grantcblair Jan 28, 2013
--- In email@example.com, Bill Gray wrote:
> Hello all,
> I cross-post because I've seen suggestions on both lists suggesting
> that, if you use heliocentric elements close to the time of the flyby,
> you can get a decent ephemeris. In particular, since the perigee is at
> 15.8 February, elements for 16 February "ought" to work Just Fine. Surely,
> the thinking goes, the perturbations can't be _that_ much over a few hours.
> I just ran a comparison of unperturbed ephemerides using heliocentric
> elements for epoch 16 February, versus an integrated ephemeris (good to
> within a couple of arcminutes at present). Of course, they agree exactly
> at 16 February 00:00 UTC, 4.5 hours after perigee.
> Two hours earlier, at 22:00 UTC on 15 February, the error is 8 arcminutes.
> An hour before that (and 1.5 hours after perigee), the error is over a degree.
> At 20:00 UTC 15 February, the error is almost six degrees.
> Geocentric elements fit the "real" ephemeris to within an arcminute or two
> at all points during the flyby, even without ephemerides. Either use them,
> or use integrated ephemerides.
> -- Bill