Hey Sky Fans! Have you had a chance to get outside at all this week? I keep trying, but, just like they’ve been all year, the clouds have been rolling in just when I have the chance to take a few quiet minutes outside. The skies were okay last night, though.
As I headed back in, reliably as always, were the bright stars of the Big Dipper and the Cassiopeia’s W. If you’re in the north, are those two asterisms are high your list of favorites? They’re pretty bright, they’re easy to find, and they stand out against the sky easily. If you’re good with a telescope, there are a lot of deep sky objects in that direction, too.
Maybe the best of all is they’re always there. They’re not seasonal like Orion or Aquila; there one day, gone the next. Night after night, month after month, year after year, they’re there. They revolve around Polaris in our sky. We get to watch them dance around each other like lovers on a ballroom floor. And so…
Cassiopeia and Ursa Major (the constellation the Big Dipper is a part of) are in a group of circumpolar constellations. Circum- comes from the same root as circle, and polar means of or relating to the poles. So, the circumpolar region of the sky is called that because the things in those parts of the sky appear to circle the poles. There’s a northern circumpolar region and a southern one. Maybe in a different life, these constellations aren’t the queen and the big bear at all, but they’re stars in an entirely different constellation called, maybe, Circumpolaris or Circumpolarium or Circumpolaritron. To make things easier, I’m going to talk of things in reference to the northern hemisphere only. The ideas are the same if you’re tuning in from the south, with obvious differences.
This touches back to culmination, from a couple weeks back. Remember, a culmination is one of the two times each day when things cross the meridian. The meridian is that imaginary line that runs all the way around the entire sky from north to south and back again, giving the sky an eastern half and a western half. The upper culmination is an object’s highest point for the day. This is the object’s southernmost point in the sky. Its lower culmination, 12 hours later, is its lowest, and northernmost point. For most things, lower culmination is below the ground.
Everything in our sky circles the north celestial pole. Most things are so far south that this illusion is lost, and they look to travel from east to west across the sky, rising, upper-culminating, then setting and lower-culminating, each day. These things don’t appear to rise in the east, travel across, and set in the west. Instead they circle Polaris, which is almost exactly at the north celestial pole (the spot where the earth’s north pole touches the sky, if it were extended off into space like a giant… um… pole), counterclockwise.
Circumpolar stars never set. Instead, they’re far enough north that they never get low enough in the sky to set. Their lower culmination happens above the horizon.
They’re there when night falls, stay in the sky all night, and stay in the sky all day, invisible, hidden by bright sun. That is, if you could turn off the sun with a snap of your fingers in the middle of the afternoon, lots of the stars you would see that night would be rising, or hard to find, or below the horizon. Not the circumpolar ones; they’d be in the sky the whole time, revolving around Polaris.
The farther north you go, the higher Polaris is in the sky, bringing the circumpolar zone with it. At the north pole, Polaris will be very close to exactly overhead, you’ll see the entire northern sky. All the stars you can see clear out to the horizons in all directions will be circumpolar. None of them will ever set. Pretty cool, huh? Conversely, the farther south you go, the less of the sky is circumpolar. What’s circumpolar for you in Oslo might not be circumpolar for me in Barcelona.
At the poles, everything is circumpolar. At the equator, nothing is.
All the same, we’re treated to having these stars there for us, helping us finding our way, reliable and steady, every night of the year, if your skies are clear.