Looking at the JPL DE421 ephemeris, and others from JPL, I’m intimidated at the prospect of figuring out how to calculate planetary positions from this data– it seems pretty complicated. Meanwhile, there’s a commercial software package called Swiss Ephemeris that’s based on the JPL DE406 ephemeris and purports to do exactly this. It seems to be written a couple of longtime dedicated astro geeks in Zurich, and informed by the Swiss national passion for accuracy and precision in timekeeping. It’s called the Swiss Ephemeris, and if you license it for your product, starting at around $680, you can also display their “Swiss Ephemeris Inside” logo.
If the planetary positions are indeed difficult to calculate, that suggests that my original idea of Arduino + SD card with JPL ephemeris might not have enough horsepower to do a good job. It might make more sense to go for a Raspberry Pi (or similar) running the Swiss Ephemeris. The whole software package fits on a CD-ROM (700MB), but I’m not sure how much of that is program code (which I think would need to fit into the Pi’s 512MB onboard) and how much is ephemeris tables that could easily on a 2GB SD card. Anyway, I’m gonna look into it. More expensive for sure, but it might be worth it to not re-invent the cosmic wheel and also have the #1 trusted name in planetary position calculation software.