Hi Larry,

I'd expect to see positional error of maybe an arcsecond or two.
Problem here is that the elements (and this will be true of MPCORB
elements, too) are set to a "standard" epoch, at 200-day intervals.
Click on an asteroid and ask for "more info", and you'll see the epoch
provided among the details.

In a better world, this wouldn't be much of a problem, because
Guide would numerically integrate the orbit from epoch to the date of
interest. I'm working on that one... I've made a bit of progress
recently, but there are still issues to be dealt with.

Seven arcseconds sounds like a lot. Could be an "unusual" case,
but it could also be the difference between a topocentric and a
geocentric position.

In any case, the "usual" asteroid elements computed by MPC are not
very good for asteroid occultations. The people who compute these on a
regular basis (Steve Preston, Jan Manek, a few others) usually start
with "high-quality" astrometry, from places such as the USNO's
Flagstaff station (FASTT) and Table Mountain. These observatories are
getting positions good to better than .1 arcsecond, and if you use only
observations of that quality, you'll get a truly exact orbit.

MPC (and, I'm pretty sure, JPL, Lowell, and NEODyS, the other
"big names" in asteroid databases) will mix in these observations with
the data from everybody else. And some of those others have astrometry
good to maybe an arcsecond... anyway, the upshot is that you will not
be able to get a "truly exact" answer unless you compute your own orbits
from observations, the way IOTA folks do.

-- Bill