Ah, thanks Bill! Entirely my own fault for forgetting Guide can use geocentric elements during conditions of close approaches. That makes so much more sense now. Quite excellent, in fact.

Cheers,

Grant

--- In guide-user@yahoogroups.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
>