![](https://i0.wp.com/sfscience.net/wp-content/uploads/2012/08/Space.png?fit=300%2C89&ssl=1)
This video, from the absolutely fantastic YouTube collaboration called Sixty Symbols, explains why the 21st of June is the longest day, why we have seasons and how Eratosthenes used differences in shadow length to calculate the circumference of the world about 2000 years ago. Watch it – it’s really good.
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