Earth & Space
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Planets, weather, oceans and the sky above them.
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How astronomers find planets by their shadow: the dip in starlight
Most planets around other stars are far too faint to see directly, so astronomers watch the star instead. When a planet crosses its face, a tiny dark disc…
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…
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…
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…