  Longitude determination at sea gained increasing commercial importance in the
late Middle Ages, spawned by a commensurate increase in long-distance merchant
shipping activity. Prior to the successful development of an accurate marine
timepiece in the late-eighteenth century, marine navigators relied
predominantly on the Moon for their time and longitude determinations. Lunar
eclipses had been used for relative position determinations since Antiquity,
but their rare occurrences precludes their routine use as reliable way markers.
Measuring lunar distances, using the projected positions on the sky of the Moon
and bright reference objects--the Sun or one or more bright stars--became the
method of choice. It gained in profile and importance through the British Board
of Longitude's endorsement in 1765 of the establishment of a Nautical Almanac.
Numerous 'projectors' jumped onto the bandwagon, leading to a proliferation of
lunar ephemeris tables. Chronometers became both more affordable and more
commonplace by the mid-nineteenth century, signaling the beginning of the end
for the lunar distance method as a means to determine one's longitude at sea.
