Sunday, November 7, 2010

The Way North

One of the most recognizable "constellations" in the sky is the Big Dipper. Most people are taught how to identify it as children and virtually all cultures have a name for this pattern of stars – in England it’s the plough, in Thailand a crocodile, and the ancient Egyptians saw the hindquarters of an ox. Technically speaking, however, it’s not really a constellation. The Big Dipper is what astronomers call an asterism – a recognizable pattern of stars. The asterism of the Big Dipper is actually part of a larger constellation called Ursa Major, the Great Bear of Greek mythology, although I must confess to having some difficulty seeing a bear in that pattern of stars!

Knowing how to identify the Big Dipper has a practical application in that it can be used as a pointer to Polaris, otherwise known as the North Star. Polaris is located almost directly above the North Pole of the Earth and is thus always found due north of your location (assuming you’re not south of the equator where it’s invisible). And, because Polaris is located directly above the rotational axis of the Earth, it doesn’t appear to move over the course of the night like all of the other stars. In the Northern Hemisphere, Polaris never rises or sets which gave it an undeserved reputation for constancy as noted by Cassius in Shakespeare’s Julius Caesar (Act III, Scene I):

But I am constant as the northern star,
Of whose true-fix'd and resting quality
There is no fellow in the firmament.

In astronomy, however, a common-sense observation like this is often incorrect. The North Star is not constant. As the Earth rotates on its axis it wobbles a little bit, like a spinning top does as it slows down, and this wobble traces out a cone-shaped path in the sky with a 26,000 year cycle. When the ancient Egyptians began building the great pyramids in Egypt a few thousand years ago, the dim star Thuban in the constellation of Draco the dragon was the North Star. A few thousand years in the future, the obscure star Alderamin in the constellation of Cepheus the king will be the North Star. The North Star is constant only on the scale of human life spans which, to be fair, is usually good enough for most purposes.

If you go outside this week at 6:00 pm or so, thanks to the time change, you’ll see the Big Dipper low in the northern sky. Polaris can be located by taking the two stars (called Merak and Dubhe) on the right side of the Dipper’s cup and extending a line from them upward in the sky about 28º where you’ll find the North Star. How do you estimate 28º in the night sky? An old astronomical trick is to make a fist and extend your arm. Your extended fist will now span about 10º of the sky. Slightly less than three fist-widths will bring you from the top corner of the Dipper’s cup to Polaris. Many people are under the mistaken impression that Polaris is a strikingly bright and prominent star but that’s not the case, it’s actually quite average looking and some stars in the Big Dipper are actually a bit brighter.

Locating Polaris enables you to tell which way is north and, by inference, the other three cardinal directions as well which is a useful skill for backyard astronomers who want to locate something in the night sky. Polaris also tells us our latitude north of the equator. In the mid-Hudson Valley, our latitude is 42º and Polaris is, not surprisingly, 42º above the horizon (a little more than four fist-widths). Is it any wonder that many of the ancient cultures that developed the science of astronomy, the Babylonians, Egyptians, and Greeks to name a few, lived near trackless deserts or on the seacoast? A little knowledge of the stars might have meant the difference between returning home or dying in the middle of nowhere.

The navigational help offered by the Big Dipper also played a key role during a sad chapter in the history of our country. In the mid-1800s, many black slaves in the southern United States learned a song called “Follow the Drinking Gourd” – a song whose lyrics described an escape route from the deep south along the Underground Railroad. Slaves commonly used a hollowed-out gourd for scooping drinking water out of buckets in the fields. The drinking gourd in the song, of course, refers to the Big Dipper and following the gourd brought one north to freedom.

I thought I heard the angels say
Follow the drinking gourd
The stars in the heavens
gonna show you the way
Follow the drinking gourd

Next time you gaze up at the Big Dipper, cherish your freedom as you try to imagine what it was like for over 60,000 American blacks who had to travel hundreds miles on foot from the South to all the way to Canada while following the direction of this well-known group of stars.

1 comment:

  1. A REALLY small point on a well written article (I’d no idea about the changes in the North Star!) You write about slaves escaping from the “deep south along the Underground Railroad.”

    The UGRR is the term for the loosely organized volunteers assisting slaves to freedom, their safe houses, the transportation they used, etc. The UGRR did not function much at all in the Deep South, which was of course, hostile territory. And most runaways were completely unassisted until they reached the north. That’s where the UGRR kicked in.

    It makes their flight to freedom even more remarkable.

    Anyway, thanks again for the informative post.

    Joel Bresler
    Independent Researcher