The romantic couple in the eastern sky

Thursday, October 18, 2012

There are many romantic couples in history. Romeo and Juliet, Pyramus and Thisbe, Tristan and Isolde, Wayne and Wanda (OK, you have to be a Muppets fan to know that one).

One such romantic couple can be seen in the eastern evening sky about an hour and a half after local sunset.

The story of Perseus and Andromeda comes to us from ancient Greek mythology. Andromeda is rescued by Perseus from the evil sea monster and they fly off together on Pegasus, the Flying Horse.

To find this couple start with the horse, Pegasus. You will need a dark sky for this.

Look east about halfway up the sky for four stars forming a square. Actually, at this time of year in the early evening it looks like a baseball diamond standing up on one corner.

Go to the star on the left corner (as you are looking at it). That is Alpheratz, and it is shared by Pegasus and Andromeda.

Starting at Alpheratz, look leftward for two strings of stars. That is the constellation. Andromeda. I like to think of it as the two arms of Andromeda reaching out toward her hero, Perseus.

Indeed, if you follow the two lines of stars they will lead you right to Perseus. He is the constellation at the end of those arms that is shaped like a capital letter "A" right below the big "W" shape of Cassiopeia, another of the story's characters.

Both constellations have interesting objects for us to look at. First, in Perseus, on the lower right side (as you are looking at it) is the second magnitude star Algol, or as some have called it, "The Demon Star."

It is supposed to represent the evil eye of Medusa's head that has the power to turn people to stone. You will recall that Perseus cut off the head of the Gorgon, Medusa and used it to slay the sea monster, Cetus.

Algol is an eclipsing binary star. It is actually a three-star system and every two days, 20 hours and 49 minutes the brightness of Algol will dim for a period of about 10 hours when one of the other stars in the system eclipses, or blocks, its light.

You can see for yourself. Find Algol and observe it from night to night over a period of a couple of weeks and watch how the star dims in brightness.

Arabic astronomers called the star "Al Ghoul," or, The Demon's Head. With the approach of Halloween, this would be a good object to find.

Andromeda's jewel next time.

SKY WATCH: First quarter moon, Sunday, Oct. 21. Tonight, Thursday, Oct. 18, a very slender crescent moon will be to the upper left of the planet Mars in the southwest. You may want to wait until about an hour after local sunset for an attempt to find the pair as the sky will be darker. While you are out there, take a gander to the lower left of Mars for the star Antares, the "Rival of Mars." Both will just fit in the same binocular field of view. Mars at two o'clock and Antares at eight o'clock. This upcoming Saturday, Oct. 20, is the date for the autumn version of Astronomy Day. So, go out and look at something.

NEXT WEEK: The jewel of Andromeda. And more astronomical blathering.

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