Name: Royal Observatory Greenwich
Address: Blackheath Ave, London SE10 8XJ, UK
Telephone: 020 8312 6608
Rating: 4.5

Wren's 18th-century astronomical observatory on the Prime Meridian, now a museum with a planetarium.

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The Royal Observatory is the historic home of the Prime Meridian, the line of 0° longitude, from which all other longitudes are measured. You can stand with one foot in the East and one in the West!

Home of Zero Longitude

The Royal Observatory was founded in 1675 by King Charles II. Its initial purpose was to improve navigation for sailors.

Founding Year

John Flamsteed was the first Astronomer Royal, appointed to run the observatory and map the stars.

First Astronomer Royal

The Shepherd Gate Clock, installed in 1852, was the first clock to show Greenwich Mean Time (GMT) to the public.

First Public Time Display

Greenwich Mean Time (GMT), the standard time against which all other time zones are measured, is named after Greenwich.

Birthplace of GMT

The Royal Observatory houses a collection of John Harrison's marine chronometers, which revolutionized navigation by allowing sailors to accurately determine longitude at sea.

Home to Marine Chronometers

The Royal Observatory is situated on a hill in Greenwich Park, offering stunning views of London.

Located in Greenwich Park

Royal Observatory Greenwich

The Royal Observatory Greenwich, perched atop a hill in Greenwich Park, is more than just a pretty view; it's the very definition of time itself. It houses the Prime Meridian, longitude zero, an imaginary line that divides the eastern and western hemispheres. Standing astride this line offers a unique photo opportunity, one foot in each hemisphere. But delve deeper, and you discover its historical significance as the home of British astronomy, commissioned by Charles II in 1675. The Shepherd Gate Clock, mounted on the outer wall, was one of the first clocks to show Greenwich Mean Time to the public, allowing ships to synchronize their chronometers. Less known is the Observatory's role in developing the Nautical Almanac, crucial for navigation at sea, transforming Britain into a maritime superpower. Its collection of astronomical instruments, including Harrison's groundbreaking marine chronometers, showcases the ingenuity required to solve the longitude problem, a challenge that once plagued seafaring nations.

Introduction

The Royal Observatory Greenwich holds a pivotal role as the historic home of the Prime Meridian, the reference line from which all global east-west distances are measured. Established by British astronomers, the observatory served as the basis for defining successive meridians, with the one passing through the Airy transit circle, first used in 1851, becoming the world's Prime Meridian in 1884. This decision, formalized at the International Meridian Conference in Washington, D.C., led to the adoption of the Greenwich Meridian as the international standard for mapping and timekeeping.

The Prime Meridian at Greenwich is physically marked in the Observatory's courtyard, initially by a brass strip and later replaced with stainless steel. Since 1999, a powerful green laser has illuminated the meridian's path across the London sky. However, GPS measurements reveal that the marking strip is slightly west of zero longitude, approximately 102 meters east of the true meridian. This discrepancy arises from localized gravity effects causing vertical deflection, rather than systematic errors in timekeeping or reference systems.

Notably, the modern Prime Meridian, known as the IERS Reference Meridian (IRM), lies 102.5 meters east of the Airy Greenwich astronomical meridian. While the Airy Transit Circle was instrumental in determining time, the IRM is the current standard defining longitude. The shift from local spheroids to a worldwide spheroid in modern geodetic reference systems caused geographical coordinates to shift, leading to the distinction between the historic and modern Prime Meridians at Greenwich.

Prime Meridian

Flamsteed House, the original building of the Royal Observatory Greenwich, was completed in 1676 and designed by Sir Christopher Wren, possibly with assistance from Robert Hooke. It holds the distinction of being Britain's first purpose-built scientific research facility. King Charles II commissioned its construction in 1675, instructing Wren, who was also an astronomer, to design the building. The location on Greenwich hill was chosen by Wren, utilizing the site of the former Greenwich Castle to avoid purchasing new land.

The building became known as Flamsteed House in honor of John Flamsteed, the first Astronomer Royal, who moved in and worked at the Observatory for 43 years. Charles II created the role of Astronomer Royal in 1675, tasking Flamsteed with creating accurate star charts for navigation. Flamsteed House served as both a residence for the Astronomer Royal, his assistant, and his family, and as a repository for scientific instruments.

Inside Flamsteed House, the Octagon Room stands out with its unusual design and large windows, initially intended for sky observation. However, its walls were not aligned with a meridian, leading to most positional observations being conducted elsewhere. The Octagon Room housed two Thomas Tompion clocks, notable for their accuracy. Today, Flamsteed House is a Grade I listed building and a key part of the Royal Observatory museum, showcasing exhibits like Harrison's sea clocks and instruments related to time and longitude.

Flamsteed House

The Royal Observatory Greenwich was established in 1675 by King Charles II, concurrently with the creation of the position of Astronomer Royal. John Flamsteed was appointed as the first Astronomer Royal, tasked with improving navigation by precisely mapping the heavens. The Observatory served as the Astronomer Royal's base of operations, with Flamsteed House becoming synonymous with the role. For nearly three centuries, the Astronomer Royal also served as the director of the Royal Observatory, Greenwich, until the roles were separated in 1972.

Early Astronomers Royal, including Flamsteed, dedicated themselves to astronomical observations that would benefit navigation. Flamsteed's work at the observatory led to the creation of a star catalog of 3,000 stars, published posthumously as Historia Coelestis Britannica, which significantly enhanced the Observatory's international reputation. Edmond Halley, another Astronomer Royal, re-equipped the observatory after Flamsteed's death. Later, Sir George Airy transformed the role from focusing solely on navigation to broader astronomical and scientific research, establishing departments for magnetic, meteorological, and solar photography.

The Astronomer Royal's observations were deemed the property of the Crown in 1765, mandating annual publication. The Royal Observatory Greenwich also relied on assistants, including women computers who performed mathematical computations, though their contributions were often overlooked. Today, the Royal Observatory Greenwich functions primarily as a museum, preserving the legacy of the Astronomers Royal and their contributions to astronomy and navigation.

Astronomer Royal

The Royal Observatory Greenwich (ROG) played a pivotal role in positional astronomy, primarily aimed at refining navigation through precise star charts and timekeeping. Tasked by King Charles II in 1675 to determine longitude, the first Astronomer Royal, John Flamsteed, initiated systematic, long-term observations of the Sun, Moon, stars, and planets. These measurements, particularly those made with meridian instruments aligned north-south, were crucial for positional astronomy. These instruments measured star positions and were intrinsically linked to time measurement, using stars as 'clock stars' to determine the local time at Greenwich.

James Bradley, the third Astronomer Royal, significantly contributed to positional astronomy between 1750 and 1762. His star charts, charting sixty thousand stars, were so accurate that they remained in use well into the 1940s. In the 19th century, under Astronomer Royal George Biddell Airy, the observatory enhanced its capabilities with improved instruments, including a transit circle installed in 1851, used for a century for positional astronomy. The transit circle measured the time a star crossed a point in the sky and its vertical angle, enabling the plotting of star locations and determination of time. Over time, the ROG's contributions facilitated advancements in understanding the motions of celestial bodies, significantly aiding navigation and astronomy.

Positional Astronomy

The Royal Observatory Greenwich (ROG) is the home of Greenwich Mean Time (GMT), holding immense significance as the origin of this pivotal time standard. GMT represents the yearly average time when the Sun crosses the Prime Meridian at the ROG, effectively standardizing time based on clock time rather than solar time. Commissioned in 1675 by King Charles II, the ROG was built with the specific purpose of improving navigation through astronomical observations, a mission intrinsically linked to the establishment of a reliable time standard.

John Flamsteed, the first Astronomer Royal, installed the best pendulum clocks at the ROG, setting them to the local time, which became known as Greenwich Mean Time. Nevil Maskelyne, the fifth Astronomer Royal, further popularized GMT through the Nautical Almanac, providing lunar distance data based on Greenwich observations and GMT as the time standard, crucial for navigators determining longitude at sea. British mariners synchronized their chronometers to GMT, enabling longitude calculations from the Greenwich meridian, which was designated as longitude zero degrees at the International Meridian Conference of 1884.

Although GMT as an international civil time standard was superseded by Coordinated Universal Time (UTC) in 1972, it remains the legal time in Britain during winter and is still used by organizations like the Met Office, Royal Navy, and BBC World Service. The Prime Meridian, passing through the Airy transit circle instrument within the ROG, symbolizes the origin of GMT, marked by a stainless steel strip and a green laser shining north across the London sky.

Royal Observatory Greenwich

Did You Know?

Home of Zero Longitude

Founding Year

First Astronomer Royal

First Public Time Display

Highlights from Reviews

Location