48 Fun Facts About Prime Meridian

What is the longitude of the prime meridian?

Nought: the prime meridian is the line of 0° longitude by definition, the starting point from which every other longitude is measured east or west.

What marked the prime meridian line in the Royal Observatory's courtyard for many years?

A brass strip: for decades a brass strip set into the courtyard of the Royal Observatory at Greenwich marked the line, and it was later replaced by stainless steel.

Which two hemispheres does the prime meridian divide the Earth into?

Eastern and Western: the prime meridian and its antimeridian split the globe into the Eastern and Western Hemispheres, a boundary fixed by convention rather than by the Earth's rotation.

In which year did the International Meridian Conference adopt Greenwich as the world's prime meridian?

Forty-one delegates met that October.

In which city was the International Meridian Conference held?

Washington, D.C.: the delegates met in the US capital that October, invited by the American government under an Act of Congress of 3 August 1882.

Which US president requested the International Meridian Conference?

Chester A. Arthur: president from 1881 to 1885, he issued the invitations under the Act of 3 August 1882 that authorised him to call the conference.

How many countries voted to adopt the Greenwich meridian at the meridian conference?

Forty-one delegates attended in total; the French abstained.

Which country abstained from the meridian conference vote and kept its own meridian for decades?

It argued for a neutral line through the Azores or the Bering Strait.

In what year did the holdout nation finally adopt Greenwich as the beginning of the universal day?

1911: a French law of March 1911 aligned the country's legal time with Greenwich, though its wording avoided the name; French marine charts followed in 1914.

What phrase did the French use instead of the word Greenwich once they gave in?

Paris mean time, retarded by 9 minutes and 21 seconds: the French law set legal time that way because the Paris Observatory lies about 2°20' east of Greenwich, a 9-minute-21-second difference in mean time.

Which Astronomer Royal fixed the Greenwich prime meridian in 1851 with his new transit circle?

Earlier transit instruments, the first bought by Halley in 1721, sat about 43 metres further west.

How far east of the Greenwich courtyard marker does a GPS receiver place the meridian?

About 102 metres: satellite-based zero longitude lies 102.5 m east of the courtyard line at Greenwich, 5.3 arcseconds away, because a plumb line there is deflected by local gravity while satellites reference the Earth's centre of mass.

What is the name of the international standard prime meridian adopted in 1984?

Its plane passes through the Earth's centre of mass rather than following a plumb line at Greenwich.

Since what year has the Earth-centred reference meridian been the international standard?

1984: that year the satellite-based, Earth-centred zero meridian took over as the international standard, a century after the Greenwich line was chosen.

How fast does Airy's transit circle drift relative to Earth-centred zero longitude?

About 2.5 centimetres a year: the Eurasian plate carries the Greenwich instrument north-eastward at roughly 2.5 cm a year relative to the satellite-defined frame, whose origin is the Earth's centre of mass.

What was installed at Greenwich in December 1999 to mark the meridian across the night sky?

A green laser: a green laser installed at the Royal Observatory for the millennium shines north along the meridian line at night, visible for miles on a clear evening.

How many countries does the Greenwich meridian pass through between the North and South Poles?

Eight: three in Europe, the United Kingdom among them, then Algeria, Mali and Burkina Faso, and two more states on the Gulf of Guinea coast before open ocean and Antarctica.

Which is the southernmost African country crossed by the Greenwich meridian?

After leaving Ghana the line runs down through the Gulf of Guinea and the Atlantic to Antarctica.

What is 'Null Island'?

The point where the prime meridian meets the equator: 0° latitude, 0° longitude is open ocean in the Gulf of Guinea, and mapmakers joke that mis-geocoded data lands on this imaginary island.

Which ancient geographer's Geographia listed coordinates for some 8,000 places from a single prime meridian?

Ptolemy: his Geographia counted longitude eastward from the Fortunate Isles, placed west of Africa so that every known place had a positive longitude, a convention Marinus of Tyre had used before him.

With which modern islands are the ancient geographer's 'Fortunate Isles' usually associated?

The Canaries: the Fortunate Isles of Greek tradition are usually identified with the Canary Islands, the westernmost land then known, so longitude counted eastward from them stayed positive.

Per the 4th-century Surya Siddhanta, India's ancient zero longitude passed through which city?

Ujjain: the Surya Siddhanta's meridian ran through Ujjain, then known as Avanti, and through Rohitaka, modern Rohtak, on its way north.

Which 1494 Spain–Portugal treaty used a mid-Atlantic line where the compass pointed due north?

The line was settled at 370 leagues west of Cape Verde.

Which island did Cardinal Richelieu's 1634 conference choose as the prime meridian?

El Hierro: the westernmost Canary island, then called Ferro, was chosen at Richelieu's 1634 conference; Delisle later fixed it at exactly 20 degrees west of the French capital's meridian.

Through which island did Mercator draw the prime meridian on his 1541 globe?

Fuerteventura: Mercator's terrestrial globe of 1541 drew the zero meridian through Fuerteventura; his later maps moved it to the Azores on the theory that magnetic north marked a natural line.

How were longitudes counted in Ortelius's first modern atlas of 1570?

From 0 to 360 degrees: Ortelius's Theatrum Orbis Terrarum numbered longitude right round the globe from its zero, a single eastward count that navigators kept well into the 18th century.

Whose Nautical Almanac, first issued for the year 1767, made Greenwich the reference point for navigators?

Nevil Maskelyne's: as Astronomer Royal he launched the Nautical Almanac, whose lunar-distance tables were computed for the Greenwich meridian, so navigators everywhere, even in French translation, reckoned from Greenwich.

Which clockmaker developed the marine chronometer that transformed the finding of longitude at sea?

John Harrison: his fourth sea clock, H4, lost only seconds on a 1761-62 voyage to Jamaica, proving a timekeeper could carry Greenwich time and so fix longitude at sea.

Which king founded the Royal Observatory at Greenwich in 1675?

Charles II: he founded the observatory by royal warrant in June 1675 to fix longitude for navigation, and Christopher Wren chose the hilltop site of the old Greenwich Castle.

At what time does the red time ball on the Greenwich observatory drop each day?

1 pm: the ball is hoisted just before the hour and falls at exactly 1 pm, a signal Astronomer Royal John Pond introduced in 1833 so ships on the Thames could set their chronometers.

Which was the first country to define its civil time by the Greenwich meridian, in 1868?

New Zealand: in 1868 it adopted New Zealand Mean Time, 11 hours 30 minutes ahead of Greenwich and based on longitude 172°30' east, the first nationwide standard time tied to Greenwich.

Whose 1876 memoir Terrestrial Time proposed a worldwide system of standard time?

Sandford Fleming: the Scottish-born Canadian railway engineer proposed a 24-hour universal time, with local time rounded to the hour shown on a second dial, in his 1876 paper Terrestrial Time.

Which of these was one of four observatories judged in 1883 to qualify for an international prime meridian?

Berlin: the International Geodetic Association's 1883 Rome conference judged only four observatories, Berlin among them, to have the required standing, and recommended Greenwich because most of the world's shipping already used it.

To where did the working Royal Greenwich Observatory move in 1957 to escape London's light pollution?

Herstmonceux: the observatory moved to Herstmonceux Castle in Sussex, a transfer completed in 1957, and moved again to Cambridge in 1990 before closing in 1998.

Which pair of astronomers, mapping Mars from 1830, chose a dark spot as its zero longitude?

Beer and Mädler: mapping Mars between 1830 and 1832, Wilhelm Beer and Johann Mädler chose a small dark spot as their zero of longitude; the region around it became Meridiani Planum, where the rover Opportunity landed in 2004.

Mercury's prime meridian was chosen to pass through what point on the planet?

The hottest point on its equator: the zero meridian runs through one of Mercury's two 'hot poles', the subsolar point at perihelion; the small crater Hun Kal, at 20° west, serves as the reference mark.

Venus's prime meridian passes through the central peak of which crater?

Ariadne: the zero meridian of Venus is defined to pass through the central peak of the crater Ariadne, a fixed radar-mapped landmark chosen as the reference point.

Where does the Moon's prime meridian lie?

Through the middle of the face visible from Earth: the lunar zero meridian runs down the centre of the near side, passing close to the small crater Bruce near the Moon's mean sub-Earth point.

The Carrington heliographic system fixes the Sun's prime meridian by the disk's centre on which date?

9 November 1853: Carrington rotation number 1 began on that date, and the solar meridian crossing the disk's centre that day became the zero of heliographic longitude.

Which line of longitude underlies most of the International Date Line?

180 degrees: the Date Line follows the 180th meridian through the Pacific, bending around Kiribati and other islands; that line is the antimeridian of Greenwich.

In which Italian city was the ancient Geographia first printed with maps, in 1477?

Bologna: the 1477 Bologna edition was the first Geographia printed with maps, spreading its author's practice of counting longitude eastward from the Fortunate Isles across Europe.

The Tordesillas line was finally fixed how far west of Cape Verde?

370 leagues: the 1494 treaty pushed the papal bull's 100-league line out to 370 leagues west of the Cape Verde islands, which later gave Portugal its claim to Brazil.

Which Astronomer Royal's star catalogue, observed at Greenwich from 1676 to 1719, underpinned lunar-distance longitude?

John Flamsteed: his Greenwich observations, published as the Historia Coelestis Britannica in 1725, gave the accurate star positions that lunar-distance longitude needed.

Besides the Azores, which waterway did France propose as a 'neutral' prime meridian in 1884?

The Bering Strait: its delegates argued for a 'neutral' line through no great nation, the Bering Strait or the Azores, rather than one through any national observatory.

How many delegates, representing 25 nations, attended the International Meridian Conference?

41: forty-one delegates from 25 countries took part, with sessions running from 1 October to 1 November under the chairmanship of US Admiral C. R. P. Rodgers.

What did Airy use in his transit circle to align the telescope with the local vertical?

A basin of mercury: pointing the telescope straight down at a mercury trough let the observer see the crosswires reflected, fixing the local vertical along which gravity pulls.

Which Antarctic territorial claim is the only one the Greenwich meridian touches?

Queen Maud Land: Norway's claim spans 20°W to 45°E, so the 0° line enters Antarctica within it after its long run south through the Atlantic.

Which technique was NOT among those that replaced optical transits to define the modern reference meridian?

Radar altimetry: the modern frame is built from very-long-baseline interferometry, satellite and lunar laser ranging and satellite navigation, which measure station positions; altimetry maps sea-surface height instead.