Earth & Space

12 breaks

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… Earth & Space4 min read 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 read 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 read 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 read Why a cannonball fired hard enough never lands: Newton's cannon and the orbit Fire a cannonball horizontally from a mountaintop and it lands some distance away. Fire it harder and it lands farther. Fire it hard enough and the Earth's… Earth & Space4 min read How astronomers measure a star's distance by looking from both sides of the Earth's orbit: parallax Hold a thumb at arm's length and blink one eye, then the other: the thumb jumps against the background. Astronomers do the same with the whole orbit of the… Earth & Space4 min read Why the Moon always shows us the same face: tidal locking The Moon does spin. It turns exactly once for every trip around the Earth, so the same face always points our way. Walk around a chair while always facing it… Earth & Space3 min read 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 read 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 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