UK Specialist in the Service and Restoration of Marine Chronometers
The marine chronometer was developed out of the necessity to have a timekeeper of great accuracy that would function at sea. The need for this was to enable the ships navigator to find Longitude out of the sight of land and thus his position on the seas and oceans of the world. Latitude is the imaginary line that runs parallel to the Equator, (the fixed datum) north and south 0-90 degrees in each direction to the Poles. This has always been relatively simple to find by means of celestial observations and was used by Columbus. Longitude is also measured in degrees 0-360 running from Pole to Pole. One hour is equal to 15 degrees thus 1 degree is equal to 4 minutes.
The Earth revolves from West to East (no fixed datum) and therefore each of these degrees will become successively opposite the Sun, (the sun highest in the sky) which we call Noon. So for every degree we travel East Noon will be 4 minutes earlier and travelling West Noon will be 4 minutes later. We know we can find Noon locally when the Sun reaches its highest point in the sky, (Latitude) if at that instant we know what time it is at the point of our departure, we can calculate how many degrees of Longitude we have travelled East or West. With the other co-ordinate of Latitude this gives us our fixed position.
One degree of Longitude is equal to 60 nautical miles at the Equator this diminishes as the lines converge towards the Poles. On a voyage of 90 days a time-keeper that has an uneven Rate (I will explain Rate late) of 4 seconds a day would be 6 minutes less or more than G.M.T. This equates to being a possible 60 to 90 miles away from your calculated position. In 1714 a prize of £20,000 was offered by The board of Longitude if Longitude could be found to within 30 geographical miles. It was not until 1761 that John Harrison collected the prize and then he did not receive the whole sum until 1773 when King George III intervened personally on his behalf. Harrison spent the majority of his working life dedicated to perfecting an instrument to find Longitude.
In October 1761 H.M.S. Deptford set sail for Jamaica and after a six week's voyage 'H.4'. was found to be 5 seconds slow of G.M.T. i.e. less than one geographical mile at that Latitude. In 1764 'H.4'. was given a second trial to Barbados and after a seven week's voyage it was 38 seconds fast which equates to 9.6 geographical miles. Greenwich (the observatory) is the point at which 0º Longitude passes through from the North to the South Pole. It is the recognized international point or Time G.M.T. to which all navigational instruments are set.
Every timepiece will have a rate, in other words it will gain or loose time indicated each day. This rate is very important to know in a Marine Chronometer as it must be added or subtracted to the navigation calculations. If an instrument gains 4 seconds a day exactly every day it would be an excellent timekeeper, as after 100 days it would be 400 seconds fast of G.M.T. a known fact that can be included in any calculation. An instrument that gains 0 second for two days then loses 2 seconds on the third, gains 3 on the fourth is unpredictable and after 100 days could be anywhere with regards to G.M.T.
The reality is that Marine Chronometers have to be set to a compromise of the best performance (steady rate) through a range of temperatures.