Cumberland Times-News

Bob Doyle - Astronomy

September 14, 2013

Start with Big Dipper to learn constellations

For most folks, learning the constellations is something they put on the back burner.

If you get a star book out of the library, it’s easy to be overwhelmed as there are dozens of constellations in view at any one time.

The best way is to start with the asterisms, star figures that are quite striking and easily recognized. Presently in the early evening, the Big Dipper can be found without too much difficulty.

Of course, trees and hills may get in the way. If you remember where the sun goes down, face that direction and raise your right arm away from your body. Your right arm is pointing approximately north, the direction of the Big Dipper throughout the year.

Find a place with a nearly flat northern horizon. You may have to walk a bit or even drive your vehicle for a good view of the Big Dipper.

Once you see what you think is the Big Dipper, count its stars. There are seven conspicuous stars — three in the handle (on the left) and four in the scoop or bowl (on the right). If the Big Dipper was real, it would be able to hold some soup in its bowl.

The two rightmost stars of the scoop are called the Pointers; a line made with these stars points at the North Star. If you find the North Star, you probably won’t be impressed by its brightness.

The North Star is a rather modest star, appearing a little less than half way from the horizon to the top of the sky. It’s special feature is that during the night all the distant sky bodies appear to revolve or circle it.

This is caused by the North Pole of the Earth very nearly pointing at the North Star.

For most of recorded history, there was no North Star, as the Earth’s polar axis has a slow wobble called precession. Precession has a cycle of 26,000 years.

During most of a precession cycle, our polar axis doesn’t point towards a visible star. Precession determines what star groups we see each season as well as which star is closest to the North Pole of the sky.

So 13,000 years from now, Orion will be a summer star group and Vega, the bright white star now nearly overhead at dusk will be the North Star then.

   Let’s return to the Big Dipper. Would this asterism look like a Dipper from some other position in our galaxy?

Knowing the distances of each of the seven stars can help us answer this question. The top star of the pointers is Dubhe and is at distance of 124 light years. The bottom star of the pointers, Merak is 79 light years away.

The next four stars (going from right to left) are: Phad (84 light years), Megrez (star that connects scoop to handle, 81 light years), Alioth (81 light years), Mizar (78 light years) and Alkaid (101 light years).

So the five inner stars of the Dipper are at nearly the same distance while the top pointer stars and the last star in the handle are about 20 light years further away.

This would cause these stars to have quite different pattern from somewhere else in our stellar neighborhood.

   If you have good eyesight, look at Mizar, the middle star in the handle. Can you spot a tiny star snuggled up next to it? This is Alcor, a star that lies a few light years away from Mizar and just happens to be very close to Mizar’s direction (from Earth).

Being able to spot Alcor was used to select soldiers from the Egyptian army as well as determining which male Native Americans could go out on the hunting parties. I will write future columns on the best asterism seen each month.

SKY SIGHTS AHEAD: At noon today, the moon will be at its closest distance from the Earth (perigee). You might see the moon in the afternoon day sky in the south.

This evening’s moon (with a shape called gibbous) will appear slightly wider than usual. At its closest, the moon distance (center of Earth to center of moon) is 225,744 miles; at its farthest the moon is 251,966 miles.

Tomorrow evening (September 16), the brilliant planet Venus will appear underneath the planet Saturn low in the 8:15 p.m. twilight. The moon will appear very full on both Sept. 18 and 19. (The actual time of full moon is 7:12 a.m. on Sept. 19.)

This full moon is called the Harvest Moon, which offers extra evening moonlight through the coming weekend.

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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Bob Doyle - Astronomy
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