NavList:

David McN, you wrote:
"The method that Brian W describes looks very similar, if not identical, to that described by John Letcher ..."

WARNING WARNING WARNING...
Yes, it looks similar, but it's useless as described. The account that Brian W provided is burdened by irrelevant detail over dLon scaling (the Mercator projection claims another victim), and it is specifically wrong, incorrect regarding the steps required to bring the lunar LOP into agreement with the other sights. Maybe I need to be more blunt??

And just a reminder (as we have discussed quite recently), Letcher seems to have independently invented this method. He was very creative, and he did a fine job explaining it in his book. But it shouldn't properly be counted as "his" method since the same procedure that Letcher described has been proposed by multiple navigators (Chichester, too) and the earliest accounts of the basic concept of acquiring Greenwich time from lunar altitudes date back over two centuries. 

And as another reminder, this trick of determining GMT/UT by a lunar line of position (or more generally a lunar altitude compared with a star or Sun altitude) is almost always inferior to a standard lunar. Standard lunars are more accurate intrinsically, and standard lunars almost always involve less math work. Note the two "almost always" modifiers in that last sentence! The exceptions can be interesting. :)

Frank Reed
My online workshops in LUNARS covering big history, practical history, modern practice, and math, too.
Two session options in November. Details here.