Around 240 BC, a librarian in Alexandria worked out the size of the planet using a stick, a shadow, and an afternoon’s worth of geometry. 

The craziest thing? He got it very nearly right. His name was Eratosthenes, and he was born in Cyrene, a Greek city on the coast of what is now eastern Libya. 

It’s the same stretch of coast where, on 2 August 2027, the Moon’s shadow will race in off the Mediterranean and turn day to night for close to six minutes in what everyone is calling the ‘eclipse of a lifetime’. 

That’s quite the circle back moment, but we’ll get into that later. 

Let’s learn about Eratosthenes. 

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Who was Eratosthenes?

Eratosthenes was born in Cyrene around 276 BC and made his career in Alexandria, where he became the third chief librarian of the greatest library the ancient world ever built. Cyrene was the intellectual centre of the Mediterranean, and about the best job going for a curious mind.

He was curious about nearly everything, which earned him the nickname Beta. You may recognise this as the second letter of the Greek alphabet. This was because he was reputed to be the second-best in the world at whatever he turned his hand to. 

Some also called him Pentathlos. This means ‘the all-rounder’, the athlete who competes in every event. 

You might notice that both of these were kind of digs; jack of all trades, master of none? I wonder who he had beef with. 

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Eratosthenes Achievements

Eratosthenes coined the word geography and effectively invented the discipline, drawing a map of the known world crossed by a grid of parallels and meridians. The inventor and ancestor of every latitude-and-longitude reference you’ve ever used. 

He devised the “sieve of Eratosthenes,” a method for finding prime numbers that is still taught to schoolchildren and still runs, in essence, inside modern computers. 

He measured the tilt of the Earth’s axis with impressive accuracy. 

And the leap year? Well, Eratosthenes was the one who proposed adding a leap day to the calendar to keep it in step with the seasons.

If that wasn’t enough for this second-best prodigy, Eratosthenes also wrote poetry and literary criticism on the side. For a man remembered as “second-best,” he had a remarkable habit of being first.

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How Eratosthenes Calculated

What sparked the interest of Eratosthenes in the first place? 

Well, Eratosthenes had heard that in the southern city of Syene, today’s Aswan, there was a well where, at noon on the summer solstice, the Sun shone straight to the bottom and cast no shadow at all. 

The Sun, on that one day, stood directly overhead. (Syene sits almost exactly on the Tropic of Cancer, which is why.)

In Alexandria, several hundred kilometres to the north, that was not the case. There, at the same moment on the same day, a vertical pole still cast a short shadow.

How did he go about doing the maths? 

Eratosthenes measured the angle of that shadow and found it to be about 7.2 degrees. 

And 7.2 degrees, as it happens, is 1/50 of a full circle.

His reasoning from there was elegant enough to have survived 22 centuries. If the Earth is a sphere, and the Sun so far away that its rays arrive effectively parallel, then the angle of that shadow in Alexandria equals the angle between the two cities at the centre of the Earth. 

If Alexandria and Syene are one-fiftieth of a circle apart, then the whole circumference of the planet must be fifty times the distance between them. 

He took that distance to be about 5,000 stadia, multiplied by fifty, and arrived at roughly 250,000 stadia for the circumference of the Earth.

Depending on exactly which “stadion” he used (the ancient world was not fussy about standard units) his answer lands within a few per cent of the real figure of just over 40,000 kilometres. 

With just a stick, a well he’d only heard about, and a walking estimate of a distance, he measured the world. 

And for the next 2000 years, no one would really do it any better. Which is truly remarkable, because it took me a while to get my head around this maths even today (OK, maths was never my forte, but…).

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Was his calculation correct?

What makes the achievement so striking isn’t the precision, which honestly was partly down to luck, but the method. And the sheer confidence of it. 

Eratosthenes never left the Nile valley. 

He didn’t sail around anything or climb anywhere to check. He reasoned that if a few things were true, that the Earth was a sphere, that the Sun was distant enough for its rays to arrive parallel, that two known cities lay roughly north–south of each other,  then a single shadow, measured carefully, contained the size of the whole planet. 

A pretty bold prediction and calculation considering the geographical and conceptual constraints of a few thousand years ago. 

In terms of maths alone, it’s one of the earliest and cleanest demonstrations that you can measure something vast and untouchable by measuring something small and near. Provided you understand the geometry connecting them. (I’m still not fully there, don’t worry).

Maths aside, it’s largely the habit of mind that set Eratosthenes apart. Much like today, the rich and powerful are there for a reason - largely, their boldness and confidence. Plus ability. 

This quality, reading the very large from the very small, is exactly the habit that lets astronomers weigh stars and time eclipses today. 

And it’s worth remembering that his answer implied a genuinely enormous world, an extremely daunting concept back then. Which is why, centuries later, a stubborn Genoese navigator preferred a smaller, wronger figure that made sailing west to Asia look survivable. 

(Eratosthenes had the better number). 

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Cyrene, The City that Made Eratosthenes

It’s easy to imagine a mind like that as a product of Alexandria alone. 

But Eratosthenes came out of Cyrene, and Cyrene was no provincial outpost.

Founded by Greek colonists from the island of Thera in 631 BC, it grew rich on the greenest corner of the Sahara. This is the Jebel Akhdar, the “Green Mountain,” where the coast catches enough rain to raise forests and crops where you’d least expect them, in an otherwise desert landscape in north Africa. 

Cyrene’s particular fortune was silphium. This was a plant so prized across the ancient Mediterranean as a medicine and seasoning that the city stamped it on its coins, harvested it to extinction, and never quite recovered the trade. 

Somehow it reminds me of the terrible fate of Nauru in the Pacific Ocean, the country that grew from rags to riches and then rags again (literally one of the poorest to the richest to the poorest countries in the world) in a matter of just a few months by running out of its one main export. Except for Nauru, this was bird poop. 

Before it ran out, this wealth of Cyrene (which lasted longer than Nauru’s, don’t worry), built a spectacular city. This wealth bought leisure, and leisure (then as now), bought thinking.

Cyrene produced a philosophical school of its own, the Cyrenaics, founded by Aristippus, a pupil of Socrates. It gave the world the poet Callimachus and the sceptic philosopher Carneades. And it gave the world Eratosthenes. 

For a few centuries, this stretch of Libyan coast was one of the places where the Greek mind did its most ambitious work.

Much of Cyrene is still standing. 

The Temple of Zeus here is larger than the Parthenon in Athens. (Yup, that’s right - plus, pretty much 0 tourists, too). 

There are sanctuaries and colonnades, a Greek agora and Roman baths and tombs cut into the hillsides. All spilling down toward the sea across the green upland. 

It is a UNESCO World Heritage Site, and it is very nearly empty of visitors. You can walk it in something close to silence, which is not a sentence you can write about many places of its stature.

Visit Cyrene and eastern Libya with Koryo Tours.

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How Eratosthenes Read the Sky?

Eratosthenes belonged to a tradition that had already done something quietly radical. It had taken the sky’s most frightening events and turned them into geometry.

An eclipse, to most ancient societies, was an omen. A swallowed Sun, a warning, a thing to be feared and appeased. Honestly, I think if the sky suddenly went dark for 2+ minutes out of nowhere midday I’d think the world was ending. 

But by Eratosthenes’ day, Greek thinkers had spent generations dismantling that fear with reason. 

The Babylonians, meticulous sky-watchers, had noticed that eclipses repeat on a cycle of a little over eighteen years (called the saros) and had learned to predict roughly when the next was due. 

Thales of Miletus is famously said to have predicted the eclipse of 585 BC, which fell in the middle of a battle between the Lydians and the Medes and so unnerved both armies that they laid down their weapons and made peace on the spot. 

Anaxagoras explained, correctly and controversially, that an eclipse was nothing more sinister than a shadow. The Moon passing in front of the Sun, or the Earth’s own shadow falling across the Moon. 

Later, Hipparchus would use observations of a solar eclipse seen as total in one place and partial in another to estimate the distance to the Moon, and get startlingly close.

That’s the intellectual world and atmosphere in which Eratosthenes grew up. 

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Back to Shadows...

Eratosthenes measured the whole Earth with a shadow. The thin dark line cast by a pole at noon. 

On 2 August 2027, the largest shadow most of us will ever witness will fall across his homeland - the Moon’s umbra, some 258 kilometres wide, racing in off the sea and sweeping over the Cyrenaican coast, holding the desert in darkness for close to six minutes.

To walk among the columns of Cyrene, and then, a day or two later, stand on the centreline near Benghazi and watch the sky go out, makes for a pretty niche bookend to more than two thousand years of human beings looking up and doing the arithmetic. 

The man born on this coast used a shadow to take the measure of the world. Now the world’s most spectacular shadow returns to the same coast, for the longest time it will linger anywhere on land for a hundred years.

Will we be standing there with you? 

Join us for the Eastern Libya Total Solar Eclipse 2027 tour. 

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