It is very easy to determine the local time at any spot by observations made at that spot.

But, as time is always changing, the knowledge of the local time gives no idea of the actual position; and still less of a moving object--say, of a ship at sea.

But if, in any locality, we know the local time, and also the local time of some other locality at that moment--say, of the Observatory at Greenwich we can, by comparing the two local times, determine the difference of local times, or, what is the same thing, the difference of longitude between the two places.

It was necessary therefore for the navigator to be in possession of a first-rate watch or chronometer, to enable him to determine accurately the position of his ship at sea, as respected the longitude.
Before the middle of the eighteenth century good watches were comparatively unknown.

The navigator mainly relied, for his approximate longitude, upon his Dead Reckoning, without any observation of the heavenly bodies.