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It's Summer Solstice and we head to Aswan Egypt, just like Eratosthanes

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Stephen Marsh Stephen Marsh | 10:30 UK time, Tuesday, 21 June 2011

Distance travelled ~ 442'521'600 km: day 172

Today Kate Humble and some of the 23 Degrees team are in Aswan, Egypt. They are there to reveal something fascinating about the Summer Solstice and the tilt of our planet.

The Earth's tilt reveals itself every time we step out in the Sun but none more so than today on the solstice. The angle between the Sun and the Earth was first worked out over 2000 years ago. It was a discovery made near the city of Aswan, at the bottom of an ancient well by a Greek scientist and philosopher, Eratosthanes.

Eratosthanes was a poet, an athlete and a true polymath. He invented the word geography and even created a map of the world. He tried to write a chronology of world history and he invented the leap day. He also proposed a simple algorithm to work out prime numbers and all the time he was the head librarian of the library at Alexandria, the greatest treasure trove of knowledge of its time.

The story goes that 250 BCE on the summer solstice Eratosthanes was in Aswan and at midday, he looked down to the bottom of a well and saw his reflection staring back at him. Now as with most geniuses he noticed something that the rest of us would have missed. Something that told him about the very nature of our relationship with the sun.

When Eratosthanes looked at his reflection in the water in the well, he noticed he threw no shadow because the Sun was directly overhead. Now to most people that wouldn't have meant much but for Eratosthames it was an important revelation. Let me explain. Eratosthames didn't live here in Aswan, he lived 500 miles north in Alexandra. You might ask why that's significant. Well history has it that he noticed that when he was in Alexandria at the same time, on the Summer solstice on a previous year he did cast a small shadow. In fact he'd measured its length and angle.

So when he saw there was no shadow in Aswan he realised that at the very same moment, midday on the solstice, the Sun's position in the sky was different in Alexandra to Aswan. So, by calculating the angle of the shadow he cast in Alexandria, and its distance to the well here, Eratosthenes was able to estimate the circumference of the earth with an astonishing 98% accuracy. And using the same data, he was able to establish the tilt of the Earth's axis, obtaining a value of 23° 51' 15". A pretty amazing result without GPS and modern computing methods.

From watching the position of the Sun, Eratosthanes observed something else. After June 21, shadows appear here once again at noon, and over the coming days, they lengthen. So this day and this place marks the most northerly apparent position of the Sun due to that 23 ½ degree tilt. But tracing a line of latitude, west to east, across the planet one can see the same thing. Aswan sits just above a hypothetical line of latitude, a line we know as the Tropic of Cancer. A line that circles the world at 23.4 degrees above the equator.

The northward drift of the Sun in our sky that begun at the Spring Equinox on March 21 ends now on the Summer Solstice. From this day on, in the northern hemisphere, the hours of daylight will start to reduce and the Sun will track a lower arc in the sky.

Kate and the team are visiting a well reputed to be the very same well that Eratosthanes looked down all those years ago, to tell the story of this remarkable man and his discovery about the tilt of our planet.

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