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Visit the Greenwich Meridian Line, London – Mean Time in the City

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London is the home of Greenwich Mean Time and the Greenwich Meridian Line your chance to time travel by nanoseconds. Well, kind of.

In the Southeast of London, you can straddle two hemispheres at the same time and stand upon the official centre of time. East is east and west is west, but the twain meets at The Royal Observatory in Greenwich – home of Greenwich Mean Time (GMT) and the Prime Meridian of the world and known in technical terms as Longitude 0º. It’s one of the core attractions in this Royal Borough, full of maritime history, art, architecture and adventure. 

A bronze line embedded into the pavement represents the division of the eastern and western hemispheres of the Earth and from this everywhere on the planet is measured according to its distance east or west of this ‘line’, just as the Equator divides the northern and southern hemispheres.

Visit the Greenwich Prime Meridian Line in London – Time Travel in the City

How to Find the Greenwich Meridian Line

Greenwich mean time  – how to officially stand in two times zones for free, what is prime meridian and why is it in london, book tickets, see more of greenwich.

Getting to the Greenwich Prime Meridian Line is easy, aside from needing a little stamina. From the Greenwich DLR station, the Royal Observatory is signposted and only a 10-15 minute walk, the majority of which is up a picturesque hill.

You will find the Prime Meridian Line instantly by the droves of people queuing for their photo, so be prepared to wait patiently while people get their snaps in various poses as they strategically place arms and legs accordingly in East and West.

The observatory even has pictures showing you how to stand on the line properly to ensure you place a foot on either side of the line.

For budget travellers and the more frugal amongst us, there is a very small section of shiny bronze line goodness you can stand on and pose for free. It’s behind a black iron gate just next to the enclosed area where the long line is.

Wall display featuring the location of he Greenwich Mean Time Prime Meridian Line.

How Much Does it Cost to Visit the Greenwich Meridian Line?

The pleasure of the bigger, more official Greenwich Meridian Line option comes at a price of £16. However, this cost is for the overall entry to The Royal Observatory’s historic Flamsteed House, Time Galleries, Meridian Line Courtyard and Meridian Building.

Inside the Royal Observatory, you can see how the discovery of GMT came about, with the display of clocks designed by John Harrison. Latitude was already measurable by the distance from the equator, and in a competition calling for an invention to solve the “longitude” problem for seafarers, these clocks brilliantly found the solution. 

Luckily the gating surrounding the larger Meridian Line has large spaces big enough to look through, stretch both arms in and take a photo. So, really, you get the best of both worlds, at no cost at all.

So why is Greenwich the Prime Meridian? It is said that before the 19th century, all towns kept their own record of time, which was probably a great excuse if you were running late for a meeting.

But then came the rise of infrastructure like public transport and so the world needed a standard measurement of international time.

And so the Greenwich Meridian was chosen as the Prime Meridian of the World in 1884, decided by 41 people from 25 nations.

Also, by the late 19th century 72% of the world’s commerce depended on sea charts which used Greenwich as the Prime Meridian, therefore choosing this location would benefit a lot of people. 

So, on your next visit to the Capital, why not come and stand at the centre of world time.

Things to Know About Visiting the Greenwich Meridian Line in London

• Opening Times: 10.00am – 17.00pm. Last admission 30 minutes before closing.
• Cost: £16 for adults. Entry for children under 16 is free. The £16 entry is an annual pass (visitors are able to return as many times as they like within 12 months for no additional charge).
• Admission to the Royal Observatory’s Astronomy Centre which houses three state-of-the-art modern astronomy galleries is free.
• Planetarium London Ticket Prices: £8 for adults, £4.50 for children and concessions.

For pre-booked tickets to ensure peace of mind, better planning and to combine sights, the respected tour platform, Get Your Guide, features various options for the Royal Observatory and Greenwich Meridian Line tours and tickets, such as:  

• The Royal Observatory Greenwich Audio Guide Tour is for those wanting extra insight as they wander around. 
• An easy-to-use and sightseeing favourite hop-on hop-off Thames Cruise , that includes a Royal Observatory ticket.
• A Day Explorer ticket , which includes the Royal Observatory and Cutty Sark. This is also a chance to visit the National Maritime Museum and the Queen’s House which are both located within a short walking distance from the Cutty Sark and are free to visit.

About Becki

Becki Enright is a British Travel Press Award-winning writer whose work focuses on changing perceptions about misunderstood aspects of destinations. Her writing combines storytelling with insight into the social, historical, political and economic factors that shape the country or place in relation to tourism. Becki has appeared live on Sky News and CNN and has contributed to high profile media including National Geographic, Time.com, Guardian online, New York Times, Grazia and Buzzfeed.

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The Meridian Line

What is the meridian line?

The meridian line in Greenwich represents the Prime Meridian of the world, Longitude Zero (0° 0' 0"). Every place on the Earth is measured in terms of its angle east or west from this line.

Since 1884, the Prime Meridian has served as the reference point for Greenwich Mean Time (GMT).

The line runs across the courtyard of the Royal Observatory and was adopted by international agreement to the irritation of the French who continued to use the Paris meridian.

Find out more about the Prime Meridian .

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Visiting the Greenwich Observatory: Time to Learn About Time

You may not have heard of the Greenwich Royal Observatory, but the research done there impacts your life on a daily basis.  It’s the home of the Prime Meridian and Greenwich Mean Time (GMT).  This topic had never really piqued my interest before, but being at the observatory and learning about the history was fascinating.  And kids will love the planetarium.  

The Observatory is part of Greenwich Maritime which made the very exclusive list of UNESCO World Heritage Sites .  If that is not enough to convince you, the Greenwich Observatory has one of the best views of London.

The Royal Greenwich Observatory needs to be on your list for London.  Let me tell you more about what there is to see there and why you must visit.

Note: This post contains affiliate links.  Please see disclosure for more information.

The Prime Meridian

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I didn’t know much about what to expect at the Greenwich Royal Observatory except that we would see the Prime Meridian. Right after you get your ticket to the London observatory, you will see a sign marking Flamsteed’s Meridian Line on the wall.  This was the first Meridian line and there was even another one before the Prime Meridian Line we use today was established.  

Like everyone else, we had to get our pictures with one foot on either side of the Greenwich Meridian line, standing in both the Eastern and Western Hemispheres.   While the Prime Meridian is supposed to be 0 degrees longitude the 0 degree point for your GPS is about 100 meters east because of the Earth’s slightly irregular shape.

Next, we checked out the camera obscura.  It is a darkened room with a pinhole that uses a mirror and lens to project an image.  Originally it was used to study the sun, but now it projects a live image of the Queen’s House .  The Queen’s House was supposedly a gift from King James I to Queen Anne of Denmark to apologize for swearing in front of her after she had accidentally killed one of his favorite dogs during a hunt.  

Then we visited the Flamsteed House.  This is where the Royal Astronomers and their families lived.  Upstairs is the Octagon Room which was designed by Christopher Wren , the architect that designed St. Paul’s and also the Naval Academy in Greenwich.  I was impressed with the trim and the ceiling. You could also see some original clocks and mock ups of telescopes and accessories.

The exhibits downstairs did a great job explaining why time and the Meridian line are tied to the Observatory.  There was a big problem with shipwrecks because it was hard to know where you were in the sea.   You could determine your latitude by using the location of the sun at noon, but longitude is not as simple and even more difficult when at sea.  You could measure how long you were gone and use that to help you determine how far you had traveled.

However, there were two issues with that method – first, factors like wind and current could affect your speed and second, the clocks used during that time were pendulum ones, which could not keep accurate time on a swaying boat.  One reason the Royal Observatory was established in 1675 was to solve this “Longitude Problem.”   After several attempts, John Harrison created a mechanical clock that would work effectively at sea.

Longitude is connected directly with time because the Earth is divided vertically into time zones.  So you can compare the time at your current location to a fixed point to determine your longitude.  

In 1884, the international community agreed that the Greenwich Meridian (Prime Meridian) would be that point and also established Greenwich Mean Time (GMT).  All time zones would be calculated from GMT.   You will notice the Greenwich Observatory clock on the gate showing GMT, known as the Shepherd Gate Clock, but it is actually controlled by a master clock inside the main building. 

In order to help ensure everyone was keeping to the correct time, in 1833 the Greenwich Royal Observatory implemented a ceremony where a red ball on a pole on top of the Flamsteed House drops to signify 1 pm.  This ceremony still takes place today, as you can see in the video.

A little before 1 pm, we went to the courtyard to watch the Greenwich time ball drop.  Just like clockwork, five minutes before the hour it started to rise up, and then two minutes to the hour reached the top.  Then at 1 pm, it dropped.  

Of course, living in NYC, this reminded me of the ball in Times Square on New Year’s Eve .  I wonder if the idea for that ball came from the Greenwich Royal Observatory?

Inside the Meridian Observatory, you get to see some of the telescopes the Royal Astronomers used.  I was pretty impressed with these until I saw the Great Equatorial Telescope.  You climb up a spiral staircase and then you will see it.  It pretty much takes up the whole dome.  

We timed it perfectly so that we could hear about the telescope and get a demonstration.  The Greenwich Observatory telescope is over 100 years old, yet it is still the best lens telescope in England.  They showed some of the images it captured on tv screens while the presenter showed how the telescope could move.  The most impressive part is that the dome rotates so that you could have the opening wherever it was needed.  As Russell said there was something very James Bondish to it.  

Then it was time for our Planetarium Show – Asteroid: Mission Extreme.  It was open seating and we sat in the middle of the second row.  It might have been better if we were further back but those seats fill up fast.  The seats were almost too comfortable.  The movie was fun, and you definitely feel like you were moving.  After the movie, we checked out the exhibits at the Planetarium, which were all very interactive and kid-friendly.

The Royal Greenwich Observatory offers several different ticket options, but to get the best prices you will want to make your purchase online . 

You can also purchase a combined ticket for the MBNA Thames Clipper and the Greenwich Observatory and arrive by boat.  For those over 60, you can get one third off entry to the main attractions and tours on Tuesdays.  If you plan to visit the Observatory often, you may want to consider becoming a member.

You will need to buy your tickets to the Planetarium shows separately.  These shows are free for members but still need to be book.  The planetarium is popular, so get tickets for the show you want to see in advance.  It’s best to book online as soon as you know you are going to Greenwich or worst case, you can try first thing when you get to the British Royal Observatory as sometimes there are cancellations.

Note: Some of the other attractions in Greenwich, like the Queen’s House and the National Maritime Museum are free!

The Greenwich Observatory is open daily from 10 am to 5 pm.  The last entry is 4:30 pm.  Please note that the Observatory is closed December 24th-26th.

Greenwich is a borough in southeast London that is accessible by DLR (part of the London Underground i.e. Tube) using the Greenwich Cutty Sark stop.  You can also take the MBNA Thames Clipper .  The National Maritime Museum has limited parking available for £15 the day.    You can easily spend a whole day as there are many things to do in Greenwich .

We got lucky with the perfect weather the day we visited, so I enjoyed the walk from our car to the Observatory.  The last bit was pretty steep, but those views are worth it.  

It was a little foggy when we first arrived, but when that cleared later in the day we could see all the way to St. Paul’s.  The grass in this park is also some of the greenest I have ever seen.  We saw lots of picnickers enjoying it.

Yes, I enjoyed my time in Greenwich and my visit to the Royal Greenwich Observatory.  Additionally, I definitely learned some great trivia facts.  Plus, I got my picture with the Prime Meridian – that’s priceless! Both kids and adults will have a good time, and it’s actually easier than you would expect to get to Greenwich from Central London.

Greenwich has so much to see, so I hope to visit again and see the Cutty Sark , the O2 , Queen’s House, and Maritime Museum.  

Have you been to Greenwich?

I would love to hear about your experience.

Pin for Later

• Buy your Royal Observatory tickets online and consider getting one of the package options to save money.
• There is no specific Greenwich Observatory parking, but you can park in the lot for the Maritime Museum.
• An audio guide comes with your admission ticket.  You can pick it up in the courtyard right outside the ticket lobby.
• Be sure to catch the Greenwich time ball drop at 1 pm GMT.
• Try to catch the talk about the Great Equatorial Telescope, so that you can see the demonstration.
• Get your tickets for the Planetarium in advance and try to sit towards the back of the theater.

Disclosure: No financial compensation was received, but the Greenwich Royal Observatory did provide complimentary tickets. As always, opinions expressed here are my own.

This post also contains affiliate links. This means we will receive a small commission for some purchases made using links in our blog with no additional cost to you. Please be assured we would not promote any product unless we believe that our readers will also benefit. The commission does not influence the editorial content of this sit e.

Last Updated on December 7, 2021

About the Author

Anisa  

Anisa is an experienced international traveler with extra pages in her passport and stamps from 41 different countries across 5 continents (and counting). She was born and raised in Texas. After a 13 year stint in NYC, she moved to England to live with her husband.

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The Royal Borough Of Greenwich London

Prime Meridian 0° 0’ 00”

Where Is The Time Zone GMT

The Time Zone for GMT (Greemwich Meantime) is in Greenwich in London, England.   Greenwich in London  has been the home of Greenwich Mean Time (GMT) since 1884.  GMT is sometimes called  Greenwich Meridian Time  because it is measured from the Greenwich Meridian Line which is at the Royal Observatory in Greenwich.  Greenwich is the place from where all time zones are measured across the world.   Read More

Greenwich Mean Time now

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Time zone: Greenwich Mean Time (GMT)

• Greenwich Mean Time is 5 hours ahead of the time in New York when New York is on standard time, and 4 hours ahead of the time in New York when New York is on daylight saving time.
• Greenwich Mean Time does not change between summer time and winter time.
• The IANA time zone identifier for Greenwich Mean Time is GMT.

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Pope opens Asia odyssey in Indonesia to rally Catholics, hail religious tolerance

The Associated Press

Pope Francis, seated in his wheelchair is welcomed as Indonesian Minister of Religious Affairs Yaqut Cholil Qoumas, center left, walks during an official welcoming ceremony at Soekarno-Hatta International Airport in Tangerang on the outskirts of Jakarta, Indonesia, Tuesday, Sept. 3, 2024. Achmad Ibrahim/AP/AP hide caption

JAKARTA, Indonesia — Pope Francis arrived in Indonesia on Tuesday at the start of the longest trip of his pontificate, hoping to encourage its Catholic community and celebrate the tradition of interfaith harmony in a country with the world’s largest Muslim population.

After an overnight flight from Rome, Francis was wheeled off the plane in his wheelchair and onto the tarmac for a welcoming ceremony under Jakarta’s perennial hazy, humid and polluted skies.

Two children wearing traditional clothes handed him a bouquet of vegetables, fruits, spices and flowers.

Francis planned to rest for the remainder of the day, given the rigors of an 11-day voyage zigzagging across time zones that will also take him to Papua New Guinea, East Timor and Singapore. However, the Vatican said the 87-year-old pope would meet later Tuesday with a group of refugees, migrants and sick people at the Vatican residence in Jakarta.

His first full day of activities begins Wednesday with visits to the country’s political leaders and meetings with Indonesian clergy who are helping to fuel the growth of the Catholic Church in Asia.

After 10 years as pope, Francis continues to reshape the Catholic Church

1162954465 Indonesian President Joko Widodo welcomed the pope, saying in a broadcast statement that “Indonesia and the Vatican have the same commitment to fostering peace and brotherhood, as well as ensuring the welfare of humanity.”

The highlight of Francis’ first stop will be his participation Thursday in an interfaith meeting in Jakarta's iconic Istiqlal mosque with representatives of the six religions that are officially recognized in Indonesia: Islam, Buddhism, Confucianism, Hinduism, Catholicism and Protestantism.

The mosque, the largest in Southeast Asia, sits across a piazza from the capital’s main Catholic cathedral, Our Lady of Assumption, and the two are so close to each another that the Muslim call to prayer can be heard during Mass.

Their proximity is not coincidental, but strongly willed as a symbol of religious freedom and tolerance that is enshrined in Indonesia’s Constitution. The buildings are also linked by an underground “Tunnel of Friendship” which Francis will visit with the grand imam, Nasaruddin Umar, before they sign a joint declaration.

While Francis will want to highlight Indonesia's tradition of religious tolerance, the country's image as a moderate Muslim nation has been undermined by flare-ups of intolerance. In 2021, a militant Islamic couple blew themselves up outside a packed Catholic cathedral on Indonesia’s Sulawesi island during a Palm Sunday Mass, injuring at least 20 people.

“We have no problem with the visit. He’s a guest and we will welcome him,” said Eldy, a 64-year-old retired government worker who uses one name and was out walking during a car-free day in Jakarta on Sunday. “He wants to visit our Istiqlal mosque, he can do it.”

Even though Catholics make up only 3% of Indonesia’s population, the sheer number of Indonesians -- 275 million -- makes the archipelago home to the third-largest Christian community in Asia, after the Philippines and China.

Pope Francis is the first pope to back the repeal of laws criminalizing homosexuality

As a result, thousands are expected to throng to Francis’ events this week, which include a Mass on Thursday afternoon at Jakarta’s main stadium expected to draw some 60,000 people. City authorities have urged residents to work from home that day given roadblocks and crowds.

“It is a joy for our country, especially for us Catholics,” said Elisabeth Damanik, a 50-year-old housewife outside a packed Mass on Sunday at Our Lady of the Assumption. “Hopefully the pope’s visit can build religious tolerance in our beloved country of Indonesia.”

Care for the environment, conflict resolution and ethically minded economic development are the major themes for the trip, and Francis may touch on them during his main speech to Indonesian authorities on Wednesday.

Francis has made caring for the environment a hallmark of his pontificate and has often used his foreign visits to press his agenda on the need to care for God’s creation, prevent exploitation of its natural resources and protect poor people who are bearing the brunt of climate extremes and pollution.

In Jakarta, he will find a metropolis of 11.3 million people choking under gray clouds of air pollution caused by coal-fired power plants, vehicle exhaust, trash burning and factories. Jakarta’s air pollution regularly registers eight to nine times above World Health Organization limits.

“Indonesia has the worst air pollution in Southeast Asia,” said Piotr Jakubowski, an air pollution expert and co-founder of Indonesian air quality monitoring company Nafas. “The visit of the pope is great because it provides a sounding board ... from another, very well-respected world leader."

Residents, too, hope Francis will speak out about the issue.

“The pollution in Jakarta is at an alarming level. That’s why the presence of the pope can provide a benefit with the discussion of environmental issues,” said government worker Erik Sebastian Naibaho, 26.

Francis is the third pope to visit Indonesia after Pope Paul VI in 1970 and St. John Paul II in 1989. Their attention underscores Indonesia’s importance to the Vatican both in terms of Christian-Muslim dialogue and Catholic vocations, since it is home to the world’s largest seminary and produces hundreds of priests and other religious workers a year.

“Indonesia is trying to grow in the faith,” said Cardinal Ignatius Suharyo Hardjoatmodjo, the archbishop of Jakarta whom Francis made a cardinal in 2019.

At a briefing last week, he said Francis wanted to express his appreciation for Indonesia’s interfaith tradition “and encourage this kind of brotherhood to continue to be maintained and developed.

• Pope Francis
• Papua New Guinea

Pope opens Asia odyssey in Indonesia to rally Catholics, hail religious tolerance

JAKARTA, Indonesia — Pope Francis arrived in Indonesia on Tuesday at the start of the longest trip of his pontificate, hoping to encourage its Catholic community and celebrate the tradition of interfaith harmony in a country with the world’s largest Muslim population.

After an overnight flight from Rome, Francis was wheeled off the plane in his wheelchair and onto the tarmac for a welcoming ceremony under Jakarta’s perennial hazy, humid and polluted skies.

Two children wearing traditional clothes handed him a bouquet of vegetables, fruits, spices and flowers.

Francis planned to rest for the remainder of the day, given the rigors of an 11-day voyage zigzagging across time zones that will also take him to Papua New Guinea, East Timor and Singapore. However, the Vatican said the 87-year-old pope would meet later Tuesday with a group of refugees, migrants and sick people at the Vatican residence in Jakarta.

His first full day of activities begins Wednesday with visits to the country’s political leaders and meetings with Indonesian clergy who are helping to fuel the growth of the Catholic Church in Asia.

1162954465 Indonesian President Joko Widodo welcomed the pope, saying in a broadcast statement that “Indonesia and the Vatican have the same commitment to fostering peace and brotherhood, as well as ensuring the welfare of humanity.”

The highlight of Francis’ first stop will be his participation Thursday in an interfaith meeting in Jakarta's iconic Istiqlal mosque with representatives of the six religions that are officially recognized in Indonesia: Islam, Buddhism, Confucianism, Hinduism, Catholicism and Protestantism.

The mosque, the largest in Southeast Asia, sits across a piazza from the capital’s main Catholic cathedral, Our Lady of Assumption, and the two are so close to each another that the Muslim call to prayer can be heard during Mass.

Their proximity is not coincidental, but strongly willed as a symbol of religious freedom and tolerance that is enshrined in Indonesia’s Constitution. The buildings are also linked by an underground “Tunnel of Friendship” which Francis will visit with the grand imam, Nasaruddin Umar, before they sign a joint declaration.

While Francis will want to highlight Indonesia's tradition of religious tolerance, the country's image as a moderate Muslim nation has been undermined by flare-ups of intolerance. In 2021, a militant Islamic couple blew themselves up outside a packed Catholic cathedral on Indonesia’s Sulawesi island during a Palm Sunday Mass, injuring at least 20 people.

“We have no problem with the visit. He’s a guest and we will welcome him,” said Eldy, a 64-year-old retired government worker who uses one name and was out walking during a car-free day in Jakarta on Sunday. “He wants to visit our Istiqlal mosque, he can do it.”

Even though Catholics make up only 3% of Indonesia’s population, the sheer number of Indonesians -- 275 million -- makes the archipelago home to the third-largest Christian community in Asia, after the Philippines and China.

As a result, thousands are expected to throng to Francis’ events this week, which include a Mass on Thursday afternoon at Jakarta’s main stadium expected to draw some 60,000 people. City authorities have urged residents to work from home that day given roadblocks and crowds.

“It is a joy for our country, especially for us Catholics,” said Elisabeth Damanik, a 50-year-old housewife outside a packed Mass on Sunday at Our Lady of the Assumption. “Hopefully the pope’s visit can build religious tolerance in our beloved country of Indonesia.”

Care for the environment, conflict resolution and ethically minded economic development are the major themes for the trip, and Francis may touch on them during his main speech to Indonesian authorities on Wednesday.

Francis has made caring for the environment a hallmark of his pontificate and has often used his foreign visits to press his agenda on the need to care for God’s creation, prevent exploitation of its natural resources and protect poor people who are bearing the brunt of climate extremes and pollution.

In Jakarta, he will find a metropolis of 11.3 million people choking under gray clouds of air pollution caused by coal-fired power plants, vehicle exhaust, trash burning and factories. Jakarta’s air pollution regularly registers eight to nine times above World Health Organization limits.

“Indonesia has the worst air pollution in Southeast Asia,” said Piotr Jakubowski, an air pollution expert and co-founder of Indonesian air quality monitoring company Nafas. “The visit of the pope is great because it provides a sounding board ... from another, very well-respected world leader."

Residents, too, hope Francis will speak out about the issue.

“The pollution in Jakarta is at an alarming level. That’s why the presence of the pope can provide a benefit with the discussion of environmental issues,” said government worker Erik Sebastian Naibaho, 26.

Francis is the third pope to visit Indonesia after Pope Paul VI in 1970 and St. John Paul II in 1989. Their attention underscores Indonesia’s importance to the Vatican both in terms of Christian-Muslim dialogue and Catholic vocations, since it is home to the world’s largest seminary and produces hundreds of priests and other religious workers a year.

“Indonesia is trying to grow in the faith,” said Cardinal Ignatius Suharyo Hardjoatmodjo, the archbishop of Jakarta whom Francis made a cardinal in 2019.

At a briefing last week, he said Francis wanted to express his appreciation for Indonesia’s interfaith tradition “and encourage this kind of brotherhood to continue to be maintained and developed.

Copyright 2024 NPR

What is the Prime Meridian - and why is it in Greenwich?

Discover more about the historic Prime Meridian of the world

Royal Observatory Greenwich  is famous around the world as the home of the historic Prime Meridian.

Just as the equator separates the northern and southern hemispheres, the Prime Meridian separates the eastern and western hemispheres. When you stand across the line , you have one foot in the east and one in the west.

But what exactly is a meridian , and why is the Prime Meridian in Greenwich ? What does all this have to do with latitude, longitude and working out where you are?

Discover more about the historic Prime Meridian of the world below, then  visit the Royal Observatory  to find your own place in the history of time and space.

Prime Meridian facts

• From pole to pole, the Prime Meridian covers a distance of 20,000 km. 
• In the northern hemisphere, the Prime Meridian passes through the UK, France and Spain in Europe, and Algeria, Mali, Burkina Faso, Togo and Ghana in Africa.  
• The only landmass crossed by the Meridian in the southern hemisphere is Antarctica. 
• Although the Prime Meridian passes through eight countries and crosses three continents, for nearly two thirds of its length it passes over the sea.
• The Prime Meridian in Greenwich is marked by a metal line fixed into the Royal Observatory courtyard , and at night by a laser beam. The beam originates from a point above the Airy Transit Circle, the telescope used to determine the Greenwich Meridian. Depending on atmospheric conditions, the beam can be seen from more than 20 km away.

Find more fascinating facts in  On the Line: the Story of the Greenwich Meridian

What is a meridian?

A meridian is an imaginary line running north to south that is used as a reference line for astronomical observations. By noting the exact time each star crosses the meridian, it's possible to build up an accurate map of the sky.

Meridian lines also help to measure distances and plot locations here on Earth by offering a way to determine  longitude.

What are latitude and longitude?

To pinpoint exactly where you are in the world, you need to know both your latitude and longitude.

Latitude is a calculation of how far north or south you are  from a common reference line. Longitude is how far east or west you are .

Latitude is (relatively) easy; the equator runs around the middle of Earth, dividing the northern and southern hemispheres. A person's latitude can therefore be calculated from this shared reference line.

Longitude is more complicated. Unlike the equator, there is no 'natural' line dividing the eastern and western hemispheres, and therefore no obvious place from which to measure longitude. In order to have an agreed way of finding locations east and west, we need a common reference: a 'prime meridian'.

What is the Prime Meridian?

Any north-south line can be a meridian. A prime meridian, however, is the meridian chosen to be zero degrees longitude for common reference: the line from which all locations east and west can be measured.

For centuries, people have chosen different meridians to be their prime meridian. The collections at Royal Museums Greenwich include many examples of historic maps that used different meridians.

By the 1880s, however, many people could see the advantages of everyone measuring from a single prime meridian. The only question was, where should it be?

Why is the Prime Meridian in Greenwich?

The Royal Observatory was founded by Charles II in 1675 , with the aim of finding a way to measure longitude at sea.

In an effort to achieve this, astronomers at Greenwich began to carefully record the time at which certain stars crossed overhead – an event known as a 'transit'.

A series of tables predicting the position of the Sun, Moon and stars throughout the year was eventually produced based on these measurements, giving sailors a way to work out their position at sea. As a result, the Royal Observatory became a vital centre for astronomy, navigation and the measurement of time.

The Meridian Observatory

This building was the most important part of the Royal Observatory, housing the instruments that defined the Greenwich Meridian. As telescopes and clocks improved, new instruments were installed, which meant that the place where transits were actually measured from moved slightly. If you visit the Royal Observatory today , you will notice there are actually three different meridian lines marked across the main courtyard, each named after the Astronomer Royal who established it.

Halley's meridian (1725–1750)

Edmond Halley, the second Astronomer Royal, measured the height of stars above the horizon and timed their transit over the meridian marked by the cross-hairs in his telescope with an accurate pendulum clock. Astronomers opened hatches in the roof to observe the stars. One of the most important observations was of the Sun at solar noon. 

Bradley's meridian (1750–1851)

The third Astronomer Royal, James Bradley,  installed a new telescope and defined a new meridian to review and correct data on over 3,000 stars. In 1801 the meridian was used by the national mapping agency Ordnance Survey as zero degrees longitude. It still defines their maps today.

Airy's meridian (1851–1954) – the world's Prime Meridian

The seventh Astronomer Royal, George Biddell Airy,  designed this large instrument in 1850 to define a new meridian with greater accuracy. It became known as the Airy Transit Circle and was used for over a century to make around 600,000 observations. This telescope defined the historic Prime Meridian of the world.  

In 1850, a new telescope known as the Airy Transit Circle (named after George Biddell Airy, the seventh Astronomer Royal) was designed for the Royal Observatory. From January 1851 it became the instrument that defined the Greenwich Meridian.

Cartographers, surveyors, navigators and astronomers around the world used observations made in Greenwich, and most shipping companies used charts based on the Greenwich Meridian. Despite this, even in the 1880s countries still used different meridians based on their own national observatories.

In 1884, representatives from different nations gathered in Washington D.C. to choose one single meridian as a common reference. 

After weeks of discussion, delegates voted to make the Greenwich Meridian the world's Prime Meridian. The cross-hairs in the eyepiece of the Airy Transit Circle precisely defined zero degrees longitude for the world.

Greenwich was chosen because it offered the least disruption. Nearly two-thirds of the world's ships were already using charts based on the Greenwich Meridian, making the transition to a single Prime Meridian easier.

Conference delegates also recommended a new global time zone system based on Greenwich Mean Time.  Greenwich found itself at the centre of world time and navigation.

'Although our lives have changed significantly with new technologies in navigation and timekeeping, the Royal Observatory is a powerful reminder of our enduring relationship with the Sun, Moon and stars. 

Using a combination of clocks and telescopes, generations of astronomers royal and their assistants spent many cold nights carefully observing the stars to translate earth’s daily rotation and motion through space as a measure of time and location.'.

- Louise Devoy, Curator of the Royal Observatory

Is the Greenwich Meridian still the Prime Meridian?

Between 1984 and 1988 an entirely new set of coordinate systems were adopted based on satellite data and other measurements, and these required a prime meridian that defined a plane passing through the centre of the Earth.

Earth's current internationally agreed prime meridian is the IERS Reference Meridian, also known as the International Reference Meridian or IRM.  The IRM passes 102.5 m to the east of the historic Prime Meridian.

Phones using satellite-based navigation systems use this meridian, which is why when you stand on the historic Prime Meridian your phone won't show exactly zero degrees longitude.

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