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It is used to determine longitude by comparing Greenwich Mean Time (GMT), and the time at the current location found from observations of celestial bodies. When first developed in the 18th century, it was a major technical achievement, as accurate knowledge of the time over a long sea voyage was vital for effective navigation , lacking ...
John Harrison (3 April [O.S. 24 March] 1693 – 24 March 1776) was an English carpenter and clockmaker who invented the marine chronometer, a long-sought-after device for solving the problem of how to calculate longitude while at sea.
Under the system time changes are required for changes of longitude in one-hour steps. The one-hour step corresponds to a time zone width of 15° longitude. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7°30′ gores that differ from GMT by ±12 hours. A nautical date line ...
It includes time for breaks and can be used to calculate how long it may take to complete a task, including required recovery times from physically strenuous work, as well as legally required breaks or other human interactions. If it includes 15-minute breaks, it is written as (PSH15). A related concept is productive machine hours (PMH). [6]
To determine "longitude by chronometer," a navigator requires a chronometer set to the local time at the Prime Meridian. Local time at the Prime Meridian has historically been called Greenwich Mean Time (GMT), but now, due to international sensitivities, has been renamed as Coordinated Universal Time (UTC), and is known colloquially as "zulu time".
When the time card hit a contact at the rear of the slot, the machine would print day and time information (a timestamp) on the card. One or more time cards could serve as a timesheet or provide the data to fill one. This allowed a timekeeper to have an official record of the hours an employee worked to calculate the pay owed an employee.
Clock on The Exchange, Bristol, showing two minute hands, one for London time and one for Bristol time (GMT minus 11 minutes).. Railway time was the standardised time arrangement first applied by the Great Western Railway in England in November 1840, the first recorded occasion when different local mean times were synchronised and a single standard time applied.
Accurate knowledge of time of day is essential for navigation, and ships carried the most accurate marine chronometers available, although they did not keep perfect time. A number of accurate audible or visible time signals were established in many seaport cities to enable navigators to set their chronometers.
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