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

Very nice, fairly efficient too.

I don't like the explicit split of Newtonian and relativistic gravity, this is often how it's presented in educational content, but it creates too much confusion; for instance it gives the illusion that they are somehow separate theories even though Newtonian gravity is a limiting case of Einsteinian gravity when v << c and gravitational fields are weak (see Poissons eq for Newtons gravitational potential.

Lastly, you should consider rendering spacetime similar to Alessandro Roussels spacetime visualization https://www.youtube.com/watch?v=wrwgIjBUYVc; probably the best and most innovative one I've seen.

I really liked your animations, but isn't step 14 incorrect? Earth's axis processes, but on a very long timescale. In the span of a day, the axis should be effectively stationary. That's why its the rotation 'axis' - it's the fixed line it rotates about. That's why Polaris is the north star: the axis of rotation points effectively directly at it at all times no matter the season. During summer in the northern hemisphere, the tilt is towards the sun, giving us more direct heating, and in the winter it's away from the sun. This isn't due to the axis moving, but due to the axis' relative position changing throughout earth's orbit around the sun.

Really cool project! I always struggled to understand the motion of the planets looking at 2D images in textbooks. So I really enjoyed the ability to click and drag to explore the motion of the planets in 3D. I think it helps me build a more accurate mental modal of our solar system.

I also loved the music and narration, I'm curious what you used to create the narration? I noticed the narrator seemed to struggle with numbers sometimes, for example on step 1: "The Sun is ~333 000× heavier" The ~333 000× portion does not seem to be read properly. Not sure if that's only a problem I am experiencing on my browser though.

>The Sun’s gravity (red arrow) pulls the Earth straight toward it the whole time — so why no collision? Because the Earth is also moving sideways (green arrow) at 29.8 km/s. Each moment it does fall toward the Sun, but its sideways speed carries it past — it keeps missing. The dashed line shows where inertia alone would send it; gravity bends that straight path into a closed loop. An orbit is simply falling, continuously, and always missing.

Reading stuff like this always makes me think "well that is fortunate." Of course there is survivorship bias so its not exactly surprising. But it also makes me wonder what could change the status quo.

I guess these are the things that could change it:

- suns becomes lighter (earth shoots into space)

- earth accelerates (earth shoots into space)

- sun becomes heavier (earth falls into sun)

- earth decelerates (earth falls into sun)

I guess in theory some large interstellar object could pass to close too earth and fling us off into space or into the sun.

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!

This was a lot of fun to explore. I find that naturally I want to click through the animations and observe them for a while before delving into the steps and reading the blurb. It's a bit like skimming around a textbook's headings & figures before reading the chapter, it builds motivation and interest.

One simply change to improve the experience is to keep the Next/Back at fixed locations. The animations and transitions are beautiful, and looking away to chase the moving buttons causes me to drop the visual context.

As a workaround I set the height of .tour-body to 900px and the the whole thing became so much more immersive, like the old planetariums.

Thank you all for the comments and showing the weaknesses in the model and visualisation. I'll try to understand the issues and fix them soon.

Very nice! Nitpick: the Moon is tidally locked, so it should always face the Earth with the same side.

It feels like touring space, and I like that it seems like a creative work. Regarding the part after the tour, my only regret is that I wish the camera had a bit more freedom. Also, when I select Mars or another planet, it seems to sometimes show the other planet instead.

For something more rigorous, I would like to take this opportunity to share rebound[1], something we use for n-body simulations in our field (planet formation). Perhaps few people here are already familiar with it. It has a Python interface but the C interface is very easy to use as well and has plethora of pre-set examples which can be visualized using GLFW. It's very very cool!

[1]https://github.com/hannorein/rebound

I'm curious: does generating the planet textures slow down older devices, and how seamless is the switch between the two physics modes?

This is neat! I love that your Step 15 shows an accurate version of the 3d helix, rather than the highly-viral "vortex" animation from a few years back[0]

It'd be awesome to scale this up to the Milk Way, and beyond, watching everything move in relation to larger time scales.

[0]https://astrorhysy.blogspot.com/2015/03/and-yet-it-moves-qui...

the way the original mathematicians figured all this out absolutely melts my brain

no computers, no calculators, barely working telescopes looking at the moons orbiting Jupiter

(don't be limited by episode title, lots of amazing astrophysics in there)

* https://www.youtube.com/watch?v=8yhk1EZq9tY

This is so cool and the music is so calming to listen to as well. I did turn off the AI voice :)

The "focus" on planets doesn't work quite correctly. I love that you included true size, but it would be great if the focus worked, and one could zoom between the planets (until the planet shows up).

I also think Saturn's rings don't wobble that fast.

Very cool!

Just one note about the moon orbit around the Earth, it is far more subtle; almost just orbiting the Sun alongside Earth. I can't explain better than minute physics, highly recommend: youtube.com/watch?v=KBcxuM-qXec

Step 5 shows the earth going as slow as 21.5 km/sec at aphelion and as fast as 39km/sec at the perihelion. I believe that should be closer to 29.3 and 30.3 km/sec, respectively.

Really cool and I love educational apps like this. One suggestion is to show the rotation of the earth as the particles come together to form it rather than a static image.

This is fun! It would be cool to get new updates that model ongoing/future satellite launches or Artemis missions.

Really cool project. I'm curious what was the most surprising thing you learned while building this that changed your own understanding of how gravity or orbits work?

Great job! 14 is misleading though - while the context is one day, the animation depicts axial precession which takes place over ~26,000 years

My physics bias would like to see earth forming while it's constituents were orbiting around the sun.

In any case, nice visualization.

That doesn’t look right: in the 7th panel (too fast it escapes) the force and velocity of earth are constant? 0_o

Neat. How about mentioning the massive black holes at the center of the galaxies?

that's amazing!

Looks great but on mobile the popover covers a quarter of the screen, obscuring the sun

I like it a lot. I'll show to my daughter. Thank you!

Super fun! I might show it to my kids later today. Thanks for making it!

> Einstein

How are you handling relativistic effects in the N-body simulation?

Very nice. My kid is going to love this. Thank you!

Oh wow I never knew Saturn was tilted so much

This is just awesome. Thank you.

Looks very cool!

wow, that's amazing—a very good way to learn this.

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