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Why the night sky is dark: Olbers' paradox and the light that hasn't arrived yet Go outside and look between the stars: it's dark. That's strange. In an endless, unchanging universe, every line of sight would end on a star, and the whole… Earth & Space4 min read3 Nov Why sunflower seeds never leave gaps: the golden angle and the Fibonacci spirals Sunflower seeds are not placed in spirals on purpose. Each new seed turns a fixed angle from the last, and only one angle, about 137.5 degrees, leaves no bare… Maths4 min read28 Oct Why there are two high tides a day: the Moon pulls the near sea harder and the far sea softer The sea rises twice a day, not once, and the second high tide happens on the side of the Earth facing away from the Moon. Why? The Moon pulls the near ocean a… Earth & Space4 min read27 Oct Why the sky is blue and the sunset red: Rayleigh scattering and the length of the air The sky is blue and the sunset is red, yet the sunlight is the same. Air scatters short blue waves about four times more strongly than long red ones, a… Physics4 min read25 Oct How a stick's shadow measured the whole Earth: Eratosthenes, a well and 5,000 stadia Two sticks, one shadow and one distance were enough for Eratosthenes to size the whole planet around 240 BC. On the summer solstice the noon Sun shone straight… History4 min read24 Oct Why the Moon changes shape: always half lit, and only part of it faces you Ask why the Moon changes shape and most people blame the Earth's shadow. It is a natural guess, and it is wrong: the Moon is half lit by the Sun all the time… Earth & Space4 min read24 Oct How a clock found longitude at sea: Harrison's H4 and the £20,000 prize In 1707 a British fleet hit the rocks off Scilly because navigators could not find longitude. Seven years later Parliament offered £20,000 for a method… History4 min read21 Oct Why planets speed up near the Sun: Kepler's equal-areas law Earth moves at 30.29 km/s near 3 January and 29.29 km/s near 4 July, with nothing pushing it along. The rule behind the change is Kepler's equal-areas law: a… Earth & Space3 min read20 Oct How a stick and a well measured the whole Earth: Eratosthenes Around 240 BC, the librarian Eratosthenes measured the size of the Earth using a well, a stick and a shadow. At noon on the summer solstice the sun shone… History4 min read11 Oct Why one day on Mercury lasts two Mercury years Mercury circles the Sun in 88 Earth days and spins once every 58.6 days. That odd pairing, three spins for every two orbits, means one sunrise to the next… Earth & Space3 min read4 Oct Why people bid more than a dollar for a dollar: Shubik's escalation trap Auction a dollar bill with one twist: the second-highest bidder pays their bid too. It sounds harmless, yet in 1971 economist Martin Shubik reported that… Economics3 min read2 Oct How honeybees give directions by dancing in the dark A honeybee that finds good flowers can tell her sisters where they are, inside a pitch-black hive, by dancing. She runs in a straight line while shaking her… Biology5 min read2 Oct