1. The chronometer method for determining longitude became practical with the successful sea trials of John Harrison's H4 chronometer on two sea voyages in 1761–62 and 1764. By comparing local time determined by celestial observations (see ref. 3) with port time maintained by the chronometer, the difference in longitude between the port and the ship could be calculated. Maskelyne's publication in 1766 of The nautical almanac and astronomical ephemeris for the year 1767 made possible the rival method of lunar distances, which determined time by observing the position of the Moon relative to certain nearby stars. The chronometer method was simpler, but often both methods were used as a check on each other. A case in point is Sir John Herschel's voyage to the Cape of Good Hope, described in his diary entry for Friday, 20 December 1833: “The Captn in a great puzzle about his Lunars. All hands at work taking obsns and at last as a final result the Lunars of yesterday and today give −27s for the Chronometer error while all Chronomrs make it +2m 21s. Bad work this. But a Lunar at Sea seems rather a bungling business.” The next day Herschel reported that “by plenty of patience and using masses of obsns “they came to a satisfactory conclusion, with the lunars and chronometers tallying within one-half minute of time or about 8 miles of longitude. Evans David S. (eds), Herschel at the Cape: Diaries and correspondence of Sir John Herschel, 1834–38 (Austin and London, 1969), 16–17.
2. The calculation was made as follows: If on 27 May at 9h a.m. the chronometer was slow 2h7m18s and on 3 June at 5h p.m. the chronometer was slow 2h6m51s then the difference in the error in 7d8h was 0 0 27s or, a daily rate of 3·7 seconds, gaining. The navigator would use this new rate for determining his longitude until another rate could be obtained. See Raper Henry, The practice of navigation and nautical astronomy (9th edn, London, 1866), 275. Note that the observation had to be made at the same meridian, or the rate could not be disentangled from a change in longitude of the ship.
3. Howse Derek, Greenwich Observatory: Origins and early history, 1635-1835 (London, 1975), 150
4. Gould R. T., The marine chronometer: Its history and development (London, 1923), 253.