Thursday, June 5, 2008

Follow the Drinking Gourd

Two weeks ago, we used the most famous star pattern in the night sky to locate an exceptional deep-sky object that lies outside our Milky Way galaxy. This week, let’s return to our galactic ‘hood and take a closer look at the notable star pattern we call the Big Dipper.

Known to every child in the Northern Hemisphere, the Big Dipper may be the most ancient of all star patterns recognized and recorded by humans. The Big Dipper is not a constellation, as many believe. The Big Dipper is the central asterism (recognizable star pattern) in the constellation Ursa Major, the Great Bear.

Ursa Major is a circumpolar constellation. In the Northern Hemisphere, a circumpolar constellation is one that circles the North Celestial Pole, the imaginary fixed point in the sky that the Earth‘s axis would intersect, were it extended from the North Pole into space. That imaginary point just happens to be extremely close to the star Polaris, which is why we call Polaris the North Star. We can use the Big Dipper to locate the North Star and geographic north; more on that in a minute.

Diagram of celestial poles by Dr. Guy Worthey

For skywatchers in latitudes above 35 degrees north, circumpolar constellations never set; they are always above the horizon as they endlessly circle the North Star counterclockwise, making one revolution per day. But although we commonly say they are "circling," constellations like Ursa Major are not really moving around the Pole. Their apparent motion--or the way they appear to move in our sky--is actually caused by our rotation, as planet Earth reliably spins 24/7 on its axis.

The Big Dipper’s proximity to the North Star has made it a favorite of travelers and navigators throughout history--a reliable guide in the night sky when plotting one’s course across land, water, or air. Let’s get oriented, shall we?

1) You’ll need to face north, so 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 left shoulder to the west, and you’ll be facing approximately north.

2) Wait at least one hour after sunset to begin observing, so that twilight’s over and your sky’s good and dark. A waxing (growing) crescent moon will be in the western sky after sunset through the weekend. First Quarter Moon, when the Moon’s face is half illuminated, occurs on Tuesday, June 10. The earlier in this observing week you look at the Dipper, the less interference you’ll have from moonlight. Or, if you’re a night owl, you can simply wait until the Moon sets.

3) Facing north, look for a distinctive seven-star asterism that looks like a giant, long-handled saucepan. It will be due north to a little west of north and somewhere between the zenith, the point directly above your head, and the northern horizon. The saucepan will be oriented upside-down. Found it? Great. This is the famous Big Dipper (water dipper), or as the French call it, “La Casserole” (saucepan).

Chart created with Your Sky

4) All seven stars of the Dipper are bright enough to be seen even in urban areas that are somewhat light polluted. In a future post, we’ll take a closer look at the individual stars.

5) Make a point of looking at the Big Dipper early in the evening and noting its orientation. Then look at it again before you turn in for the evening, or if you get up before sunrise, look at it again before dawn lights the sky. Do you see how its orientation has changed, how it has rotated counterclockwise and is approaching right-side-up? It’s exciting when you first realize how the stars above the North Pole will 'move' in a predictable, circular motion every night of the year, simply because we live on a spinning globe.

They are all ‘moving’ around the North Star. You can locate Polaris using the two stars on the right side of the right-side-up Dipper’s bowl. These two stars are called the pointer stars, because they point the way to Polaris.

Chart created with Your Sky

6) Draw an imaginary line connecting the two pointer stars. Then extend that line about five times its length, above the right-side-up Dipper. The first fairly bright star you come to is Polaris; it’s slightly dimmer than the top pointer star. Beginning stargazers are often surprised to learn that the North Star isn’t one of the brightest stars in the sky. Its importance to us is its location, not its magnitude of brightness. Now that you’ve found Polaris, face it square and you will be facing geographic north, also known as true north.

Our ability to recognize the Big Dipper enables us to reliably find north at night--without a compass, a map, location familiarity, or knowledge of where the sun set earlier. This is a celestial landmark that might even spell the difference between life and death, or captivity and freedom. According to American folklore, runaway slaves traveling under cover of night oriented to the Big Dipper and followed it north to freedom. They called it the “Drinking Gourd,” a reference to the hollowed-out gourd used in rural areas as a water dipper.

A hauntingly beautiful folk song of uncertain origin called “Follow the Drinking Gourd” may have been used by the Underground Railroad to provide coded directions in its lyrics, guiding the slaves to freedom. Click here to play an MP3 of “Follow the Drinking Gourd,” courtesy of Roger McGuinn’s Folk Den.

When the sun comes back and the first quail calls,
Follow the Drinking Gourd.
For the old man's waiting for to carry you to freedom,
If you follow the Drinking Gourd.

Reward notice for runaway slaves Harriet Tubman and her brothers