• Dan

Why is the Royal Observatory so important, and why is the 'mean time' even in Greenwich anyway?

Updated: Jan 14, 2019

The Royal Observatory was designed by Christopher Wren in the seventeenth century, at a time when all of Europe was obsessed with one scientific problem – how to navigate at sea.


The discovery of the Americas had opened the oceans up but all those ships once they lost sight of land were essentially sailing blind


This was dangerous, they could hit reefs or run aground

  • It was costly, in that time was lost

  • With all the shipping hugging well-worn naval pathways it was an invitation to pirates.

  • There was the cost of lost opportunities, what if there was a North West Passage to Far East?, what if there was a Great Southern Continent?


Each country in Europe wanted to be the first to solve this problem


Working out how far North or South you were had long been accomplished by measuring the height of the sun over the horizon at midday, or the Pole Star’s height over the horizon in the night time.


The question was working out how far East or West you were; the longitude problem.


The French were already onto this problem, founding the Paris Observatory in 1667. They proposed using the thousands of eclipses of the moons of Jupiter that are visible from Earth as a clock in the sky. Tables of eclipses would enable navigators to compare their location with the time of the eclipse at the Observatory in Paris and thus how far east or west they were of Paris.


The English thought it more sensible to use the position of the moon against the stars, however to do this they would need an accurate map of the night sky.


King Charles II appointed John Flamsteed as his astronomical observator and various locations were mooted as a site for the Royal Observatory; Hyde Park, Chelsea. The idea of Greenwich came from the architect appointed to build it, Sir Christopher Wren. It is not known exactly why, perhaps for clearer skies?


The foundation stone was lain at 3.44 pm on 10th of August 1675 with John Flamsteed casting a mock horoscope for the new building and the next year he moved in prior to a partial eclipse of the Sun.


Over the next one hundred years much work was done at Greenwich mapping the Heavens and working out the exact time at Greenwich. However the brown brick buildings to your left are where most of the observations took place, Christopher Wren’s red-brick building being built more for ornamentation and show than as a practical observatory.


Much of this was accomplished with the Royal Observatory’s publication of tables of the celestial bodies’ locations and the mathematical formulas for the navigators to work out how far east or west they were of Greenwich. Greenwich was thus the prime meridian.


From the late eighteenth century Greenwich came to be taken as zero longitude on maps, whether they were British or not.


The red bobble on top of the Royal Observatory would drop every day at one o’clock, being on a high hill above the docks of London it allowed sea captains below look up and set their clocks to Greenwich mean time.


The International Geographical Congress at Antwerp in 1871 saw the first move to establish Greenwich as the internationally accepted prime meridian. By that time about ¾ of the world’s floating commerce were using Greenwich as their prime meridian.


Not everyone agreed it should be Greenwich. The Azores and Tenerife, the Baring Strait, the Pyramids and Jerusalem were also proposed as well as observatories in Paris, Berlin and Washington. The French offered to accept Greenwich only if Britain accepted the metric system.


But countries slowly fell into line (although France stubbornly held out until 1978).






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