Tuesday, August 24, 2010

The Ellensburg sky for the week of 8/28/10

Saturday: 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.

Sunday: Ah, the beauty of classification. A large three-sided figure such as the Summer Triangle is a triangle. Hence the name “Summer Triangle” and not “Summer Sandwich”. Although those little triangle-shaped sandwiches are quite tasty. Where was I? Oh yes, classification. Small three-sided figures are also called triangles. This week, the planets Mars & Venus and the bright star Spica make a tiny triangle low in the western sky right after sunset. At 8:30 tonight, Venus is at the bottom of the triangle, less than half a fist above the west-southwest horizon. Spica is a finger width to the upper left of Venus and Mars is about two finger widths to the upper right of Venus.

Monday: When Galileo looked at Venus nearly 400 years ago, he discovered that Venus goes through phases from new to quarter to full just like our moon does. Thanks to this and Galileo’s many other observations of moons and planets in our Solar System, support for an Earth-centered universe was greatly diminished. To commemorate these findings, NASA has made planets, dwarf planets, and moons this month’s “Hot Topic” for the International Year of Astronomy. For more information, go to http://astronomy2009.nasa.gov/topics_sep.htm.

Tuesday: “I’m a little teapot, short and stout. The galactic center, I pour it out.” (I’m a Little Teapot, astronomy version, 2010.) 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 if 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 see the actual galactic center, you can gaze in the direction of the center by looking 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.

Wednesday: This morning’s last quarter moon is in the constellation Taurus the bull. While the quarter moon phases look smaller than the full moon at a glance, all phases are the same size. Recently, astronomers discovered that the moon has not always been the same size. It contracted about 0.2 km in diameter, out of a total diameter of 3,470 km, as it cooled over three billion years ago.

Thursday: Jupiter is one and a half fists above the east-southeast horizon at 10 p.m.

Friday: 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 25th 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.

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

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