Date: October 21, 2009

Title: 125 Years of the Prime Meridian at Greenwich


Podcaster: Marek Kukula

Organization: The Royal Observatory, Greenwich

Description: In October 1884, 41 delegates from 25 countries met in Washington, DC, for the International Meridian Conference. By the end of the month, after heated debate, the meridian line defined by the Airy Transit Circle telescope at the Royal Observatory in Greenwich had been selected as longitude 0 degrees. We look back on 125 years of the Prime Meridian to find out how a telescope defined the co-ordinate system for the whole world.

Bio: The Royal Observatory, Greenwich was founded by King Charles II in 1675. Now part of the UK’s National Maritime Museum the observatory site is open daily to the public with exhibitions on the history of astronomy and contemporary space science, as well as regular shows in the Peter Harrison Planetarium.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Frances Day who wants to thank the Open University in the UK for the amazing Astronomy and Physics courses they provide. Also for giving her a first taste of using a real telescope – even if that did turn her into an addict. Some addictions are good!


365 Days of Astronomy
125 Years of the prime Meridian – transcript

Marek Kukula: Hello, I’m Marek Kukula, Public Astronomer at the Royal Observatory, Greenwich. The Observatory is known around the world as the home of Greenwich Mean Time and of zero degrees longitude and this month marks the 125th anniversary of Greenwich being chosen as the site of the Prime Meridian. But why Greenwich and what on Earth does longitude have to do with astronomy?

With me is Mike Dryland, tour guide at the Observatory. Mike, we’re currently standing in the main courtyard of the Observatory, with Christopher Wren’s original buildings to the side and a view North towards the River Thames and Canary Wharf. A meridian is basically a North-South line – you can define a meridian anywhere you like – so what does that have to do with an astronomical observatory?

Mike Dryland: Well Marek, a meridian or north-south line, is what we use to mark position east-west – or longitude – on the Earth, both on land and sea. What’s it got to do with astronomy? Well there’s only one reason why there’s an observatory here in Greenwich and that’s because back in the 17th Century mariners at sea were basically lost. They could find their latitude north or south of the equator but they couldn’t find longitude and lots of voyages were ending in shipwreck.

There’s only one way to find longitude: you have to work out the time difference between where you are and, say, Greenwich. Every hour’s difference is the same as 15 degrees of longitude. Now it’s easy to find local time from the sun but from a bouncing sailing ship in the middle of the ocean how can you find out what the time is at Greenwich?

The obvious answer was to take an accurate clock on the ship and set the clock to Greenwich time before you set sail. The clock would always show you Greenwich time. But the only clocks accurate enough for navigation were pendulum clocks and a pendulum wont work on a bouncing ship. But it took a long time to devise an accurate clock that would work on board a ship.

The other way to find Greenwich time was – guess what? – to use astronomy. You can read the night sky as if it were a huge clock – the Moon is the hand of the clock and the bright stars are the numbers on the face of the clock but of course you need a set of tables, an almanac, to look up the Moon’s position and find the time in Greenwich, but in the 1670s that almanac didn’t exist. That’s why the Royal Observatory was founded in 1675. John Flamsteed, the first Astronomer Royal, began work here to chart the stars accurately and observe the motion of the Moon to be able to print that book of tables, the almanac.

But in 1714 he still hadn’t finished and a prize of £20,000 was offered by the British Government – that’s worth around £5 million in modern values – to the first person who could solve this problem.

MK: So behind us is a long low building that contains many of the observatory’s historic telescopes. In the roof I can see several slots that clearly used to open up to allow telescopes to peer out at the sky. So Mike maybe you can tell us a little bit about the meaning of those slots in the roof?

MD: Most people are aware of this line across the courtyard here – the Prime Meridian of the world or zero degrees longitude, but there are three earlier lines marked here at Greenwich, set up by the first Astronomers Royal.

The building we’re looking at is called the Meridian Building and it’s where much of the work of astronomy was done while this was an active observatory. Inside we’ll find the first Greenwich Meridian which was the place where the first Astronomer Royal, John Flamsteed, set up his measuring equipment, a mural quadrant, a telescope to chart the stars.

About a metre to the east is the second meridian set up by the great Edmund Halley, second Astronomer Royal, of comet fame. Flamsteed’s equipment had been sold when he died and Halley had to move the measuring position when he installed replacements because Flamsteed had built too close to the hill and the ground was subsiding there.

When Halley died the third Astronomer Royal James Bradley wanted to install much better equipment for higher precision measurements and he moved the meridian about another three metres east.

The astronomers worked at Bradley’s Meridian for a hundred years until 1851. While they were there a clockmaker from Lincolnshire walked off with the £20,000 prize. His name was John Harrison and his wonderful marine clocks are still on display here at the Observatory – wonderful things to see.

Also, while they were working at Bradley’s Meridian people started making the Ordnance Survey – the very accurate maps of Britain. And the Ordnance Survey used Bradley’s meridian as their zero longitude. But when the Victorian Astronomer Royal, George Airy, arrived he found a lot of the equipment was worn out. He got new equipment and had a new measuring position built at what we call Airy’s Meridian or the 1851 meridian, about 3 or 4 metres to the east.

MK: We’re now standing inside the Meridian Building by George Airy’s telescope from the 1850s. Can you tell us a little bit more about this instrument?

MD: Well this is a serious astronomical telescope: eleven foot focus and ten inch objective lens. You can see asteroids through this. But why was it important? Well, this was the main observing position in Greenwich in 1884. If you’d bought a map or chart before 1884, where you’d find the zero point on the map would depend on where the map was made. British charts used Greenwich but French charts used Paris and some others used Cadiz and other places.

By 1884 this was very confusing if you wanted to compare positions and also if you wanted to compare times very accurately – when you talk about the time you need to say which meridian you’re measuring from. It would be a lot easier if there was just one place in the world which everyone would use for their prime meridian, their reference meridian, zero degrees.

In 1884 the US Government was persuaded to host a conference in Washington, DC, to see if they could get agreement to that. 25 nations sent delegates and after a lot of debate they did vote for Greenwich to be the Prime Meridian of the world. But the British Government had to agree to introduce the metric system into Britain and in 1884 they promised to do that – and as you know we’re almost there. All we’ve got to do is sort out road distances, milk and beer and we’ll be fine!

So Greenwich is now home to the Prime Meridian of the world, zero degrees longitude. To commemorate the new millennium in the year 2000 we mounted a laser beam which you can see at night shining north up the meridian. Over a million people visit each year to stand with a foot in each hemisphere and take their photo. Of course the line stretches all the way from the North Pole to the South, but there’s something special about this bit!

MK: We’ve moved about a hundred and two metres east of the Observatory into the middle of Greenwich Park and this brings us to the latest chapter in the story of meridians here in Greenwich because we’re now standing on the meridian defined by the satellites of the Global Positioning System. Now if you want to find out about that I’m afraid you’ll have to come to the Observatory, but Mike you and the other guides will be giving special Meridian Tours of the site later this month won’t you?

MD: Yes – to commemorate the 125th anniversary of the meridian conference the guides team will be doing tours daily from October 24th to October 30th so if anybody wants to know more about it we’d be delighted to see them and we’ll be offering tours hourly during that week.

MK: Mike, thank you very much. And if you want to know more about the Observatory and our events for the International Year of Astronomy check out the Observatory website. Goodbye.

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.

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