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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 a falling pole can be held up: feedback, and the PID controller A pole on a cart wants to fall, and a controller that watches the lean and shoves the cart can hold it up. But how hard it pushes matters: too weak and it… Engineering4 min read29 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 Why Taipei 101 hangs a 660-tonne ball in its upper floors: the tuned mass damper At the top of Taipei 101 a 660-tonne steel ball hangs in plain view. It is not decoration: it is a tuned mass damper that swings against the building and can… Engineering4 min read19 Oct Why rockets drop their tanks on the way up: the rocket equation and staging Orbit isn't about height. It's about speed: roughly 7.8 km/s sideways, and about 9 to 10 km/s of total speed gain once gravity and air drag take their cut. The… Earth & Space4 min read16 Oct How a 2,000-year-old bronze machine predicted eclipses with the number 223 In 1901, divers pulled a corroded bronze machine from a shipwreck off the Greek island of Antikythera. One dial ended in the number 223, written in Greek… History4 min read4 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 a 660-tonne steel ball swings to keep Taipei 101 still Skyscrapers sway in the wind, and gusts that match the tower's own rhythm can make it sway more and more. Taipei 101's answer is a 660-tonne steel pendulum… Engineering4 min read3 Oct Earth is closest to the Sun in January. So why is that winter? Summer can't be caused by being closer to the Sun: Earth makes its closest pass around 3 January and is farthest around 4 July. The distance changes by only… Earth & Space4 min read29 Sep Why the sky is blue and sunsets are red: it's the same light The blue sky and the red sunset are the same effect, seen from two angles. Air molecules scatter short, blue wavelengths far more than long, red ones: about… Physics4 min read27 Sep Why Mars sometimes walks backwards across the sky Every 26 months or so, Mars slows down, stops, and drifts backwards against the stars for a couple of months before carrying on. Ancient astronomers built… Earth & Space4 min read27 Sep Why a rainbow is always a circle 42° around your shadow Every rainbow is part of a circle centred on the shadow of your head, with its red edge about 42° away. That number comes from inside a single raindrop… Physics5 min read26 Sep