Nautical Time - Nautical Day

Nautical Day

Up to late 1805 the Royal Navy used three days: nautical, civil (or "natural"), and astronomical. A nautical day entered in a ship's log as 10 July, for example, in fact commenced at noon on 9 July civil reckoning, PM therefore coming before AM. The astronomical day of 10 July, on the other hand, commenced at noon of 10 July civil reckoning, and ended at noon on 11 July. The astronomical day was brought into use following the introduction of the Nautical Almanac in 1767, and the British Admiralty issued an order ending the use of the nautical day on 11 October 1805. The US did not follow suit until 1848, while many foreign vessels carried on using it until the 1880s.

Prior to 1920, all ships kept local apparent time on the high seas by setting their clocks at night or at the morning sight so that, given the ship's speed and direction, it would be 12 o'clock when the sun crossed the ship's meridian. The local apparent noon is 12 noon.