A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Lars Bergman
Date: 2016 Dec 1, 09:15 -0800
Regarding the confusing longitudes my guess is that they are reckoned eastwards from the meridian of Ferro (El Hierro).
The meridian of Ferro was 20°W of the meridian of Paris. Paris is 2°20'E of Greenwich, thus Ferro is 17°40'W of Gwch. La Rochelle is 1°10'W of Gwch, or 16°30'E of Ferro. The longitude of day 1, 12°45' (E of Ferro, I suppose) indicates a westerly longitude shift of 3°45' from La Rochelle. Assuming a westerly course, that corresponds to a distance of 156 miles (nautical) sailed (at latitude 46°). If they sailed for say 26 hours (anchors aweigh two hours before noon of day 0), this corresponds to an average speed of 6 knots, which seems reasonable. The longitude of day 30, 320°28'E of Ferro, corresponds to 39°32'W of Ferro; or 57°12'W of Gwch.
Lars, 59°N 18°E (of Gwch)