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    Lunar Occultation continued...
    From: Bob Crawley
    Date: 2015 Nov 2, 09:23 -0800

    As with Geoffrey I missed the Occultation, thick cloud in Suffolk. Anyway out of interest I used the prediction from the nearby Orwell Observatory [N52d 00.6m E001d 13.8m] and assume that as Astronomers they are spot on for an Observed distance of 0.

    Geoffrey’s graphical method is elegant but it’s a lot of work and requires a high precision Azimuth to work well which requires a lot of calculation. Following Frank’s observation that it’s actually a special case of a Lunar distance I tried calculating with the Stark tables and then using the calculator on reednavigation. This was a good refresher in doing Lunars. The results came out as follows.

    Orwell Observatory

    Stark tables

    reednavigation site corrected

    reednavigation site uncorrected


    Time 214949





    Time 224824




    Immersion seems all very well but the difference in the results for emmersion is interesting. Having gone through the Stark tables again there is no input for Latitude so he can’t allow for the Earth’s oblateness and it is this that seems to make the difference on the reednavigation site. Obviously rounding errors can be significant in this sort of comparison. I’ll try to do some higher precision calculations to dig deeper as it seems unlikely that Oblateness would make that much difference.  Stark does mention problems at short distances but this does not seem to an issue here.

    I wondered about  the method that Bruce Stark used? Looking through the archive I found that Fred Hebard had reverse engineered it. Here’s the post


    My first thought was that this would be a quick, high precision method of finding Longitude with only a stopwatch, it wasn’t but has been a very good learning exercise, As a method it’s not much use since I still don’t know my Longitude from it given the September and October apparitions have been cloudy. Good fun though.


    Bob C

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