Tidal Forces (Why There Are Two Bulges)
Put a planet next to a moon or a star and see how gravity pulls the near side harder than the far side — the stretch that raises two tides a day, locks the Moon's face to us, and can tear a moon apart if it strays too close.
Preview — sign up free to change the numbers yourself
What you can adjust
- System
- 0 – 4
- Tide-raiser mass (Custom only)
- 20 – 30.477121 log₁₀(kg)
- Stretched body's radius (Custom only)
- 1 – 5.90309 log₁₀(km)
- Stretched body's mass (Custom only)
- 20 – 27.30103 log₁₀(kg)
- Separation
- 0.30103 – 5.477121 log₁₀(× tide-raiser radii)
- Animation duration
- 4 – 8 s
Pick a real planet/moon/star pair, or go Custom and set every mass, radius and separation yourself.
How massive the body doing the pulling is, on a log scale (10^value kg) — only used when System is set to Custom.
The radius of the body being stretched, on a log scale (10^value km) — only used when System is set to Custom.
The mass of the body being stretched, on a log scale (10^value kg) — only used when System is set to Custom.
How far apart the two bodies are, as a log-scale multiple of the tide-raising body's own radius — drag it down far enough and the stretched body crosses its own Roche limit.
How long the clip runs — long enough to watch the stretched body complete one full orbit.
About this simulation
What it shows
A body pulls on a nearby planet or moon and stretches it, and the stretched body completes one orbit. You see the two tidal bulges and whether it is inside its Roche limit.
The key idea
Gravity pulls harder on the near side than the far side, and that difference stretches the body in two directions. It grows as mass over distance cubed, so it rises very steeply as the bodies get closer, and too close it can tear a body apart.
Try this
Pick Earth–Moon in System, then drag Separation down: the stretch grows steeply, and below the Roche limit the stretched body would be pulled apart.