A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2016 Nov 28, 23:06 -0500
Longitude in that era was "calculated" by dead reckoning. Fundamentally, it's vector analysis, in which the new "calculated" longitude is based on the old longitude plus the sum of the vectors of the days sailing. Even one day's estimate of vector sums can be off due to currents, angular direction errors and all sorts of other factors. Whilst somewhat better than a WAG, It suffers from lack of accuracy. Needless to say, the accumulation of error over days and weeks in such "calculation" can be rather large.
Not to say it can't be done, but reliably and repeatedly? Nope!
I have a copy of the logbook from a French man-of-war my ancestor was aboard in 1687. The captain of the ship Arc-en-Ciel recorded both latitude and longitude in the journal. Latitude gave a fairly good reckoning in those days, but the decription of longitude confuses me.
1. How was longitude calculated on board a ship in 1687?
2. Here are typical longitude readings leaving from La Rochelle France heading for Canada. . Day 1: 12°45', Day 2: 9°59', Day 3: 8°5',. . . . . Day 8: 2°35', Day 10: 1°9', Day 11: 359°49', Day 12: 358°31' . . . . . . Day 20:341°55' . . . . Day 30: 320° 28' etc.
When I use latitude and leagues travelled (according to the logbook), I can estimate a longitude, but the original longitude numbers in the logbook are confusing.
Can anyone help me out in understanding both how it might have been calculated and what those longitude numbers written in the logbook represent.?
Thanks for your help - George in Toronto Canada