An observation of the angle between the edge of the moon or sun, or a planet, in order to find Greenwich Mean Time (GMT) at sea and so determine the longitude. The moon has a relatively rapid motion across the heavens in relation to the fixed stars, so that the angular distance between the moon and any fixed star that lies in the moon's path changes comparatively rapidly. The earlier nautical almanacs provided the navigator with tables of predicted lunar distances (the angles between the moon and certain fixed stars, as well as the sun) against GMT. By observing a lunar distance it was possible to ascertain, by interpolation between tabulated values of lunar distances against GMT, the precise GMT of the observation. By comparing this with the local time of the observation, the longitude of the ship could be ascertained. The introduction of the chronometer, bringing GMT permanently on board, made lunar observations no longer necessary for the accurate determination of longitude. See also board of longitude.
Subjects: Maritime History.