Orbital Motion (Gravity)
Orbit two bodies about their barycenter — pick each body's mass (planets through stars) and the starting radius and speed to see circular, elliptical, and escaping (hyperbolic) trajectories; a body only glows like a star once it passes the fusion mass threshold.
Preview — sign up free to change the numbers yourself
What you can adjust
- Central mass (custom, kg)
- 100000000000000000000 – 4e+31 kg
- Initial radius
- 57900000000 – 2872000000000 m
- Initial (tangential) speed
- 500 – 80000 m/s
- Orbiter mass (custom, kg)
- 100000000000000000000 – 4e+31 kg
- Orbiter mass
- 0 – 19
- Central mass
- 0 – 19
- Central body color
- 0 – 8
- Orbiter color
- 0 – 8
Real kilograms — defaults to the Sun's mass (1.989e30 kg). Only used when the Central mass dropdown is set to Custom.
0.39 AU (Mercury) to 19.2 AU (Uranus). Earth = 1 AU ~= 1.496e11 m.
Real Earth orbital speed is ~29,780 m/s. Try the circular speed for your radius, then go faster (ellipse, then escape) or slower (tighter ellipse).
Defaults to Earth's mass (5.97e24 kg). Both bodies orbit their shared barycenter — raise this toward the central mass (up to 4e31 kg, star-scale) to make the two bodies visibly orbit each other as a binary, with the central body no longer near-fixed. Only used when the Orbiter mass dropdown is set to Custom.
Pick a real body (Ceres through Blue giant) to use its real mass instead of the Orbiter mass (custom, kg) slider — pick Custom to use that slider.
Pick a star (or any body) to use its real mass instead of the Central mass (custom, kg) slider — pick Custom to use that slider. Masses are independently reversible: try Central mass = Ceres with Orbiter mass = Jupiter to see the 'central' body do most of the wobbling.
Drawn color of the central body — purely visual, independent of its mass preset.
Drawn color of the orbiting body — purely visual, independent of its mass preset.
About this simulation
What it shows
Two bodies orbit their shared centre of mass under gravity. You pick each mass, the starting distance and the sideways starting speed, and the path is drawn.
The key idea
Gravity falls off as 1/r². At exactly the circular speed the path is a circle, a little faster gives an ellipse, and past the escape speed (circular speed × √2) the orbiter leaves for good.
Try this
Raise Initial (tangential) speed from 29,780 m/s to 35,000 m/s for a stretched ellipse; above about 42,100 m/s the orbiter escapes.
Real orbits take months or years, so playback is sped up to fit about 30 seconds. The on-screen clock shows the real time that has passed.