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    Re: Accuracy of ephemeris
    From: Ken Muldrew
    Date: 2017 Oct 16, 19:17 -0600
    If you use Skyfield for python then you can load the JPL ephemeris. In fact, you can load several different versions of JPL's ephemerides depending on your needs. Here are some of them from the docs:
    EphemerisSizeYearsIssued
    de405.bsp63 MB1600 to 2200May 1997
    de406.bsp287 MB−3000 to 3000May 1997
    de421.bsp17 MB1900 to 2050February 2008
    de422.bsp623 MB−3000 to 3000September 2009
    de430.bsp128 MB1550 to 2650February 2010
    jup310.bsp932 MB1900 to 2100December 2013
    It's pretty easy to build an almanac generator with this package although you do have to spend some time thinking about time to get it right.

    Ken Muldrew.

    On 2017-10-16, at 7:33 AM, Bill Lionheart wrote:


    I have been using the python ephem package for ephemeris and it is
    based on code from Xephem, with a discussion of accuracy here
    http://www.clearskyinstitute.com/xephem/
    
    I have seen other discussions of accuracy of ephemeris software on
    NavList and I am sorry if this has been covered before
    
    1) Where can I find an analysis of the effect of errors on the
    intercept  method? Hence the answer to the question "How accurate does
    ephemeris need to be for celestial navigation?" . I know we are
    limited by time and angular resolution in taking sights but we can
    average over a series of sights to reduce this error, but after errors
    in reading the sextant and chronometer is the next biggest error going
    to be variations in the atmosphere? Suppose we are using celestial
    navigation at a fixed point on land and we can average over different
    atmospheric conditions.
    
    2) Is there a "gold standard" ephemeris used in astronomy? It strikes
    me in meteorology the gold standard must be to check predictions with
    observations. Presumably there are observatories making high precision
    measurements with minimal errors (for example when objects are near
    the zenith to reduce refractions).
    
    --
    Professor of Applied Mathematics
    University of Manchester
    http://www.maths.manchester.ac.uk/bl
    

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