Ursa Major Never Sets

The stars move with our seasons. For most of us, some move below the horizon and we lose sight of them for part of the year. But the circumpolar stars stay above the horizon all hours of the day, every day of the year. They are there now, even if it is daylight as you read this, they are there. there’s not a lot you can count on here on Earth – or even in the heavens – but you can count on them.

The Big Dipper asterism is the best known of the circumpolar groups at all latitudes north of 41 degrees north latitude. (That is the northern half of the mainland United States and most of Europe.)

The Big Dipper is part of a bigger constellation, Ursa Major or the Great Bear.

In Greek mythology, the god Zeus had fallen in love with the maiden Callisto. In a story that would make the news today, and get Zeus some bad headlines, Zeus got her pregnant. Callisto was a nymph in the retinue of the goddess Artemis. But she would not be with anyone but Artemis. Zeus disguised himself as Artemis and seduced Callisto. When the child Arcas was born, Zeus’ wife Hera turned Callisto into a bear in revenge.

Callisto wandered the forest for years in bear form, until a chance meeting with her son, Arcas. He was the king of Arcadia and a great hunter. He raised his spear to strike at the bear, not knowing it was his mother. Zeus stepped in and sent them up to the heavens with Callisto as the Great Bear and Arcas as Bootes the Herdsman. (Or maybe he is Ursa Minor, the Little Bear,  depending on whose mythology you follow.) Hera was not pleased that Zeus stepped in, so she wever, and conspired with the gods of the sea so that the Bear could never swim in the ocean. That is one explanation – totally unscientific – for why Ursa Major never sets

The Big Dipper is circumpolar, so it is visible year round. It is up in the spring and down in the fall. The pointer stars in the bowl of the Big Dipper always point to Polaris, the North Star |  |  Image via Chris Mihos, Department of Astronomy, Case Western Reserve University

Where are you? If you’re with me in the Northern Hemisphere, every star north of the celestial equator is circumpolar, and every star south of the celestial equator is below the horizon. At the Earth’s South Pole, every star south of the celestial equator is circumpolar, whereas every star north of the celestial equator remains beneath the horizon.

And at the Earth’s equator, no star is circumpolar because all the stars rise and set daily in that part of the world. You can actually see every star in the night sky over the course of one year.

Follow the Drinking Gourd

For those of us in the northern U.S. or Canada or at a similar latitude, the Big Dipper is always above the horizon. That means it is described as circumpolar. The mnemonic to remember for the Big Dipper is “spring up and fall down” to describe its appearance in our northern sky.

The Big Dipper’s location at around midnight in each season. Image via burro.astr.cwru.edu

The Big and Little Dippers are asterisms – a prominent pattern or group of stars, typically having a popular name but smaller than a constellation. The Big Dipper ascends in the northeast on spring evenings, and it descends in the northwest on fall evenings.

“Follow the Drinking Gourd” is an American folk song that used “Drinking Gourd” as another name for the Big Dipper. The lyrics, according to legend, came from a conductor of the Underground Railroad, called Peg Leg Joe, as a way to guide some fugitive slaves.

The “drinkin’ gou’d” alludes to the hollowed out gourd used by slaves (and other rural Americans) as a water dipper. Used in the song, it was a code name for the Big Dipper which points to Polaris, the Pole Star, and to the North and freedom.

Polaris is a special star because it always stays in the same spot in the northern sky. The entire northern sky appears to turn around it because Polaris is located more or less above the northern axis (pole) of the Earth, and the wheeling of the stars across the dome of night is really due to Earth’s turning, after all. Polaris is part of the harder to find star pattern known as the Little Dipper.

Dipping Into the Stars

Many people learned about the Big Dipper when they were children. Perhaps a parent pointed it out, or it was in school or at a planetarium show.  Lesser known is the Little Dipper.

Are they constellations? No. They are asterisms which are star patterns.  These Dippers are part of the constellations of the Big Bear and Little Bear (Ursa Major and Minor).

In all my years of stargazing, I still can only see those constellation shapes on a chart or planetarium show when someone connects the dots – and even then it is a stretch of the imagination!

But the shape of a dipper (once used to get a drink of water from a larger vessel or well) is pretty easy to see. This month you can find the Big Dipper high in the northern sky. The two outer stars in its bowl are referred to as pointer stars because they point to the North Star (Polaris) which is the end of the Little Dipper’s handle.

In Star Names: Their Lore and Meaning, it says that the ancient Greek writers (like Homer) didn’t mention Ursa Minor or the dipper shape.  I always marveled at seeing the stars and planets as a child thinking that I am looking at the same sky that ancient people saw.

This group of stars became the “wings” of the constellation Draco the Dragon. When, around 600 B.C., the Phoenicians showed the Greek philosopher Thales how to navigate by the stars, he supposedly used the Dragon’s wings to create a new constellation. This might have been to make it easier to show them how to locate the north celestial pole.

Ursa Minor was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations. The Bears’ tails are the dippers’ handles.

Further falling away of my childhood star knowledge came when I learned that our Polaris,  which marks the north celestial pole in the sky, was not the star those ancients would have used to navigate. Kochab and Pherkad at the end of the Little Dipper were closer to the north celestial pole in 600 B.C.

Learning how our sky view of the heavens has changed over the centuries isn’t at all disappointing to me, but rather a reminder that everything is changing.

The Liitle Bear with The Dragon looping around it, as depicted in Urania’s Mirror, a set of constellation cards published in London c.1825

 

“UrsaMinorCC” by Till Credner – Own work: AlltheSky.com.
Licensed under CC BY-SA 3.0 via Wikimedia Commons.