I am finalizing an Orrery that used Arduino to move planets into their respective orbital position based on the year and month from a users input. I am looking for a table or database that has the location of each planet as represented in a concentric plane. For example January 2018, Earth was 9.81° from 0 (arbitrary I know, got to start somewhere).

Planet Angle

Orrery nearing completion

Right now I am using an extremely crude formula to calculate the planets location angle based on the orbital period and a fixed date (Jan 2018). Of course this will result in gross errors when extrapolating out to dates more than a few years forward or backwards (orbits are elliptical). What I would like to do, is have a table with each planets location based on January of their respective year. I will program the system to lookup the value for each planet and move them to their respective position. Now I imagine the mathematics for producing this is WELL outside of my expertise and mathematical prowess.

Orrery in motion


1 Answer 1


The concept you're looking for is that of an ephemeris: from Wikipedia,

an ephemeris (plural: ephemerides) gives the positions of naturally occurring astronomical objects as well as artificial satellites in the sky at a given time or times.

There are a large number of published ephemerides, and they're easy to find once you use that search term (and, particularly, if you look for heliocentric planetary ephemerides). If you're looking for a good place to start with, I understand the JPL Horizons system is a solid starting ground, and it seems to be sufficiently well documented.

To be honest, though, given the look of your device, I seriously doubt that using hard-coded year-by-year data will significantly increase its accuracy as compared to just hardcoded rotation rates. As you point out, planetary orbits are elliptical - but your device does not actually implement this from the look of it, and neither does it get the planetary orbital radii correct (which is a hard problem, because of the wide range of scales, but something I'd class as more important than the mild ellipticities and the various elliptical anomalies). But ultimately, it's your project, and it's your opinions on what's important that count.

  • $\begingroup$ Thank you very much. Knowing the correct search term is more than half the battle! I know that accuracy is out the window with this type of device, its a very basic machine that is obviously not more than a desktop trinket. I will dig into the data available for ephemeris that pertains to my project. Thank you again. $\endgroup$
    – Bryan Lord
    Aug 29, 2018 at 14:25

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