Thursday, January 22, 2009

The Purloined Pentagon

Just north of Orion the Hunter and the horns of Taurus the Bull lies a distinctive pentagon of stars. You may recall from murky memories of high school geometry that a pentagon is a five-sided closed figure. This starry pattern is an irregular pentagon, since its five sides are not equal in length.

The celestial Pentagon is an asterism (recognizable star pattern) containing the brightest stars in the constellation Auriga the Charioteer (aw-RYE-guh). Let’s take a closer look.

1) About an hour after sunset, face south. 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.

Star maps created with Your Sky

2) Locate the Hourglass asterism of Orion, rising in the southeastern sky. Above it are the business ends of Taurus’s deadly horns. Keep going north past them to locate the Pentagon.

3) The brightest star in the Pentagon is a tad brighter than both Betelgeuse and Rigel, the two brightest stars in the Orion hourglass. This is Capella, the sixth brightest star in the entire night sky as seen from Earth. Capella (kuh-PELL-luh) is from the Latin for she-goat. It’s a yellow-white star, only 42 light years away.

4) Next to Capella, moving counterclockwise around the Pentagon, is the second brightest star in Auriga, Menkalinan (menn-KAH-lih-nan). Its name is from the Arabic for shoulder of the rein-holder. Menkalinan is a white star.

5) The second brightest star in the Pentagon is not Menkalinan, and it’s not in Auriga. To make the Pentagon shape, we must purloin a star from Taurus. This star is Elnath, the tip of Taurus’s northern horn. Elnath (EL-noth) is from the Arabic for butting with the horns. Elnath is a blue-white giant star.

We’ll continue our exploration of Auriga in more depth next week. See you then!

Astronomy Essential: There is no dark side of the Moon.

Pink Floyd megahit notwithstanding, there is no such thing. The Moon has no side that is always dark. The entire surface of the Moon is illuminated by the Sun at one time or another during its month-long orbit around the Earth. However, only half of the Moon’s sphere can be illuminated by the Sun at any given time (shine a flashlight at a baseball, and you'll get the idea).

Although the Moon has no dark side, it does have a far side that always faces away from Earth. Because the Moon’s rotation (spin around its axis) is in lockstep with its revolution (orbit around the Earth), the same lunar hemisphere always faces us earthbound viewers.

If you could instead perch on the Sun awhile to observe the Earth/Moon system, you’d watch Moon circling Earth, turning just enough on its axis to keep the same face toward Earth as it went around. And from your vantage point on the Sun, you’d notice that— over the course of a month— your view of the Moon’s surface continually changed until you had seen it all.