# How the problem of measuring longitude was solved with Harrison's Clock

Measuring latitude, how far you are north or south of the equator, was not a problem in 1675 because it could be calculated from the altitude of the sun at noon. Longitude, the position east or west, was still not accurate but was theoretically possible to measure in terms of time.

We knew in the 17th century that the earth turns 360° every day, or 15° every hour. So if you travelled 15° eastward, the local time moves one hour ahead and similarly, travelling West, the local time moves back one hour. The imaginary lines drawn on a globe representing each 15° are called the meridian lines.

Therefore, using the meridian lines you could see how far you have travelled east or west.

For example, if it was noon where you were and you knew it to be 3pm in Greenwich (0°) you would have sailed west for 3 X 15º and be at longitude 45°W. If it was noon where you were and you knew it to be 9am in Greenwich you would have sailed east to be at longitude 45°E.

You know from using a compass if you have travelled east or west but knowing the exact time back in Greenwich was the problem.

How do you know if you have travelled 2 hours 59 minutes or 3 hours 02 minutes?

Getting a clock to work accurately at sea was very difficult - just a few seconds too fast or too slow could result in the ship ending up miles off course. The difficulty was producing a clock which could maintain accurate time in widely-varying temperature, pressure and humidity.

If such a clock were built and set at noon in London at the start of a voyage, it would subsequently always tell you how far from noon it was in London at that second, regardless of where you had traveled. By referring to the clock when it is noon locally (i.e. the sun is at its highest in the sky where you are) you can read, almost directly from the clock face how far around the world you are from London. For instance, if the clock shows that it is midnight in London when it is noon locally, then you are half way round the world, (i.e. 180 degrees of longitude) from London.

John Harrison, a working class clock maker form Yorkshire, solved the problem of longitude by inventing a timepiece that could tell the right time at sea.

His chronometer, H4, built in 1759 after years of experimentation, was the first marine timekeeper accurate enough to be used with confidence. H4 eventually won John Harrison the Longitude prize from the British Government.

Prime Meridian Line

The other problem was noon for one part of the world was not noon for another.

We needed to have a prime meridian from where all longitude could be calculated. In 1884 a conference in Washington of 25 nations agreed that Greenwich would be the world's Prime Meridian of world time and time zones.

Every place on the Earth is measured in terms of its distance east or west from this line. The line itself divides the eastern and western hemispheres of the Earth - just as the Equator divides the northern and southern hemispheres.

Up until 1884 a majority of the world used Greenwich to set their clocks by. Although it did not influence the decision it seemed a fitting tribute to the hard work of John Flamstead and the Royal Observatory.

Time and GMT

Two regulator clocks, built by Thomas Tompion, were used in 1676 by John Flamstead to chart the position of the stars.

One of these clocks can be seen today at the Observatory along with John Harrison's clocks. Scientific clocks were later used and the Observatory became a testing centre for new timepieces.

'Greenwich Time' was used by ships in London's docks from 1833.

They would look at Flamstead House where a time-ball would drop at 1pm and then set their own chronometres by it.

Later the railways and the electric telegraph would also use Greenwich Time. Time signals would be transmitted by telegraph in August 1852 using the new Shepherd electric clock.

This 24 hour public clock still works today and can be seen set in the outside wall.

The Greenwich Meridian (Prime Meridian or Longitude 0°) marks the starting point of every time zone in the World.

GMT stands for Greenwich Mean (or Meridian) Time and is the average time that the earth takes to rotate from noon-to-noon. It is an average because at certain times of the year the Earth's orbit around the Sun is not perfectly circular.

Adaptation and Update Source: BBC London History Greenwich, Longitude, Prime Meridian & GMT