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On 2016-02-22 7:03, Dave Walden wrote:
> ************************
> 
http://aa.usno.navy.mil/cgi-bin/aa_geocentric.pl?ID=AA&task=6&body=1&year=2016&month=2&day=1&hr=11&min=0&sec=0.0&intv_mag=1.0&intv_unit=1&reps=5
> 2016 Feb 01 11:00:00.0     19 14 15.421     - 20 38 03.89
> ************************
> http://ssd.jpl.nasa.gov/horizons.cgi#results
> 2016-Feb-01 11:00     19 14 15.4172 -20 38 03.896
> ***************
> http://vo.imcce.fr/webservices/miriade/?formsMercury2016-02-01T11:00:00.0019 14 15.42394
> -20 38 3.8918


Nobody is curious about the discrepancies between those positions? For
example, the format of the USNO coordinates implies about 10 mas (milli
arc second) accuracy. But compared to JPL HORIZONS, the difference is 54
mas. OK, that's not actually unreasonable. Maybe the USNO values carry
an extra decimal place to ensure negligible loss of accuracy due to
roundoff. I can go along with that.

But what about HORIZONS vs. IMCCE? The coordinates differ by 95 mas.
That's far more than a few counts in the last place!

Maybe the time scale is not the same for all three sources? Mercury
moves about 1.75 arc seconds per minute of time, so 95 mas is equivalent
a time error of 3.2 seconds. UT1 is about .026 s ahead of UTC and 68
behind TT. Neither number is close to 3.2, so a time scale mistake
doesn't look like a good explanation for the position discrepancy.

No do I think we can blame differences between ephemerides. Here is the
geocentric geometric place of Mercury at 2016 Feb 1 11:00:00 UTC from
three major JPL ephemerides going as far back as 1997:

19h13m19.3193s -20°39'56.415  DE430
19h13m19.3193s -20°39'56.416  DE422
19h13m19.3193s -20°39'56.412  DE406

I think IMCCE has their own ephemeris, but could it differ from JPL by
95 mas? Of course that's insignificant for navigation. But when precise
computations differ by that much, frequently there's a mistake
somewhere. I know that from painful experience!

   

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