Notice a familiar star pattern missing these late fall nights? How about the Big Dipper. It’s still there for some of us but you might just need to stand on your tippy toes to see it. For observers like myself who live in the northern U.S. the Dipper never sets. Provided you find an open view to the north, it’s our constant companion, slowly wheeling around the North Star like the hour hand on a clock.
Like Cassiopeia, Cepheus and several other constellation that lie near North Star or Polaris, the Big Dipper is circumpolar. It goes round and round Polaris in a big circle and never sets. Any star that fits within a circle with a radius equal to or less your latitude is circumpolar. For example, in Minneapolis, located at 45° N latitude, stars within 45° (four-and-a-half fists) of the North Star never dip below the horizon. That includes the Big Dipper.
From Tucson, where Polaris stands just 32° above the horizon, the circumpolar radius shrinks to 32° and the Dipper disappears from view. At the North Pole the North Star is directly overhead and every star within 90° of the North Star — the entire sky — is circumpolar! Can you guess how many stars are circumpolar at the equator? Correct — none!
From the northern U.S. the Big Dipper dips low in November but remains circumpolar. But the further south you travel the closer those seven famous stars approach the northern horizon. In Des Moines (40° N latitude), all the Dipper stars stay up but not by much: Alkaid, the star at the end of the Dipper’s handle, stands just 1° high at its lowest point. In Atlanta (33.7° N) half the Dipper plunges below the horizon and disappears from view, while in Tucson Dubhe remains the sole circumpolar representative. Tucsonians have to wait until about 1:30 a.m. for the rotating Earth to bring the Dipper back into view.
I’ve also marked an imaginary line called the meridian on the maps. The meridian starts at the due south point on the southern horizon, arcs through the zenith (overhead) and meets the horizon again at the due north point on the northern horizon. Most stars rise in the east, cross the meridian — when they’re highest or reach culmination — and then set in the west. But circumpolar stars cross the meridian twice, once above the North Star and a second time below it. Six months from now during the evening hours, the Dipper will culminate above Polaris and climb nearly at the top of the sky. In November it crosses the meridian below the North Star when it’s said to be at its nadir (NAY-der), the bottom of its daily circuit.
That’s why the Big Dipper or Ursa Major (the Great Bear) takes a bit of effort to find this time of year. Like a bear the constellation represents, it’s gone into temporary hibernation. Come January it returns with so much enthusiasm, you’ll find it literally standing on its tail in the northeastern sky.