Thanks Bill and Alan

I was using Steve Preston's data to enter the (New) Juno and using geocentric
position I stll get the 6' error. But then the elements for the Guide Juno are
almost 2 years old. Maybe I should install the Mycorb data but I can live with the
few minute discrepincy.
If the Mycorb data reduces the flexibility I will stay with the original Guide cd.

Larry Wood

Bill J Gray wrote:

> 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
>
>
>
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