The Ellensburg sky for the week of 8/25/12

Saturday: Let’s all sing along with that fabulous oldie: Blue Moon.

Blue moon, you see it up in the sky.

The second moon in a month.

But that description’s a lie.

The term “blue moon” has had a number of meanings throughout history. In the earliest meaning, blue moon means never as in “The astronomy columnist’s next invitation to a Daily Record party will happen when the moon turns blue.” By the nineteenth century, astronomers noticed that the moon really does turn blue. For example, when the Indonesian volcano Krakatoa erupted in 1883, its dust turned the moon blue for two years. Thus, once in a blue moon came to mean “infrequent” rather than “never”. Most months have only one full moon and most seasons have only three full moons. The Maine Farmer’s Almanac lists the third of four full moons in a season as the blue moon. In 1946, James Hugh Pruett of Eugene, Oregon misquoted the Maine Farmer’s Almanac in a Sky and Telescope magazine article. He wrote that the blue moon was the second full moon in a month, which is not how the Maine Farmer’s Almanac defined it. For years after that, Sky and Telescope magazine as well as other media outlets used the incorrect description. The moral of the story: Always carefully read the Maine Farmer’s Almanac.

Sunday: School starts next week so it is time for a little geometry review. Go outside at 10 p.m. tonight with notebook in hand. Ready? A triangle is a polygon with three corners and three line segments as sides. A good example is the Summer Triangle made up of the bright stars Vega, Deneb and Altair. Vega, the brightest star in the triangle is a little bit west of straight overhead. Deneb is a little bit east of straight overhead and Altair is five fists held upright and at arm’s length above the south horizon.

Monday: Now that you know the general definition of a triangle, it is time for some specifics. A right triangle. At 8:30 p.m., Mars, Saturn, and the star Spica make a right triangle about one fist above the west-southwest horizon.

Tuesday: At 6 a.m., Venus is three fists above the east horizon and Jupiter is five and a half fists above the southeast horizon.

Wednesday: The Ellensburg Rodeo is a “Top-25” rodeo. What does it take to be a “Top-25” star? There are many ways to rank stars. The most obvious way for a casual observer to rank stars is by apparent brightness. The apparent brightness is the brightness of a star as seen from Earth, regardless of its distance from the Earth. Shaula (pronounced Show’-la) is the 25^th brightest star in the nighttime sky as seen from Earth. It represents the stinger of Scorpius the scorpion. In fact, Shaula means stinger in Arabic. Shaula has a visual brightness rating of 1.62. Sirius, the brightest star has a visual brightness rating of -1.46. (Smaller numbers mean brighter objects.) The dimmest objects that can be seen with the naked eye have a visual brightness rating of about 6. There are approximately 6,000 stars with a lower visual brightness rating than 6 meaning there are 6,000 stars visible to the naked eye. Shaula is a blue sub-giant star that radiates 35,000 times more energy than the Sun. It is 700 light years away making it one of the most distant bright stars. Shaula is a challenge to find because it never gets more than a half a fist above the horizon. Look for it tonight about a half a fist above the south horizon, a little bit west of due south, at 8:30.

Thursday: Tonight’s moon is full making it difficult to see dim objects in the sky because of the sky glow. While that might make you initially feel blue, you should really feel happy because there is so much to see on the moon itself. In fact, the lunar crater called Tycho is best seen during a full Moon. Tycho was formed about 109 million years ago when an asteroid struck the Moon, leaving a crater over 50 miles in diameter and ejected dust trails that radiate out hundreds of miles in all directions. For more lunar highlights and a basic lunar map, go to http://goo.gl/eJn5p, a resource of the Night Sky Network.

Friday: “I’m a little teapot, short and stout. The galactic center, I pour it out.” (I’m a Little Teapot, astronomy version, 2011.) Despite its great size and importance, the center of our Milky Way galaxy and its giant black hole remains hidden to the naked eye behind thick clouds of gas and dust. By plotting the orbits of stars near the middle of the galaxy, astronomers have determined that the black hole’s mass is equal to about 4.5 million Suns. While you can’t see the actual galactic center, you can gaze in the direction of the center by looking just to the right of the teapot asterism in the constellation Sagittarius. This point is about one fist above the south-southwest horizon at 9 p.m.

The positional information in this column about stars and planets is typically accurate for the entire week.