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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 read 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 read How Eratosthenes measured the whole Earth with a stick and a shadow Around 240 BC, Eratosthenes, head of the Library of Alexandria, measured the size of the whole Earth with a stick and a shadow. At noon on midsummer, sticks in… History4 min read 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 read 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 read How sailors found longitude before satellites: a race against the clock Latitude is easy: measure the height of the Sun or the Pole Star. Longitude is hard, because the Earth turns one degree every four minutes. To know how far… History6 min read