Thursday, March 6, 2008

The Winter Hexagon

If you’re a regular reader of my blog, then you know I’m a fan of using asterisms (recognizable star patterns) rather than constellation figures to navigate the night sky. This method is widely used in the amateur astronomy community, and it works.

Most amateur astronomers couldn’t tell you off the top of their heads where one constellation ends and another begins. They couldn’t tell you precisely where the official constellation boundaries lie, that is, the ones designated by the International Astronomical Union in 1930. And they probably couldn’t tell you how the stars of a particular constellation connect in order to resemble a charioteer, a winged horse, or a whale. Now, I have my own theory that quaffing a large quantity of high-octane mead is required to see the star pictures that the ancients saw, but I digress.

What amateur astronomers can tell you is how to spot the Sickle, the Great Square, the Teapot, and the Lazy W--and in which constellation each is. In other words, amateurs use asterisms as celestial landmarks to let them know which 'country' they're in, even if they couldn’t tell you precisely when they crossed the border.

This week I invite you to locate and explore a unique asterism--one that does not lie within a constellation, but rather spans six constellations. Learning it will help you master the winter sky. This marvel is called the Winter Hexagon.

The Winter Hexagon stars (marked in yellow)

Pull on your long johns and your bunny boots, and let’s head out.

1) You’ll need to face south, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

2) Wait at least one hour after sunset to begin observing, so your sky is good and dark. New Moon--when the moon does not appear at all in the night sky--occurs on Friday the 7th, so the weekend through Monday night would be the best time to hunt the Hexagon without interference from moonlight. After that, you would have to wait too long for the waxing (growing) crescent moon to set, and the Hexagon stars would begin setting also.

3) I use a memory prompt to remember the six stars that make up the Winter Hexagon. It is CARS-PP, that is, the word “CARS” followed by two P’s. I can remember this easily, and it gives me the first letter of each of the six stars, in order as they are positioned around the Hexagon. The stars are Capella, Aldebaran, Rigel, Sirius, Procyon, and Pollux. All six are bright, easily-seen stars, and each is in a different constellation.

Feel free to start anywhere you like on the Hexagon. Use the star map at the top of this post as your guide. You will find all six constellations somewhere between the zenith (the point directly above your head) and the southern horizon. If you’re a regular reader, you'll recognize a number of these constellations from previous posts. You may find that you recognize certain constellations more easily than others, so you may want to leapfrog around the Hexagon until you are sure you have located all six stars. If nothing else, CARS-PP will simply ensure you don’t forget one.

4) Capella is the brightest star in Auriga the Charioteer. Capella (pronounced cah PELL uh) is actually a binary with two yellow component stars orbiting each other. We see their combined light as one star. Auriga’s defining asterism is a pentagon-like shape, and it connects with the sharp end of Taurus the Bull’s upper horn.

5) Aldebaran is the brightest star in Taurus the Bull. Aldebaran (pronounced ahl DEBB er ahn) is an orange giant star found in the distinctive “V” asterism that outlines the bull’s face. Aldebaran marks the position of the bull’s eye.

6) Rigel is the brightest star in Orion the Hunter. Rigel (pronounced RYE jull) is a blue supergiant. It can be found at the lower right corner of the large, hourglass-shaped asterism that defines Orion.

7) Sirius is the brightest star in Canis Major the Big Dog, and it is also the brightest star in the night sky. Blue-white Sirius (pronounced SEER ee us) marks the top of the Upside Down Y asterism that defines Canis Major.

The Winter Triangle

8) Procyon is the brightest star in Canis Minor the Little Dog. Procyon (pronounced PRO see ahn) is a yellow-white subgiant star. Procyon, along with Sirius and the red supergiant Betelgeuse in Orion, mark the corners of the Winter Triangle asterism.

9) Pollux is the brightest star in Gemini the Twins. Pollux (pronounced PAH lucks) is an orange giant star with a planet three times the size of Jupiter orbiting it. Currently, if you draw a line between Aldebaran and the copper-colored planet Mars (which is slightly brighter than Aldebaran) and continue past Mars, you will come to a pair of bright stars. The southernmost of the pair is Pollux, slightly brighter than the other star, Castor. Castor and Pollux are the names of the Gemini twins.

So there you have it, the best and the brightest that the winter sky has to offer. Collect all six.