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On Sunday, August 01, 2004 8:38 PM,
Jim Thompson  asked:
> Why does the Nautical Almanac include data for lower branch moon
> transit? Perhaps the moon gets high enough in the sky so that we
> can observe its lower branch transit often enough to make it
> worthwhile to include in the NA?  Then the navigator could use
> it like an upper transit, to estimate latitude?

The other reason for including the time of the Moon's lower transit is to
allow the navigator to predict tides. Nowadays, daily tidal prediction
tables are printed for all major ports, with offsets for secondary ports
that follow the same pattern. However, it used to be that such detailed
predictions were not widely available. Pilot books would give the
"establishment" of various ports, being the average time between the Moon's
upper or lower meridian transit and the next high tide. For areas with two
tides a day and an uncomplicated monthly tidal pattern, this will suffice.
For places with one-a-day or "disappearing" tides, there would be some
footnotes.

If you have a 19th century coast pilot or something like Blunt's
_Shipmaster's Assistant_, take a look at the port information and you should
find lists of establishments and tidal heights.

 -- Peter

   

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