Today is the Northern Hemisphere's Summer Solstice, which means it's the longest day of the year. Woohoo!
In celebration of the solstice, take a look at this great little clip from Carl Sagan's Cosmos, which describes the Greek mathematician Eratosthenes' calculation of the circumference of the Earth some 2,200 years ago. The summer solstice played a key role in Eratosthenes' discovery:
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