Module 8 · Travelling by gravity, and beyond
What if everything stopped?
Interactive 7 min
An orbit is a fall that keeps missing, and sideways speed does the missing. Stop every planet and nothing misses: Mercury hits the Sun in about 15½ days, the Earth in about 65. A fall from rest lasts 1/(4√2), about 18%, of the orbital period.
Try this. Press Stop everything. Why do the outer planets take so much longer to fall in?
Step 1 of 1
The stopped galaxy
Every orbit is a fall that keeps missing, and the only thing doing the missing is sideways speed. So take the speed away. Press Stop everything and every planet loses its orbital velocity in the same instant. Gravity is no stronger than it was — but now nothing misses. Mercury hits the Sun in 15½ days, Venus in 40, Earth in 65, Mars in 121. Jupiter takes 2 years and Neptune 29: a fall from rest always lasts 1/(4√2), about 18% of the orbital period, so distant planets fall slowly for the same reason they orbit slowly. Now switch to the Milky Way and stop its stars. The Sun, 26,000 light-years out, reaches the centre in about 44 million years — yet stars are so small and so far apart that almost none collide. They plunge straight through the middle and swing out the other side, and the flat spiral dissolves into a swarm on long radial orbits. (Here the dark-matter halo stays put, so its pull doesn’t change.)
Text version of this interactive
The stopped galaxy
Every orbit is a fall that keeps missing, and the only thing doing the missing is sideways speed. So take the speed away. Press Stop everything and every planet loses its orbital velocity in the same instant. Gravity is no stronger than it was — but now nothing misses. Mercury hits the Sun in 15½ days, Venus in 40, Earth in 65, Mars in 121. Jupiter takes 2 years and Neptune 29: a fall from rest always lasts 1/(4√2), about 18% of the orbital period, so distant planets fall slowly for the same reason they orbit slowly. Now switch to the Milky Way and stop its stars. The Sun, 26,000 light-years out, reaches the centre in about 44 million years — yet stars are so small and so far apart that almost none collide. They plunge straight through the middle and swing out the other side, and the flat spiral dissolves into a swarm on long radial orbits. (Here the dark-matter halo stays put, so its pull doesn’t change.)
About this interactive
- Licence
- MIT
- Credits
- © 2026 Mateusz Wojczal. Earth texture: Solar System Scope, CC BY 4.0. Three.js (MIT). Inter and Roboto Mono (SIL OFL 1.1).
- Source
- https://github.com/ulams-dev/ulams/tree/main/demo-content/gravity