Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight.

- Greenwich Mean TimeIt played a major role in the history of astronomy and navigation, and because the Prime Meridian passes through it, it gave its name to Greenwich Mean Time, the precursor to today's Coordinated Universal Time (UTC).

- Royal Observatory, Greenwich2 related topics

## Universal Time

ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians

ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians

Starting in 1847, Britain established Greenwich Mean Time, the mean solar time on the Prime Meridian at Greenwich, England, to solve this problem: all clocks in Britain were set to this time regardless of local solar noon.

Using telescopes, GMT was calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK.

## Marine chronometer

Precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation.

Precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation.

It is used to determine longitude by comparing Greenwich Mean Time (GMT), or in the modern world its successor Coordinated Universal Time (UTC), and the time at the current location found from observations of celestial bodies.

It was common for ships at the time to observe a time ball, such as the one at the Royal Observatory, Greenwich, to check their chronometers before departing on a long voyage.