'H4' (left), a marine chronometer completed in 1759 by English clockmaker John Harrison
Measuring latitude, how far you are north or south of the equator, was not a problem in 1675.
However, measuring Longitude accurately, the position east or west, was proving difficult.
We knew in the 17th century that the earth turned 360° every day, or 15° every hour.
Therefore 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.
Using the meridian lines you could see how far you have travelled east or west.
For example, if it was noon at your current location and you knew it to be 9am in Greenwich (0°) you would have sailed west for three hours and you would be at longitude 45°.
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.
John Harrison, a working class clock maker form Yorkshire, solved the problem of longitude by inventing a timepiece that could tell the correct 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.
The Prime Meridian line at the Greenwich Observatory
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 Flamsteed and the Royal Observatory.
Visitors to the Observatory can stand in both the eastern and western hemispheres simultaneously by placing their feet either side of the Prime Meridian.
Time and GMT
Two regulator clocks, built by Thomas Tompion, were used in 1676 by John Flamsteed to chart the position of the stars.
The orange time-ball drops each day at 1pm
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 Flamsteed House where a time-ball would drop at 1pm and then set their own chronometres by it.
This time-ball still operates today.
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.