If you look up into the clear night sky in the Northern Hemisphere, you’ll spot Polaris, more commonly known as the North Star.
Polaris is special because it’s almost directly above the geographic North Pole, making it a trusty guide for navigation for centuries.
However, Polaris hasn’t always been our North Star. Long ago, from 3942 to 1793 BCE, a star system called Thuban held that title.
Thuban is part of a binary star system, which means it has two stars. In Ancient Egypt, Thuban was known as the “head of the serpent,” which sounds pretty cool and also scary.
The main star in Thuban is a white giant that’s 2.8 times the size of our Sun.
The second star in the system is likely an A-type main-sequence star, about 2.6 times the Sun’s mass.
But why do the North Stars change?
“Forces associated with the rotation of Earth cause the planet to be slightly oblate, displaying a bulge at the equator. The moon’s gravity primarily, and to a lesser degree the Sun’s gravity, act on Earth’s oblateness to move the axis perpendicular to the plane of Earth’s orbit,” NASA explains.
“However, due to gyroscopic action, Earth’s poles do not ‘right themselves’ to a position perpendicular to the orbital plane. Instead, they precess at 90 degrees to the force applied. This precession causes the axis of Earth to describe a circle having a 23.4 degree radius relative to a fixed point in space over about 26,000 years, a slow wobble reminiscent of the axis of a spinning top swinging around before it falls over.”
Put simply, it’s because of a slow wobble in Earth’s rotation, kind of like how a spinning top wobbles as it slows down.
This wobble, called precession, takes about 26,000 years to complete one cycle. Right now, we’re in a part of that cycle where Polaris is our North Star.
But in the distant future, around the year 20346 CE, Thuban will once again become the North Star.
So, if you live long enough, you might get to see Thuban taking over Polaris’ spot and be our North Star once again.
For those in the Southern Hemisphere, there’s no single bright star near the South Pole. Instead, they use a constellation called the Southern Cross to find their way south.
It’s not as straightforward as having a North Star, but it works well enough if you are navigating down under.
Next time you look up at the night sky, remember that our North Star, Polaris, is a temporary guide.
Future explorers–well, those that are on Earth–will use completely different stars to find their way.
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