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    Re: William Bligh's navigation and mapping
    From: Don Seltzer
    Date: 2016 Aug 26, 15:42 -0400

    On Fri, Aug 26, 2016 at 3:03 PM, Frank Reed <NoReply_FrankReed@fer3.com> wrote:

    ...The navigation in the boat after the mutiny would have been essentially identical to the navigation of Joshua Slocum a century later. Bligh used dead reckoning for longitude, and Noon Sun sights for latitude.

    Dead reckoning in higher latitudes involves the mathematical complexity of dealing with the differing sizes of latitude and longitude degrees, but in the tropics where Bligh and his compatriots were sailing, it's simple: longitude and latitude are just like x and y on a basic graph. You work out how many miles you've travelled west and how many miles north and you add (or subtract as appropriate) directly from the previous day's latitude and longitude. Dead reckoning usually requires a simple timepiece to keep track of the number of hours spent on each heading, if there is any change of heading. Bligh, like Slocum, had a simple watch. I've also seen speculation that Bligh could have kept track of elapsed time using time sights for local time. I consider that an improbable fantasy.

    This can be seen in Bligh's notebook, available online at http://www.fatefulvoyage.com/notebook/index.html

    On May 5, a log line was fashioned.  On May 7, Bligh began keeping a regular log with hourly entries of course and speed, with typically an observed latitude entered on the bottom.  The pages labelled Figs show the math that Bligh used for dead reckoning.  I believe that in a few cases he obtained a longitude from passing a known island listed in his copy of Moore.

    Don Seltzer
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