Why a cannonball fired hard enough never lands: Newton's cannon and the orbit
30seconds
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 surface curves away as fast as the ball falls: that is an orbit.
From 200 km up the speed needed is 7.79 km/s, and past 11.0 km/s the ball never comes back. Fire Newton's cannon at any speed you like, and see why astronauts float while gravity is still about 88% as strong as on the ground.
Deep dive · 4 min
More in Earth & Space
View all →
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…
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…
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 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…
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…
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…