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Why a tossed tennis racket flips: the intermediate axis theorem Spin a tennis racket about its handle or its face and it flies cleanly. Spin it about the third axis and it flips, even in orbit where nothing pushes on it… Physics3 min read5 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 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 guitar frets get closer together as you go up the neck Guitar frets crowd together toward the body because of one fact about hearing: we hear pitch by ratio, not by difference. Each fret has to raise the note by… Music & Sound5 min read28 Sep Why astronauts float: the space station never stops falling Astronauts on the International Space Station look weightless, but at 400 km up, Earth's gravity is still about 90% as strong as on the ground. The trick is… Earth & Space4 min read28 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 we only ever see one side of the Moon The Moon always shows us the same face, so it's easy to assume it doesn't spin. It does. It turns exactly once for every orbit, about every 27.3 days, so the… Earth & Space6 min read26 Sep