"But I am as constant as the Northern Star", says Julius Caesar in the play Shakespeare named after him.

He was making a strong statement that he was reliable and unchanging in his opinions. Shakespeare regarded the North Star as a permanent feature of the heavens. He was wrong. He should have picked something else.

All of us have seen, at some time or another, one of those time exposures of the northern sky, where all the stars leave circular tracks, and in the centre of this circling lies a star that is hardly moving. That is because those circular tracks are due to the rotation of our planet.

If we were at the North Pole, there would be one star overhead not moving because that is where the Earth's axis of rotation meets the sky. We call that star the "North Star.”

That star, named Polaris, is the brightest in the constellation of Ursa Minor, "The Little Bear".

For millennia, Polaris has been used for navigation. If you are sailing the ocean, out of sight of........

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