[twic]

I remember spending a lot of time playing with a planets-and-gravity simulator on the Mac when i was a kid. I wrote a crappy clone of it as an applet for one of my first jobs!

This is the best web-based equivalent i found with a quick search:

https://hermann.is/gravity/

And this is a rather fine tutorial on writing your own:

https://css-tricks.com/creating-your-own-gravity-and-space-s...

[perl4ever]

I played with a simple program like that for the Amiga when I was younger, and then I discovered that there was an O(n log n) algorithm[1] rather than the obvious O(n^2) one. Never was up to writing a program to use it.

https://en.wikipedia.org/wiki/Barnes%E2%80%93Hut_simulation

[m0skit0]

I highly recommend Space Engine https://store.steampowered.com/app/314650/SpaceEngine/