Just for fun and self education, I've built this over a weekend to teach myself why orbits exist, not just show planets going around. Something that was never clearly explain to me in school. It opens with a guided tour that builds the idea up step by step: two bodies and the equal/opposite force, inertia (the Sun is removed and Earth just drifts straight), then "an orbit is falling and continuously missing," cosmic velocities with a little rocket, Voyager 1 & 2's real gravity assists (the clock runs the actual 1977–1989 dates so the planets orbit into their grand-tour alignment and the slingshots line up), and it ends on Einstein — gravity as curved spacetime, the classic rubber-sheet well. What's real: every body uses its real radius/mass and J2000 orbital elements; positions come from solving Kepler's equation each frame. You can toggle to an N-body mode (symplectic leapfrog) that shows live energy drift (~1e-6%) so you can see the integrator is honest. The only thing faked is scale — at true scale you can't see anything — so there's a toggle between true scale and a log-remapped "visual" scale, with physics always running in real AU. Tech: TypeScript + Three.js + Vite, fully client-side, no backend, works offline (surface textures are generated procedurally from value-noise; only Earth uses a real image). Source: https://github.com/qunabu/Gravity
Happy to answer questions — and feedback on the physics or the explanations is very welcome. This project might be totally inaccurate in terms of real physics, this is how i do understand this on my own - i'm happy to confront this with reality
VikingCoder31 minutes ago
This is nice.
I did laugh at how the Gravity built the Earth, with a tiny North America and all, and then as more mass was accumulated, North America got to get bigger and bigger and bigger!
Iolaum13 minutes ago
My physics bias would like to see earth forming while it's constituents were orbiting around the sun.
In any case, nice visualization.
BigTuna28 minutes ago
Great job! 14 is misleading though - while the context is one day, the animation depicts axial precession which takes place over ~26,000 years
genpfault37 minutes ago
> Einstein
How are you handling relativistic effects in the N-body simulation?
qunabuop28 minutes ago
Not in the sim right now — it's purely Newtonian (symplectic leapfrog, classical gravity). I show the concept on the last slide ("Einstein: gravity is curved spacetime") — a curve in space wrapping around a star/planet that pulls nearby objects into the well. The quantitative case, Mercury's ~43″/century perihelion precession, I'd add next as a 1PN correction — haven't gotten to it yet. Will try to figure it out how to show this
Brendinoooan hour ago
Super fun! I might show it to my kids later today. Thanks for making it!
stevenalowean hour ago
Looks great but on mobile the popover covers a quarter of the screen, obscuring the sun
qunabuopan hour ago
I should have mentioned that its not mobile friendly so far. I will try to fix this.
qunabuopan hour ago
It should be better now
iainmerrick17 minutes ago
It works pretty well on iPhone, except the descriptive text fills most of the bottom half of the screen, overlapping the sim which is centered on the screen.
If the sim were instead centered on the free space (the top half of the screen) it’d be perfect.
qunabuop12 minutes ago
There a toggle button to show hide description if you missed it