The Pole Star Is Not Permanent

Polaris is commonly called the North Star. It sits so close to the point in the sky that the whole northern sky appears to rotate around it. For centuries, navigators and stargazers have used it as a fixed reference point. However, Polaris has not always held this position, and it will not hold it forever. Earth's axis does not point in a perfectly fixed direction. Instead, it slowly traces a wide circle over roughly 26,000 years. This wobble is called axial precession. It is caused by the gravitational pull of the sun and moon on Earth's slightly flattened shape. As the axis wobbles, the point it points toward in the sky slowly changes. This means the star nearest that point also changes over time. Around 2787 BCE, during the era of the Egyptian pyramids, the star Thuban held the position now occupied by Polaris. About 14,000 years ago, the bright star Vega served the same role. Astronomers calculate that Vega will become the pole star again in about 12,000 years. The ancient Greek astronomer Hipparchus first identified this slow cycle, around 150 BC. He did this long before telescopes existed, using only careful comparisons of star positions recorded over time.