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.

Top Ten Moon Myths

As you gaze at this week’s big moon--and frolic in the light of the Full Moon on Sunday, April 20--ponder my picks for Numbers 10 through 6 of the top ten moon myths of all time:

10) The Moon’s surface is completely smooth. Early science through the time of the Renaissance was based upon the teachings of the influential Greek philosopher Aristotle. According to his worldview, the Moon was a perfect sphere with a perfectly smooth surface. The invention of the telescope in the 17th century blasted that myth into oblivion. When the pioneering Italian astronomer Galileo turned his homemade telescope skyward in 1609, he discovered that the Moon was pockmarked with craters and crisscrossed with mountains and ridges.

Unfortunately, Galileo misinterpreted one type of lunar feature he saw: the dark, basalt-filled craters. He thought they were water features similar to Earth’s seas. Consequently, lunar mapmakers of the time gave them names beginning with “mare,” the Latin word for sea, and those names persist to this day.

9) The Moon does not rotate. Many think that, because the same side of the Moon always faces the Earth, it doesn't spin. In fact, if it did not rotate on its axis, we'd eventually see all parts of the Moon parade by us over the course of its month-long orbit around Earth. But we don’t, do we?

Millions of years ago, the strong pull of Earth’s gravity put the brakes on the Moon’s spin, slowing it down until its rotation became synchronous with its orbital motion. Now the Moon makes one complete rotation on its axis in the same time it takes to complete one orbit around the Earth. You can picture this slow rotation best if you imagine observing the Earth/Moon system from the Sun. From your vantage point on the Sun, you could watch Moon circling Earth, turning just enough on its axis to keep the same face toward Earth as it went around. And from the Sun you’d notice that, over the course of a month, your view of the Moon’s surface continually changed until you'd seen it all, proving that it was rotating.

8) Dogs and wolves howl at the moon. This is a common belief--perpetuated in popular art, music, and film--but there's no scientific evidence that it is so. Dogs and wolves howl to communicate with one another. A couple theories as to why humans have imagined this causal relationship between the Moon and howling animals are: a) Animals, particularly predators, may be more active on moonlit nights when they can see better, and b) When we hear animals howl at night, we're more likely to notice a big moon in the sky if it’s plainly visible. However, we may not recall all the times we heard animals howl when there was a crescent moon or no moon in the sky. This is called selective memory, and we use it to perpetuate our already-held beliefs.

7) The Full Moon is larger when it’s near the horizon than when it’s overhead. This effect, known as the Moon illusion, has been recognized since antiquity. How many times have you seen the rising Full Moon hanging just above the eastern horizon and thought to yourself, “The Moon looks huge!”? In fact, the Moon is a little nearer to us and therefore slightly larger in apparent size when it’s high in the sky. Think about it: as the Earth spins east, we are brought toward the Moon.

So what causes us to believe otherwise? The second century mathematician and astronomer Ptolemy first postulated that foreground objects near the horizon, such as trees and buildings, give us something with which to compare the low Moon, making it seem larger. Modern scientific researchers call these distance cues. They are still considered in current theories about the Moon illusion, along with the physiology of our eye muscles and the neurological processes in our brains. Some believe evolutionary selections were made for useful human survival skills like seeing eye-level objects better than high-in-the-sky objects. This skill might improve the chances of finding shelter and food, as well as detecting danger. But there is still no one definitive answer as to why we experience this optical illusion.

6) The Moon has a “dark side.” 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.

Stay tuned for next month’s Full Moon to find out which moon myths made my top ten list in the Numbers 5 through 1 slots. Until then, happy howling!