Celestia 1.3.2 prerelease

[chris]

My understanding is that their databases are extremely large, and Chris may still be debugging them, so they aren't included with the prerelease. Hopefully he'll publicise them and make them available on SourceForge when they're ready. I'd expect they could be made available separately, perhaps sooner than the "final" release of 1.3.2.

Have a look at http://www.celestiaproject.net/~claurel/celestia/files/jpleph/

There are three JPL ephemeris files. The .405 files cover a timespace of 50 years. The .406 file covers 300 years from 1800-2100. The max interpolation error of the DE406 ephemeris is slightly greater (up to 25m versus 1mm for DE405), but this shouldn't matter for Celestia. To experiment with the JPL ephemerides, just copy one of the files (probably unxp1800.406) into Celestia's data directory as jpleph.dat. Then, change the CustomOrbits in solarsys.dat from vsop87-<planet> to <planet>-jpl. There's still a slight discrepancy between the positions Horizons returns and the positions I compute from the JPL ephemerides--for Jupiter, it's up to 200km. At this point, I suspect the problem may be related to a slightly different time standard.

--Chris

[Falck]

There's still a slight discrepancy between the positions Horizons returns and the positions I compute from the JPL ephemerides--for Jupiter, it's up to 200km. At this point, I suspect the problem may be related to a slightly different time standard.

Chris,

There are a few things you may want to look into. The most likely reason for the discrepency is the delta between universal time and ephemeris time. This is affected by the number of leap seconds added to the calendar. I believe the current value is 62.174 (or thereabouts). Possibility 2 is that the VSOP87 is reporting the observational position, not the true position (the difference being the time it takes for the light to propagate to Earth for an observation to be made). This will probably be a minor effect if its there at all.