Cassiopeia's and the Dipper's unending ballet

High in the northeastern sky these October evenings, over trees turning autumn brown, sparkles one of the best-known and easiest-to-spot constellations, Cassiopeia. Look for a broad W-shaped pattern of five fairly bright stars. The W looks like it was drawn by a diligent third-grader: uneven but clear. At this time of year, the right side of the W (its brighter side) is tilting upward.

Below Cassiopeia and a bit to the right is a scattering of stars forming the constellation Perseus. And lower left of Perseus shines Capella, the Goat Star, one of the brightest in the sky. If it's still too low behind buildings or trees after dusk, wait another hour or two for it to rise higher.

Cassiopeia is always an evening constellation of autumn and winter. It wheels around the northern sky in a coincidental balance with another famous star pattern: the Big Dipper. The two are opposite numbers, forever trading places.

For instance, when Cassiopeia is high in the northeast, the Big Dipper is low in the northwest. Look for it if you have a clear view low in that direction. The Dipper is lying nearly level, with its handle to the left and its bowl to the right.

The Big Dipper swings high overhead in spring and summer, when Cassiopeia retires down to the horizon to rest up for a while.

Actually, this round-and-round routine happens not just once a year but once a day - or, to be exact, every 23 hours and 56 minutes. Those missing four minutes a day add up to make the constellations gradually shift positions through the seasons. So if you look up at the same time of night all around the calendar, like a strobe light flashing on a spinning wheel, you'll see the sky complete one full rotation in a year.

Located midway between Cassiopeia and the Big Dipper is Polaris, the North Star, due north. It's not very bright, but it's the cynosure around which the whole celestial drama rotates.

To picture it, imagine you're holding an open umbrella, with the Big Dipper drawn on the inside near one edge and Cassiopeia drawn near the other edge. The North Star is on the axis in the middle. Aim the point of the umbrella diagonally up at the North Star, and turn the handle counterclockwise. You've now got a miniature planetarium, showing how Cassiopeia and the Dipper go through their sky-cycles every day and every year.

Countless astronomically-minded parents have done this little project with the household umbrella and a marker or dabs of paint. Shown this way, a third-grader grasps how the sky turns around the North Star immediately.

Of course, the inside of the umbrella is just one part of the sky - the sky's northern polar cap. But imagine the umbrella extending out beyond its edges to form a giant balloon all around you, with stars painted on the inside and you in the middle. Now turn the handle. The whole celestial sphere moves as a unit, with stars coming up diagonally in the east and going down diagonally in the west while the Dipper and Cassiopeia do their thing in the north.

And what do umbrellas and imaginary balloons that are centered on you personally have to do with the real universe, in which a round Earth is circling the Sun in a galaxy of scattered stars?

Try puzzling it out, and you may get a new appreciation for how hard it was for people who thought they lived on a flat Earth under a hemispherical, divinely painted sky to grasp our true place on a rotating ball in endless space. No wonder they went after Copernicus and Galileo.

Like a lot of things, the whole picture is obvious once you get it - but try telling that to someone who doesn't.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge. His Star Watch column appears the first Saturday of every month.