Summer solstice: the perfect day to bask in a dazzling scientific feat

Wednesday is the longest day of the northern hemisphere's year - but few realise that it also marks a monumental achievement in rational thinking

If you live in the northern hemisphere, Wednesday is the summer solstice - the longest day of the year. In London, the sun will rise at 04:43 and then creep across the sky for 16 hours, 38 minutes and 18 seconds before setting at 21:21. For some cultures, the solstice is seen as the beginning of the summer, while others think of it more as midsummer. It is marked with celebrations across the northern hemisphere, most famously at Stonehenge.

Traditionally, such revels were conducted in skyclad fashion. But these days turning up starkers at an English heritage site is probably not the best course of action. Not that I object to the odd bit of pagan revelry, but for me 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.