Comment by krisoft
14 hours ago
I have once created a pendant to my friends’ wedding following a similar idea. A silver disk engraved one one side with the position of the planets and major moons at the moment of the ceremony. Fun thing is that the Galilean moons orbit fast enough that you can even read the intended minute. https://www.instagram.com/reel/DIpFTPOIP60/
If you have a blog post with a few more technical details, it may be a nice submission for HN. (Do you have a few photos of the intermediate steps to share?)
Some ideas/questions: How is it painted? Is it laser cut or by hand? Did you designed it? How did you do the calculations? Does Saturn have rings? Where is the cutoff? (No Neptune/Uranus/Fobos/Deimos/...) Have you tried to give a different size to each planet?
PS: I showed the video to my older daughter that is interested in astronomy and she likes it.
> If you have a blog post with a few more technical details, it may be a nice submission for HN.
Oh. That is very kind of you. I do have many more pictures and details. I will try to collect them together, and will publish it once it is done. But can’t promise that it will happen soon. So i will answer your questions here in the meantime.
> How is it painted?
The shapes are recessed and the recesses are filled with black nail polish. The excess nail polish was then scraped off from the flat upper surfaces leaving it only in the recesses.
It was very fiddly, and i don’t necessarily recommend this method for anyone. I have since learned how to enamel by melting glass powders onto the metal surface which is both easier and gives a better result. That is how i would do it today. (On my instagram the last reel i posted is showing that process, even though with a different design.)
> Is it laser cut or by hand?
A third and a fourth option. The planet side is machined on a cnc. First I etched the orbits with a v-bit, then cut the planets with a 0.8mm flat endmill, then cut the hole, and finally cut the outline. After that i etched the initials side chemically. As a resist i used self-adhesive vinyl which i cut with a plotter.
To be honest. I wouldn’t recommend this process either. It was super finicky, slow, and error prone. Today i would just etch and cut the metal with a fiber laser. In fact i bought a fiber laser because i got sick of the chemical etching and mechanical machining during this project. :)
> Did you designed it? How did you do the calculations?
I did design it! I’m very proud of it. The initials side was designed in inkscape while the planet side was generated with a python script. The script used the super handy skyfield python library for the calculations. (Which in turn uses the planetary ephemeris files published by the Jet Propulsion Laboratory.)
> Does Saturn have rings?
No ring of Saturn unfortunately. But it would be a cool idea!
> Where is the cutoff? (No Neptune/Uranus/Fobos/Deimos/...)
Unfortunately I don’t have a real good principled answer to this. Because of the machining I had a hard limit on the smallest details I could put on the metal. I did know that i wanted to put the Gallilean moons on there because their short periods meant that they provide good basis for the minutes and hours part of the date. I did know that i also wanted one of the gas giants to provide a “slow hand” to the clock to show the years, and to hopefully stretch out the period before the next time the solar system is in a similar position to very far into the future. And i wanted the inner planets and the Moon so people and future alien minds will recognise it as the solar system. Everything else was just futzing around with the script and finding a good compromise between not making it too large to wear and not making it too crowded either.
> Have you tried to give a different size to each planet?
I did, but it looked uneven and too haphazard to my eyes. Not saying it is impossible to make it neat with different planet sizes but I liked the diagram simplicity of keeping all the planets one size and the moons an other smaller size.
> I showed the video to my older daughter that is interested in astronomy and she likes it.
Oh thank you! That is lovely!
> On my instagram the last reel i posted is showing that process, even though with a different design.
Permalink: https://www.instagram.com/cogs_and_curios/reel/DTNtEFPjEGQ/
> And i wanted the inner planets and the Moon so people and future alien minds will recognise it as the solar system.
I think it was successful.
Out of curiosity were the positions (especially of the Galilean moons) actual simultaneous positions, or positions as seen from Earth, given the ~40 light-minutes distance between the Earth and Jupiter?
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If others are interested in getting something like this — there's an Australian firm already doing a good job at scale (but slightly different to parent).
https://www.thenightsky.com/
That's so cool! Is there a calculator somewhere that can convert to/from dates and solar system position charts?
To calculate the orbital positions i used the skyfield python library. https://rhodesmill.org/skyfield/
They have a very handy example right on the landing page how one can calculate the positions and angles of a planet from a date.
The inverse was a bit trickier. But I also implemented a script which could “solve” a given picture backwards and give us a date. I believe i used binary search to narrow the date down first for the planet with the slowest period, and then refined the date around that timestamp using the position of the planet one faster. That way the estimate got more and more accurate and i didn’t need to brute force search a large time interval. (I applied the assumption that the date to be found is within half a saturn year from our current date, but if that assumption were incorrect it would have resulted in a solver failure during the refinement and thus detected.)
Positions at a given time could be simulated in e.g. Celestia (and then projected). The other direction, I don't know.