Summer Solstice: The day we first calculated the size of the Earth.

Summer Solstice in this country is marked with celebrations, most famously at Stonehenge.

Traditionally, such revels are conducted skyclad.summer solstice stonehenge  But these days turning up starkers at an English heritage site is probably not advisable!

But revelry aside did you know the summer solstice is the anniversary of one of the greatest achievements of the human mind: it marks the day we first calculated the size of the Earth.

This amazing feat took place not in the modern age, but way back, over 2000 years ago, in the time of the classical Greeks.

Eratosthenes of Cyrene was the chief librarian at the great library of Alexandria in the third century BC. He read in one of the library’s many manuscripts an account of the sun being directly overhead on the summer solstice as seen from Syene (Egypt). earthThis was known because the shadows disappeared at noon, when the sun was directly overhead. This sparked his curiosity and he set out to make the same observation in Alexandria. On the next solstice, he watched as the shadows grew small – but did not disappear, even at noon.

The length of the shadows in Alexandria indicated that the sun was seven degrees away from being directly overhead. Eratosthenes realised that the only way for the shadow to disappear at Syene but not at Alexandria was if the Earth’s surface was curved. Since a full circle contains 360 degrees, it meant that Syene and Alexandria were roughly one fiftieth of the Earth’s circumference away from each other.

Knowing that Syene is roughly 5000 stadia away from Alexandria, Eratosthenes calculated that the circumference of the Earth was about 250,000 stadia. In modern distance measurements, that’s about 44,000km – which is remarkably close to today’s measurement of 40,075km.

What a stunning piece of deduction, based only on a few simple observations and an ocean of clear thinking. So pause a moment n your revelry pause a moment to think about  the power of rational thought. That too is something to celebrate. skyclad solstice

Now I enjoy more than the odd bit of pagan revelry so skyclad I will dance in the golden rays of dawn. How will you celebrate the solstice?

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