The Royal Observatory Greenwich: Astronomy, Time And Longitude

Outside the Royal Observatory

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Why does the Prime Meridian, the imaginary line that divides east from west, run through Greenwich? And why does this small town, a few miles from central London, set the standard for all of the world’s timekeeping? The answers to these questions, and more, can be found at the Royal Observatory in Greenwich, a fascinating centre of astronomical history and discovery.

History Of The Royal Observatory Greenwich

In the 17th century European nations were busy exploring new lands and setting up trade routes to distant places. Ships needed reliable charts and navigation, which meant they had to be  able to work out, and document, their exact location. Although the movement of the sun had allowed sailors to calculate their latitude since ancient times, longitude (the distance from a given meridian, or north-south line) was much more complicated. This was linked with the measurement of time, and the only way to solve the puzzle was through astronomical observation.

Outside the Royal Observatory
The buildings of the Royal Observatory in Greenwich

King Charles II set up a commission to try to find ways of measuring time and longitude. As a result he decided to establish a Royal Observatory at Greenwich. He chose this site because the land was already a royal park (Henry VIII had had a palace at Greenwich Castle, although it was now ruined). It had the additional benefit of being on a hill, with clear views of the horizon. John Flamsteed, the first Astronomer Royal, took up post at the new building in 1676.

The observatory remained in this location for 300 years, until light pollution and bomb damage necessitated the relocation of its work elsewhere. The site subsequently reopened as a tourist attraction and education facility.

The Measurement Of Time And Place

The Time and Longitude Gallery in the Royal Observatory tells the story of the search for time and longitude, and the astronomical activity that facilitated it. It also shows how by the 19th century the measurement of time had acquired a new imperative: the coming of the railways necessitated timetables, and local variations in time could no longer be tolerated. An international conference in Washington DC in 1884 agreed that the Prime Meridian (0 degrees longitude) should be in Greenwich, and that this should be the baseline for world time.

The Burgess Clock in the Royal Observatory
The Burgess Clock – one of many historic timepieces

A line on the floor outside the observatory marks the position of the Prime Meridian, a popular spot for visitors to capture pictures of themselves with one foot in the Western Hemisphere, and the other foot in the East. (Although purists should note that seismic movements mean that this is no longer the exact location of the meridian.)

One foot on either side of the Meridian Line
Standing with one foot in each hemisphere

Nearby you can see the Time Ball. Since 1838 this has been hoisted to the top of the mast and dropped at precisely 1pm each day. This was the first public time signal, originally a cue for ships on the Thames to set their clocks accurately.

From 1852 a clock inside the observatory (the Shepherd Clock) sent time signals by telegraph to major British cities, and later extended the signals to the US. Today’s visitors can set their watches by the Shepherd Gate Clock (outside the observatory and close to the entrance), which is linked to the master clock. Note also the yardstick and other measures beneath the clock, a reminder that time is only one of many important measurements.

Astronomy And Observation

None of this discovery of time and place would have been possible without careful astronomical observation and measurement. The Astronomers Royal lived in Flamsteed House, a purpose designed building that combined family accommodation with large windows for watching the sky. As well as being an important part of astronomical history, Flamsteed House is notable for its architecture. It was designed by Sir Christopher Wren, and includes the elegant Octagon Room, with its telescopes, clocks and 13ft high windows.

The Octagon Room in Flamsteed House
The Octagon Room is equipped with telescopes and tall windows

In practice, much observation was done outdoors. The astronomers also used a camera obscura – a darkened room which combined lenses and mirrors to create an image of the outside surroundings – to watch eclipses without damage to the eyes. There is still a camera obscura in the courtyard, and visitors can see a remarkably clear projection of the nearby Royal Naval College (make sure you stay inside the room for a few minutes to allow your eyes to adjust to the darkness).

Circular image in the camera obscura
The camera obscura gives a remarkably clear image

When you leave the Observatory there is a separate area that houses the Altazimuth (built in the 1890s for lunar observation), the Astronomy Centre Galleries and a Planetarium. The Galleries have displays on various subjects including the formation of the universe and modern astronomy, and there are lots of family activities.

Vsiting The Royal Observatory

The nearest stations are Cutty Sark (Docklands Light Railway), and Greenwich or Maze Hill (overland railway). Note that, whichever station you arrive at, there is an uphill walk to the observatory.

Royal Observatory and Octagon Room
Pinnable image of the Royal Observatory and the Octagon Room

You will find it less crowded if you are able to visit outside of weekends or school holidays. Alternatively, early mornings tend to be quieter. You can save time by booking your tickets in advance.

If you are planning to visit the Cutty Sark on the same day it is cheaper to buy a combined ticket. The Astronomy Centre Galleries are free, but you will need a separate ticket for the Planetarium. Food and drink are available at the Astronomy Café.

Thanks to Royal Museums Greenwich for providing me with tickets for the Royal Observatory.

This article is now included in a mobile app. Go to GPSmyCity to download the app for GPS-assisted travel directions to the attractions featured in this article.


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WorldWideWriter is owned and managed by Karen Warren.

I have been writing and travelling for many years (almost 70 countries at the last count), and I’ve visited every continent except Antarctica. This website is my attempt to inform and inspire other travellers, and to share some of the things I’ve discovered along the way. Read more…


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