NavList:

   

Reply

Greg asked-

| To the List,
|   All - when doing a time sight at sea in period say circa 1780,
| I seem to have gleaned "the latitude used was the estimated latitude"?
| I assume this means latitude from a noon sight was applied (with DR
| corrections)to the time sight that was done 2-3 hours before or after
| the noon sight? while they certainly would not have traveled far at
| say 6 to 12 knots some error would have undoubtably crept in due to
| unknown currents etc. and it would not be impossible for the actual
| lattitude to be off by 2-4 miles. How did they compensate for this
| any ideas?

=================

You're right, as is Robert Gainer when he commented-

"I would think that if you don't know the amount of error (drift from unknown 
current) you couldn't build in a correction.  The
unknown is just that, unknown."

It was a limiting factor. But not much of a limiting factor, in practice. 
Remember, in 1780 there were no more than a handful of
experimental (and temperamental) chronometers at sea. Longitude navigation, if 
practiced at all, was done by lunars. Because of the
slow motion of the Moon across the sky, even if a lunar distance could be 
established to 1 arc-minute, and even if the Lunar tables
were accurate (which they weren't), the resulting error in Greenwich time 
would give rise to a longitude error of about 30
arc-minutes. Compared with that, the error in estimating change in latitude 
since noon, which was estimated to be 2-4 miles, was
rather insignificant. If a time-sight could be taken when the Sun was near to 
due East or West (possible in Summer, not Winter) then
errors in estimated latitude had little or no effect.

Peter Fogg commented-

"The major ocean currents are well known, although of varying speed. Tidal
movements around coasts and counter currents (running in the opposite
direction to the main one close to the coast) can be bafflingly
unpredictable."

I think that was somewhat off the mark. The question was about circa 1780, 
when ocean currents were only very imperfectly
understood.. It was mostly by the work of Maury, US navy, in the mid 1800s, 
that navigators became more enlightened about ocean
currents. Peter is right about the tides.

   

Reply