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The Question

(Submitted February 03, 1998)

I'm at the "watches NOVA and sometimes reads Scientific American" sort of level.

Shakespeare has Brutus say "I am as constant as the northern star." Given the precession of the Earth's axis, was Polaris close to the celestial north pole in 44BC? Was another bright star closer? Or is this an anachronism on Shakespeare's part?

The Answer

The Earth precesses much like a top, the projected rotation axis of the Earth to trace out two small circles (North and South) on the sky, of radius 23 degrees, taking about 26,000 years to complete one precession.

We do know that about 5,000 years ago, around the time the Great Pyramid at Giza and Stonehenge were being constructed, the Earth's north pole pointed near the star Thuban, in Draco. 2000 years ago, the north celestial pole will have been some 12 degrees from Polaris and the nearest bright star will have been Kochab (in the tail of Ursa Minor) 8.5 degrees away.

(In about 12,000 years, the Earth's axis will be directed towards the star Vega, which is in the constellation Lyra.)

I think it's safe to say that Shakespeare didn't know about precession!

Paul Butterworth and Maggie Masetti
for the Ask an Astrophysicist team

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