The chronometer is an accurate time keeping instrument carried on board. It always shows the GMT.
Use of Chronometer Onboard
The primary use of a chronometer on board of a ship is for Astro-Navigation. Sun is the most commonly used heavenly body for this purpose. The altitude of the Sun changes rapidly and an error of four seconds can create an error of one nautical mile in the observed position.
Accurate time is also essential for running a ships routine and a chronometer provides such accuracy.Currently the GPS is used for both these purposes, but as an alternate measure, a chronometer is still used.
Types of Chronometers
- The Spring Tension Chronometer (winding Type): this chronometer employed a balance wheel and a spiral spring for regulation, instead of a pendulum
- 2-day variety
- 8-day variety (So named because of the number of days it would work after winding once. The former is more popular on merchant ships.)
- Electronic Quartz crystal chronometer: Quartz Crystal Chronometer is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than mechanical clocks.
Errors of chronometer
- No chronometer is expected to keep perfect time.
- But if a chronometer gains or loses the same amount each day (i.e., if it has a constant daily rate),
- Daily rate may change due to temperature variation, vibration, shock, magnetic influence, irregular winding, age, etc.
- It is suitable for navigation because the precise value of the chronometer error at any instant can be obtained by simple extrapolation.
- The chronometer error is noted each day at sea by means of radio time signals and the error noted in the ‘chronometer error book‘
- A chronometer is landed ashore for repairs or overhaul when
- Its daily rate is erratic
- Or the daily rate of a spring tension chronometer exceeds 6 seconds.
History of chronometer
A marine chronometer was a high tech product of its era which can be compared to other landmark inventions like the telegraph, steel making, railways, steamships and so forth. The chronometer was the life work of one man, John Harrison, spanning 31 years of persistent trial and error that revolutionized naval (and later aerial) navigation. The British beat France, Spain, Germany, Holland and Belgium as colonisers of the world because of superiority of the Royal Navy over all other navies. Royal Navy proved to be superior to other navies of the era because they always had a technological edge over the others. The Chronometer was one such invention. British fleets had the surety of navigation given by the chronometer, and their Portuguese, Dutch, and French opponents did not.
Until the mid 1750s, navigation at sea was an unsolved problem due to the difficulty in calculating longitudinal position. Navigators could determine their latitude by measuring the sun’s angle at noon (i.e., when it reached its highest point in the sky). To find their longitude, however, they needed a portable time standard that would work aboard a ship. Chronometer provided such time. By comparing local high noon to the chronometer’s time, a navigator could use the time difference to determine the ship’s present longitude. Since the Earth rotates 360 degrees every day (that is, 24 hours or 1,440 minutes), the time difference between the two points reveals how many degrees separate them.
The creation of a seaworthy timepiece was difficult. Until the 20th century, the best timekeepers were pendulum clocks, but the rolling of a ship at sea rendered the ordinary, gravity-based pendulum useless. John Harrison, a Yorkshire carpenter, invented a clock based on a pair of counter-oscillating weighted beams connected by springs whose motion was not influenced by gravity or the motion of a ship. In 1761 Harrison submitted his invention for the £20,000 longitude prize that had been offered by the British government in 1714. By 1825, the British Navy had begun routinely supplying its vessels with chronometers. Till the 2nd World War, chronometer manufacture was hand made and craft based. With the onset of the war, the Hamilton Watch Company in the US perfected the process of mass production, which enabled them to produce thousands of their superb Hamilton Model 21 chronometers for the US Navy and other Allied navies. Despite Hamilton’s success, chronometers made in the old way never disappeared from the marketplace during the era of mechanical timekeepers. Mercer, in St. Albans, England, for instance, continued to produce high-quality chronometers by traditional production methods well into the 1970s.
The most complete international collection of marine chronometers, including Harrison’s H1 to H4, is at the National Maritime Museum, Greenwich, England.