The NASA HORIZONS[1] system is a telnet service that provides planetary data and ephemeris tables, which are basically coordinate-time plots for celestial bodies.

It's pretty archaic and somewhat difficult to use, especially since it just dumps out text tables and you have to parse it to do anything with it.

Some projects, e.g. KDE Marble, use it as a data source and typically just consume it with a little procedural code to get what they need.

I wanted to provide a full-fledged library for it so that you can easily mash it up in a web app, so I started a Gem called Tengai[2]. It has a very minimal set of features right now, but I have a lot of ideas and I'd appreciate feedback (and forking!).

This little Sinatra app[3] uses Tengai to provide a JSON interface to the ephemerides. Check it out:

http://horizons.herokuapp.com/bodies/499/ephemeris?start_time=2013-3-23%2000:00:00&stop_time=2013-3-23%2023:59:00&interval=720

[1]: http://ssd.jpl.nasa.gov/?horizons

[2]: https://github.com/zacstewart/tengai

[3]: https://github.com/zacstewart/restful-horizons