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    Re: Latitude by two stars ...on a Party Boat
    From: Antoine Couëtte
    Date: 2018 Jan 10, 03:31 -0800

    Oops ! I forgot to include a copy of my old and trusty 1981 Ephémérides Nautiques Mercator Chart yesterday.

    • From Frank's picture I eyeballed the Moon position the best I could and I drew Lady Moon slightly "higher" over the apparent horizon in order to account for Parallax. I was able to easily guesstimate reliable Moon Equatorial Coordinates (RA = 08h20m Dec 19°N), hence recovering the Party date from a quick search for the end of January 2018.

    For those interested, I performed my computations with Height of Eye = 17' as specifically indicated by Frank, and also with T = +2°C and P = 1030 mb as a best guess to adequately match Frank's chilly clear sky high pressure environment.

    While I performed my computations yesterday with rounded figures, I ran again these data to fully published figures to-day and got a final result extremely close from my previous one. Here is such refined result:

    • Party Date : 30 Jan 2018 (unchanged ... good on me !)
    • Stopwatch correction : +18h07m25.5s (vs. 18h07m19s , a totally insignificant 6 second difference), hence the averaged Moon shot observed at Stopwatch 04:12m02.5s occurred at 22h19m28s GMT
    • Observed position : N34°29.9' W064°40.7' (1 NM or so away from the previously published position), which also is a totally insignificant change to our starting course and distance to Bermuda: unchanged course 202° and distance slightly less than 140 Nm to reach the shore since TXKF/BDA Airport (N32°21.8' W064°40.7') is a bit further down from our computed position.
      •  In this specific example it is interesting to see that the 3 plan intersection dives quite fast from outer space towards the Sea, thus enabling a good determination of the time it hit the Earth Surface with a quite reliable Observer's actual position if we consider the Sextant observations to be reliable, which of course they are. And also:
        • I also attempted to work afterwards onto the early uncorrected data published on Dec 27th:
          • After a hypersonic shallow dive towards the Earth Surface the 3 plan intersection makes a huge "many G" pitch up correction somewhere over the South Indian Ocean as I can recall. It almost grazes the Sea Surface down to D=1.0104 (some 65 km of Altitude) or so - but apparently no direct hit - before it soon regains very high altitudes in outer space. Hence I could not identify a solution - at least within the Observer's published environment - which also may explain the difficulties initially encountered by Greg and Bob. One good point though: no need to warn the populations in La Réunion Island, Madagascar or South East Africa.

    And last, but not least, given the close time span (only 7 seconds) between extreme mean observations and with the Observer on a slow Sailboat, we can simplify our search for a solution through assuming all observations to occur at the very same time, e.g. the one for Sirius. The ensuing final result should not be skewed in any appreciable manner. From your long practice of Lunars Frank you have smartly timed your observations.

    And very special thanks for this quite interesting exercise.

    Antoine Couëtte



    File:
    180109-Moon-Approximate-Posi.pdf

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