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Sky watch: Precession's ancient north star

Wouldn't it be great to have a time machine? We could peer into the same starry heavens pondered by such luminaries as Aristotle, Copernicus and Galileo.

Well, if you'd like to see very same sky viewed by anyone throughout recorded history, look up. For the most part, the heavens haven't changed much.

Orion is still Orion; the dippers are still the dippers.

change that occurs in our sky called "precession." Precession is the wobbling of the Earth's axis -- the same kind of wobbling experienced by a spinning top. But while a top's precession is easy to see, the Earth's requires some 25,800 years to complete just one cycle.

Earth's axis to point out in different directions over the millennia. Right now, our planet's north axis aims toward Polaris, making it our North Star. But when ancient Egyptians were around, our northern axis pointed to the star Thuban in the constellation of Draco.

Draco is a large and ancient star grouping that appears nightly in our northern sky.It wraps itself around the north celestial pole and remains perpetually above the horizon for much of the U.S.

After dark this week, search for Draco between the Big and Little dippers in the northern sky. Look for its long string of stars beginning between the Big Dipper's "pointers" and the North Star. Then follow it down until it snakes back to Polaris, where it turns and heads down again.

At the lower end of its long body lie four stars that form Draco's head, also known as the "lozenge." And back to the tail of the dragon, midway between the Little Dipper's bowl and the Big Dipper's handle, lies the star known as Thuban -- Arabic for "dragon."

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