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Astronomical Data Accuracy


Hi,
Does anyone know what the accuracy is for the AstronomicalData 
calculations for "Altitude" for the sun, moon, planets?  I'm writing a 
Mathematica program to solve celestial navigation problems.  I used the 
following code:

AstronomicalData["Sun", {"Altitude", {2010, 10, 1, 12, 0, 0}, {39., 
-77.}}, TimeZone -> 0]

This gives a result of 9.83748 deg.

However, when I ran the same computation using The United States Naval 
Observatory (USNO) online data service (www.usno.navy.mil/USNO) the 
altitude was given as 9.93889 deg.  This is a difference of 0.1 deg.  
I'm sure the Mathematica AstronomicalData functions were never intended 
to replicate the accuracy and precision of the USNO computation.  
However, 0.1 deg seems a bit much, especially when doing 
navigation/astronomy calculations.  Does Mathematica use low accuracy 
formulae, or did I do something wrong?  Any help would be appreciated.  
Thanks,
Tim


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