morning twilight. Look southeast for a slightly yellow point of light.

While you’re at it, grab your binoculars and check out its four brightest moons, the Galilean satellites. Depending on the quality of your binos, at least two of them should be visible lined up closely around the planet.

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Tom Burns

Stargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.