Oh wow I never knew Saturn was tilted so much
Show HN: Gravity – Interactive solar-system simulator, from Newton to Einstein
31–40 of 63 posts
Re: Show HN: Gravity – Interactive solar-system simulator, from Newton to Einstein
#32I 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 hemisph…
This is exactly right; the phenomenon is known as axial parallelism.
Oh neat, didn't realize there was a term for it!
Re: Show HN: Gravity – Interactive solar-system simulator, from Newton to Einstein
#33Thank 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.
I've just published the first batch of patches and new features. I've learnt a lot during the process and from the comments which was one of the main goals, so I'm really happy about this process. Thanks again!
Re: Show HN: Gravity – Interactive solar-system simulator, from Newton to Einstein
#34Really 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?
Re: Show HN: Gravity – Interactive solar-system simulator, from Newton to Einstein
#35the 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
The equations required to calculate perturbations, and the effort required to do it by hand with just paper, really are brain melting.
https://descanso.jpl.nasa.gov/monograph/series2/Descanso2_S0...
Basically pages and pages of differential equations, either modelled analytically or approximated (as accurately as possible) with Chebyshev polynomials.
Aside from the basic Kepler orbits, everything influences everything else. This doesn't make much of a different in the short term, but space is biiiiig and it doesn't take much for tiny influences to have a measurable effect.
There's a slightly simpler introduction to detailed perturbative planetary orbit calculations in Feynman's Lectures on Physics.
FWIW the solar system isn't unconditionally stable. Even without wandering visitors, there's a small chance Mercury might drift outwards and collide with one of the other Inners in the next few billion years.
Re: Show HN: Gravity – Interactive solar-system simulator, from Newton to Einstein
#36I 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 hemisph…
Everything you're saying is right, but I'm not seeing what's wrong with step 14. Did they edit it?
> Earth turns once every 23 h 56 min (one sidereal day) about an axis tilted 23.4° (the blue line). That spin gives us day and night; the tilt gives us the seasons.
Nothing in step 14 to me implies s procession of the axis.
Re: Show HN: Gravity – Interactive solar-system simulator, from Newton to Einstein
#37>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, continuo…
The tutorial made it seem a little too much like there is only one speed that would keep us in orbit. Any slower and we'd crash, any faster and we'd leave.
In fact, though, if you've ever played any game with orbiting mechanics you'd see that it's extremely difficult to get out of orbit if you're in orbit. Going faster simply increases the size of your orbit, and going slower simply shrinks it.
Note that no space program has ever managed (or tried) to send an object into the sun. We're already starting off with such a high orbital velocity, 30km/s, that we'd need to send a rocket backwards at nearly that speed just to slow it down enough to make it crash into the sun. That would require massively more energy than anything we've ever done before.
Re: Show HN: Gravity – Interactive solar-system simulator, from Newton to Einstein
#38Step 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.
Re: Show HN: Gravity – Interactive solar-system simulator, from Newton to Einstein
#39I 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 hemisph…
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Re: Show HN: Gravity – Interactive solar-system simulator, from Newton to Einstein
#40>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, continuo…
The tutorial made it seem a little too much like there is only one speed that would keep us in orbit. Any slower and we'd crash, any faster and we'd leave. In fact, though, if you've ever played any game with orbiting mechanics you'd see that it's extremely difficult to get out of orbit if you're in orbit. Going faster simply increases the size of your orbit, and going slower simply shrinks it. Note that no space pro…
Seems like somehow orbiting bodies finally come to an "equilibrium point"... where orbital speed cancels out gravitational pull towards the sun, so a balance is achieved ?