On Wed, 1 Nov 2017 11:06:44 +0000, Martin Brown
wrote:

Until there was a reliable means to keep accurate mechanical time on
board ships observations of the sun were useless for determining
longitude...

It is interesting to read about the methods used by Lewis and Clark in
their journey across North America at the beginning of the 19th
century. The did have a chronometer, but had to reset it from time to
time. They did this using complex astronomical observations- the
distance between the Sun and Moon, or the positions of Jupiter's
moons.