Gravity – Interactive solar-system simulator, from Newton to Einstein

Someone who goes by qunabu on GitHub looked at orbital mechanics and realized something was missing from every simulation they'd ever used—they showed you planets going around but never explained WHY. So over a single weekend, they built Gravity, an interactive solar system simulator that actually teaches you the physics behind orbits rather than just pretty animations.

It starts with a guided tour that builds the concept piece by piece: two bodies and the equal-and-opposite force, inertia (watch what happens when you remove the Sun and Earth just drifts in a straight line), then the wild realization that "an orbit is just falling and continuously missing." It throws in cosmic velocities with a little rocket and references Voyager 1 and 2's actual trajectory. No textbooks, no lectures—just a hands-on way to feel why gravity works. The Earth texture credits Solar System Scope under CC BY 4.0.

This isn't a startup. It's not raising money. It's someone who was curious, built something to teach themselves, and shared it on Hacker News because they thought it was cool. That's it. That's the whole vibe. And honestly? More people should build things just because they want to understand them.