Last week in Starwatch we did a big celestial bear hunt for the constellation Ursa Major. That’s the formal Latin moniker for the Big Bear.
The Big Dipper that we know and love in the northern sky is not actually a stand-alone constellation, but does possess the brightest stars that make up the derriere and tail of the Big Bear. This time of year the Big Bear is flying upside down, high in the northern sky, and with not much eye strain it is easy to see, even in moderately light-polluted skies.
This week, it’s Ursa Minor’s turn, a constellation with a dual identity. Not only is it known as the Little Bear, it’s also the Little Dipper, and without a doubt that’s actually more what it looks like.
The problem is that it’s not nearly as bright as the nearby Big Dipper. The sad fact is that in a lot of city — and even suburban skies — it’s difficult to see completely. The good news is that you can use the Big Dipper to find and see as much as you can of the Little Dipper.
All you have to do is use the stars Merak and Dubhe, which make up the side of the Big Dipper’s pot opposite its handle, as “pointer stars” to Polaris, the brightest star of the Little Dipper. As you see on the diagram, draw a line from Merak to Dubhe and continue that line nearly straight down. Extend your clenched fist at arm’s length, and three of your fist widths should get you from Dubhe to Polaris. Polaris resides at the end of the Little Dipper, which is more or less hanging right side up.
Once you find Polaris, look for the next two brightest stars you can see to the right of Polaris. They’re Kochab and Pherkad, on the far side of the pot section of the Little Dipper. The really neat trick is trying to find the other two much dimmer pot stars to the left of Kochab and Pherkad, and the two equally dim handle stars to the right of Polaris. That may require a trip out into the darker countryside to enjoy.
Even though Ursa Minor is not exactly a marquee constellation, it contains a very important star. That star is Polaris, also known as the North Star. It’s certainly not the brightest star in the sky, but at my star parties, I like to refer to it as the “Lynchpin” of the heavens, because all celestial objects in the sky — the stars and even the sun and moon — circle around the stationary North Star every 24 hours as the Earth spins on its axis. (If you want a plain linchpin, throw your own party.)
It happens that Polaris shines directly above the Earth’s terrestrial axis. As we ride around the spinning Earth it appears to us that all of the stars make one complete counter-clockwise circle around the North Star in that 24-hour period. At our latitude of 47 degrees north in Everett, Polaris shines in the northern sky about 47 degrees above the northern horizon.
Stars and constellations close to the North Star make their circuit around Polaris completely above the horizon. They are known as circumpolar constellations. The rest of the stars and constellations farther away from the North Star have only part of their circular path above the horizon. The northern part of their circuit takes these stars below our northern horizon. The result for us is that these stars appear to rise above the eastern horizon and set in the west, much like our sun and moon.
Astronomically, Polaris is considered a supergiant star about 432 light-years distant, or just over 2,500 trillion miles away. It’s about 30 times the width of the sun and kicks out more than 2,200 times the light of our wimpy little home star. Like a lot of stars that appear as solo acts in the sky, Polaris is actually the brightest star of a three-star system.
Don’t count on Polaris to be our North Star forever, though. That’s because the Earth’s axis very slowly wobbles over a 26,000-year period. Thousands of years from now, Polaris won’t be shining above the Earth’s North Pole, but for now it’s the Lynchpin of the night sky.
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and author of the book, “Washington Starwatch,” available at bookstores and at his Web site, www.lynchandthestars.com
The Everett Astronomical Society welcomes new members and puts on public star parties. The Web site is members.tripod.com/everett_astronomy.
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