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    Re: Skyfield
    From: Lars Bergman
    Date: 2018 Oct 23, 14:00 -0700

    John H.,

    I am fully aware that the Nautical Almanac of today is not that accurate.

    But it was different in 1915. For the Moon, RA and declination were given for each hour. Also given were the variation in 10 minutes, given to 0.001 seconds of time and 0.01 seconds of arc respectively, providing full information for performing a quadratic interpolation for times in between. For the Moon, "The numbers (which contain Newcomb's Correction to Hansen's Tables) represent the position of the Moon, with respect to the true equinox and the equator, as it would appear from the centre of the Earth at the respective Greenwich times."

    My point is that the differences, as shown by Robin, between Skyfield and NA is showing a very regular pattern, indicating some kind of systematic "error" somewhere, it is not random noise. Different coordinate system? Time offset? I don't know.


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