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Why Is Polaris the North Star?
The Earth spins on its "axis". This axis is an imaginary line running through the Earth. If you were to be high above the Earth, looking straight down along the axis, all the points on Earth would appear to move in circles around the axis. If you followed this axis out into space from the northern hemisphere on Earth, it would point toward a particular star in the sky. We call that star the "North Star" since it sits in the direction that the spin axis from the northern hemisphere of Earth points.
At present, the star known as Polaris is the North Star. However, Polaris has not always been the North Star and will not always be the North Star. To understand that, we need to look at how the Earth spins on its axis.
The spin axis of the Earth undergoes a motion called precession. If you have ever watched a spinning top, you know that its spin axis tends to stay pointed in the same direction. However, if you give it a slight nudge, the axis will start to change its direction, and its motion traces out a cone. This changing of direction of the spin axis is called precession. So what gave the Earth the "nudge" it needed to start precessing? The Earth bulges out at its equator, and the gravitational attraction of the Moon and Sun on the bulge provided the "nudge" which made the Earth precess. It was the ancient Greek astronomer and mathematician Hipparchus who first estimated the precession of the Earth's axis around 130 B.C. The period of precession is about 26,000 years. In other words, it takes 26,000 years for the axis to trace out the cone one complete time.
You can see precession of the spin axis in a spinning top
Polaris will not always be aligned with the north spin axis of the Earth, because that axis is slowly changing the direction in which it points! Right now, the Earth's rotation axis happens to be pointing almost exactly at Polaris. But in the year 3000 B.C., the North Star was a star called Thuban (also known as Alpha Draconis), and in about 13,000 years from now the precession of the rotation axis will mean that the bright star Vega will be the North Star. Don't feel bad for Polaris, however, because in 26,000 more years it will once again be the North Star!
Earth's spin axis also precesses. It takes 26,000 years to go around once!
The North Star Helped Slaves Escape to Freedom
Follow the Drinking Gourd
Escaping slaves had to find their way north. Northern states such as New York and Massachusetts had strongabolitionist societies and benevolent groups — both black and white — that would help escaping slaves. The slaves' final goal was Canada, north of the U.S. border. Slavery was not permitted there, and American laws that allowed people to capture runaway slaves had no effect.People trying to escape slavery had many clues they could rely on to find out where "north" actually was. They knew moss usually grew on the north sides of trees. They also observed that migrating birds flew north in the summer.
What are some other ways escaping slaves could determine where “north” was?
One of the best clues they could use to find north was to locate the North Star. The North Star is also called Polaris. Unlike other stars, it stays in pretty much the same position in the night sky. It always points to the north.People have always used a group of stars to help them find the North Star. They have called this group of stars many names, depending on how they saw the “picture” created by the stars. Sometimes people thought the group looked like a Big Bear. In fact, the Latin name for this star group was Ursa Major, or Big Bear.
At other times, people thought the group of stars looked like a dipper — with a cup that had a very long handle. Slaves knew this group of stars as the Drinking Gourd. They sometimes used hollowed-out gourds to dip and drink water. The gourds looked just like long-handled cups. Two stars on the cup’s edge always point to the North Star. By finding the “drinking gourd” in the sky, people traveling at night could always find the North Star.
Many escaping slaves — including Harriet Tubman, James Pennington, and Josiah Henson — may have “followed the Drinking Gourd” to freedom. Today, many people know of a song called “Follow the Drinking Gourd.” It talks about people escaping slavery by following clues to go north to freedom. The song is a combination of old ideas and new words by many musicians.
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