The Time and Longitude gallery explores two British solutions to the longitude problem. One was based on detailed mapping measurements of the night sky, done here at Greenwich. The other was the development of an accurate, portable clock that worked on board ships – the celebrated Harrison marine timekeepers.

Islamic astrolabe
Repro ID E5555-3Location: Flamsteed House, Royal Observatory (see floor plans). Please note that an entrance charges applies for Flamsteed House - see times and admission.  

Key objects in the gallery

Islamic astrolabe

The Greeks had been great astronomers and much of their knowledge was preserved by the Arabs after the fall of the Greek and Roman Empires. The Arabs developed instruments like the astrolabe to record and measure the positions of the sun, moon and stars. Muslims used them to find prayer times.

A fishing boat off a rocky coast in a storm with a wreck. Repro ID BHC0837. Acquired with the assistance of the Art Fund.

Wreck painting

This Dutch painting dramatises the danger to lives at sea, as people from the sinking fishing boat cling to pieces of wreckage in the water.

Ships that did not know their exact position were often lost when they came too close to rocky coasts. In 1700, skilled seamen still lacked accurate instruments or methods to calculate their east-west position, known as longitude.

Bird sextant, D7080

Bird sextant

This is almost certainly the 20-inch sextant made by John Bird for Captain John Campbell's sea trials of the astronomical method of finding longitude.

As such, it is the first marine sextant ever made. The mirrors on sextants and octants gave much more accurate sightings than was possible on earlier instruments.

John Harrison's Fourth Marine Timekeeper, H4

H4

This is the most important timekeeper ever made. It is the machine that solved the problem of keeping accurate time at sea and finally won Harrison the Longitude Prize.

H4's high-energy watch balance was less affected by the movement of a ship and was the design breakthrough that Harrison needed. He believed his fourth timekeeper was the most perfect and beautiful machine ever constructed.

H4's predecessors H1, H2 and H3 are also on display in the Time galleries.

• Read about John Harrison and the longitude problem
• Listen to John Harrison's clocks gallery favourite talk
• See Harrison's timekeepers in Collections online

Post-Harrison chronometer – Arnold 36
Repro ID D8566

Post-Harrison chronometer – Arnold 36

John Arnold gradually evolved Harrison's ideas into simple, practical designs, making large numbers of timekeepers. Arnold No. 36 was the first timekeeper known as a chronometer.

Pocket chronometers were cheaper but less accurate than the larger machines. They were popular with captains who had to buy their own timekeepers.

Acknowledgements

The Time galleries are supported by: