Re: [guide-user] asteroid positions - slightly out?

Bob Elliott Mar 8, 2003

At 08:54 2003-03-08 +0000, Lawrence Harris wrote:
>I have twice measured and imaged two asteroids that were virtually
>occulting stars, but despite setting the time/lat-long/ and using the
>MPCorb file I am finding instances where the actual plotted position of
>an asteroid is not where it actually is. Can anyone suggest more checks
>that I can try to get an exact match?

Lawrence.

You may never get an exact match. That is one reason why we do the
astrometry.
All orbits are not updated each day. Take a look at the more info provided
with each minor planet and you will see that many orbits were computed
years ago for lower numbered MPs. Also look at the epoch of the orbit.
Many of the ones I just looked at had an epoch nearly a month old. I don't
update my MPCorb file each day.

You might try Bill's numerical integration routine to update the elements
to the night of your observations and see if that helps.

You didn't mention how much discrepancy you have between your observations
and Guide's calculated position. There is also a possibility of error in
the catalog position of the star.

Paul Comba gave an interesting talk at the MPAPW workshop in 1999 where he
showed the effect of parallax over several hours on his observations of
1996 PA1 that amounted to between two and three arc seconds. This was for
a minor planet that was 1.5 AU from the Earth. When the MPC corrects for
the observers position on the earth some of your observational errors may
be reduced.

Maybe Bill will comment on how Guide deals with Parallax of NEOs. I have
seen trails on Guide that show a wavy line for some objects over several
hours.

You seem to be having a run of good observing weather. I am getting about
15cm of snow today.

Keep up the good work.

Bob Elliott
750 Fall Creek