Brightest Stars

Thuban was Pole Star for the ancient Egyptians

Huge stone Egyptian Pyramid with tiny tourists in the foreground.
The Great Pyramid of Giza. Egyptologists believe that it was built as a tomb for a 4th Dynasty Egyptian pharaoh, Khufu, in the 26th century BC. During the reign of Khufu, Thuban was the Pole Star. Image via Nina Aldin Thune/ Wikimedia Commons (CC 3.0).

Thuban isn’t a particularly bright star, but it holds a special place in the hearts of stargazers. That’s because Thuban – a relatively inconspicuous star in the constellation Draco the Dragon – was the Pole Star some 5,000 years ago, when the Egyptians were building the pyramids.

And there’s evidence that Thuban helped guide the ancient pyramid-builders. Some features inside the pyramids do align with the stars.

But while the pyramids appear to us as an enduring monument of ancient Egypt. … the sky slowly changes. And, because Earth’s axis wobbles slowly over 26,000 years, the identity of our Pole Star changes. So Thuban is no longer our Pole Star.

But it’ll be the Pole Star again some 20,000 years from now. What will humanity be doing then?

The pyramids and Thuban

Among the many mysteries surrounding Egypt’s pyramids are the so-called “air shafts” in the Great Pyramid of Giza. Originally, experts believed these narrow passageways were for ventilation while building the pyramids. In the 1960s, though, we realized the air shafts aligned with stars or areas of sky as the sky appeared for the pyramids’ builders 5,000 years ago.

One of the “air shafts” follows a crooked course through the Great Pyramid, so you couldn’t have sighted stars through it. To this day, the purpose of these passageways inside the Great Pyramid isn’t clear, although they might have been connected to rituals associated with the king’s ascension to the heavens. Whatever their purpose, the Great Pyramid of Giza reveals that its builders knew the starry skies intimately.

They surely knew Thuban was their Pole Star, the point around which the heavens appeared to turn.

Star chart showing a circle around a red dot and some constellations.
The 26,000-year precession cycle causes the north celestial pole to move counterclockwise relative to the background stars. Whichever star is closest to the north celestial pole, at any given time, is called the North Star. Image via Wikimedia. Used with permission.

Past and future Pole Stars

Indeed, Thuban at times made a better Pole Star than our modern Polaris. Various sources claim that Thuban almost exactly pinpointed the position of the north celestial pole in the year 2787 BCE.

Meanwhile, our modern Polaris – which many centuries ago was an ordinary star known by the name Phoenice – won’t match Thuban’s precision when it most closely aligns with the north celestial pole on March 24, 2100. Polaris will be 27′ 09″ (0.4525 degrees) from the north celestial pole at that time (a little less than the angular diameter of the moon when at its farthest from Earth), according to the computational wizard Jean Meeus.

The Northern Hemisphere also has had long stretches without a Pole Star. After the reign of Thuban but before that of Polaris, Kochab in the Little Dipper served as a rather poor Pole Star in 1100 BCE. Kochab was only half again as close to the north celestial pole as it is today.

Looking into the future, Errai will become the northern Pole Star around 4000 CE, and Alderamin will take its turn around 7500 CE.

Animation of spinning top, bottom of axis fixed, top of axis making a circle.
Earth never changes its axial tilt, but its axis does point at different Pole Stars. Many compare this movement of Earth to the wobble of a spinning top before it falls. Animation via Astro Bob. Used with permission.

Why does the identity of the Pole Star keep changing?

Earth’s axis maintains a tilt that varies from about 22 degrees to 24 degrees from perpendicular every 41,000 years with respect to the plane of our orbit around the sun. But, over a period of 26,000 years, Earth’s axis points out at different Pole Stars, tracing out a slow circle in the heavens. Whichever star lies on or near that circle will eventually be a Pole Star.

Many compare this motion of Earth – called precession or sometimes precession of the equinoxes – to that which you sometimes see in a spinning top wobbling before it falls.

A line from Earth's axis. Its far end moves around a circle on the stellar background.
The 26,000-year cycle of precession. It’s caused by a wobble of Earth. Over this cycle, Earth’s northern axis can be imagined to trace out a circle on the celestial sphere. In reality, precession causes Earth’s northern axis to point to different stars. Thus, the identity of Earth’s Pole Star, or North Star, shifts over the cycle of 26,000 years. Image via Tfr000/ Wikimedia Commons. Used with permission.

How to see Thuban

Thuban is part of the constellation Draco the Dragon. Although it’s not a super bright star, it is bright enough to see with relative ease on a dark night.

Most people star-hop to Thuban from the Big and Little Dippers.

Sky chart showing Big and Little Dippers in July.
Orientation of Dippers on July and August evenings. Note that Polaris is the end star in the handle of the Little Dipper. The star Thuban, in the constellation Draco, lies between the 2 Dippers. Chart via Chelynne Campion/ EarthSky.

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Bottom line: The Pole Stars change over time. Thuban was the Pole Star some 5,000 years ago, when the Egyptians were building the pyramids.

August 9, 2023
Brightest Stars

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Bruce McClure

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