The guardian of the bears

Find this constellation up all night in April

SkyCaramba astronomy blog for the week ending April 16, 2011

Ancient stories tell of a herdsman whose animals are tethered to the celestial poles. The herdsman drives the animals forward and they pull the heavens ‘round the earth. Their endless haul causes the stars to move and day to follow night. The herdsman is Boötes (boh-OH-teez).

Another story says the stars that go around the north celestial pole are a flock of animals that Boötes keeps together. Still another says those circumpolar stars have an evil influence and Boötes is a good being who keeps them from bothering us on Earth too much.

People all over the world made this ancient constellation out to be something, undoubtedly because of its bright orange star Arcturus. The star’s name means bear watcher.

Ancient Greek legends say Zeus, while married to Hera, fathered a child with Callisto. Not wanting Hera to find out, Zeus turned Callisto into a bear. Callisto longed to be with her child and searched for him for many years. Finally, she found him. He was a grown man by then and he didn’t recognize his mother as a bear. Zeus was still keeping an eye on things. As the son was about to slay the bear, Zeus took pity and turned both into constellations. Despite all Zeus did to cover his tracks, Hera found out what he’d done anyway. And Hera didn’t see a problem with taking her anger out on Callisto and son, even after they’d been turned into constellations. Zeus put Arcturus in the sky to protect these victims of circumstance.

Arcturus is the fourth brightest star in the sky. That’s the first thing that works in favor of this star being so well storied. It’s also 19 degrees north of the celestial equator. So it’s easily visible from most of the inhabited world.

In 1933, the organizers of the World’s Fair in Chicago connected an Arcturus related past to the future. Light from the star, focused onto photo sensors at several astronomical observatories, created an electrical current that triggered a switch in Chicago. All the lights at the fair went on and the celebration entitled A Century of Progress began.

Arcturus was chosen because the last time Chicago had a World’s Fair was 1893 and astronomers had calculated Arcturus to be 40 light years away. So the new fair was to be started with light energy that started its journey to Earth during the last fair in Chicago. Astronomers have since recalculated the distance of Arcturus to be 36.7 light years.

What star should have been used? Capella, 42.2 light years away, is almost as bright. If a candidate closer to 40 years is desired, a star known to astronomers as HR1925 has a distance of 39.93 light years.

You’ll find Boötes up all night this time of year. So look in the east not long after sunset, overhead around midnight, and in the west as dawn approaches. Let the bright orange Arcturus inspire your imagination now as it did for our ancestors.