Astrod is built with support for PyEphem and Pyswisseph.
PyEphem is based on XEphem. This is based on based on the following ephemerides:
- Inner planets: VSOP87D
- Outer planets: NASA JPL DE200
Pyswisseph is based on the Swiss Ephemeris. This is based the newer:
We performed a comparison of the two libraries, and note the following:
- Pyswisseph is faster than PyEphem by about a factor of 4
- PyEphem feels easier to code with and better documented than Pyswisseph
We then examined each library’s predictions for the position of each planet on the ecliptic between 2000BC and 2000AD.
- The predictions of the two libraries generally differ by less than a tenth of a degree for all planets except Pluto
- For Pluto, the newer Swiss Ephemeris gives markedly different predictions for times outside recent history
Overall, Pyswisseph seems to be the superior library, and PyEphem the easier one. Currently Astrod can make use of both libraries for all of its calculations.
For the record, the results are shown below. In all cases, the x-axis is the year, and the y-axis the difference in prediction of position on the ecliptic in degrees between the two libraries.