50 Fun Facts About Prime Meridian

What is the longitude of the prime meridian?

Longitude is defined as 0 there; with its anti-meridian it splits the globe into two halves.

What long symbolised the prime meridian in the courtyard of the Royal Observatory?

It has since been replaced by stainless steel.

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

Unlike the equator, the choice of prime meridian is astronomically arbitrary.

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?

Congress had authorised the president to call it by an Act of 3 August 1882.

Which US president requested the International Meridian Conference?

Congress had authorised the invitation by an Act of 3 August 1882.

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?

Even then it refused to say Greenwich, calling it Paris mean time retarded by 9 minutes 21 seconds.

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

Navigation charts switched to the Greenwich meridian in 1914.

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?

That is 5.3 seconds of arc, explained by the deflection of the vertical at Airy's transit circle.

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?

Greenwich had held the role for exactly a century.

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

The drift is northeast and measured from observations of satellites and quasars.

How has the prime meridian been marked across the London night sky since 16 December 1999?

The ground marker is now stainless steel.

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

The list runs from England down through the Sahara to the Gulf of Guinea, plus Antarctica's Queen Maud Land.

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'?

It is open ocean about 600 km south of West Africa, and a running joke among mapmakers about bad coordinates.

What was moored at Null Island until 2021?

Every buoy in the PIRATA array is named after a musical genre.

Which ancient geographer was the first to use a consistent meridian for a world map, in his Geographia?

He based it on the Fortunate Isles, comfortably west of Africa because negative numbers were not yet in use.

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

His maps actually fit the Cape Verde islands better.

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

The city was then called Avanti; the line also passed through Rohitaka, modern Rohtak.

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 Canary island did Cardinal Richelieu choose as the meridian in 1634?

Delisle rounded its position to exactly 20 degrees west of Paris, making it the Paris meridian in disguise.

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

His later maps switched to the Azores, following the hypothesis that magnetic north marked a natural line.

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

Navigators kept the 360-degree habit well into the 18th century.

Whose Nautical Almanac, published from 1765, made Greenwich the universal reference point for navigators?

He published 49 issues; even the French translations kept his Greenwich calculations.

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

His H4 and three earlier timekeepers are on show at Greenwich; Godfrey and Hadley developed the octant.

Which king founded the Royal Observatory at Greenwich in 1675?

Christopher Wren picked the hilltop site of Greenwich Castle and John Flamsteed became the first Astronomer Royal.

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

Astronomer Royal John Pond installed it in 1833 so ships on the Thames could set their clocks.

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

It set its clocks 11 and a half hours ahead of GMT to coordinate telegraph offices.

Which Canadian railway engineer's 1876 memoir Terrestrial Time first proposed a worldwide system of time?

He proposed a 24-hour universal time with a second dial for local time rounded to the hour.

How many time zones did North American railroad executives agree to adopt in Chicago on 11 October 1883?

Standard railway time began at noon on 18 November 1883, a year before the Washington conference.

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

The others were Greenwich, Paris and Washington; the geodesists recommended Greenwich.

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

It moved again to Cambridge in 1990 and closed in 1998; Greenwich is now essentially a museum.

The prime meridian of Mars runs through the centre of which crater?

It is named for the same Astronomer Royal, and pinned by the longitude of the Viking 1 lander.

Mercury's prime meridian, defined in 1975, was chosen to pass through what?

It sits 20 degrees east of the crater Hun Kal, where the sun briefly reverses at noon during perihelion.

Venus's prime meridian, defined in 1992, passes through the central peak of which crater?

The choice was entirely arbitrary.

Where does the Moon's prime meridian lie?

It passes near the crater Bruce; Io, Titan and Pluto follow the same tidally-locked convention.

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

That is when Richard Carrington began his sunspot observations.

Which line of longitude, opposite the prime meridian, underlies the International Date Line?

A Canadian engineer had proposed the anti-meridian of Greenwich as the prime meridian itself, but Britain would not back it.

In which Italian city was Ptolemy's Geographia first printed with maps, in 1477?

Many 16th-century globes followed Ptolemy's lead in placing zero longitude at the Fortunate Isles.

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

That is roughly 2,190 km; Columbus had reported the compass pointing due north somewhere mid-Atlantic in 1493.

Which first Astronomer Royal's star charts, compiled 1680-1719, let navigators use the lunar method of longitude?

Halley, his successor, disseminated them; the octant of Godfrey and Hadley made the sightings practical.

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

France abstained in the vote and kept the Paris meridian until 1911.

How many delegates, representing 25 nations, attended the 1884 International Meridian Conference?

Twenty-two of the countries voted for Greenwich as the common zero of longitude and standard of time reckoning.

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

That plumb-line alignment is deflected slightly from the ellipsoid normal, which is why GPS places zero longitude to the east.

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

Queen Maud Land is claimed by Norway; the line reaches it after crossing Ghana and the Atlantic.

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

The change began in 1973 and put the meridian plane through Earth's centre of mass rather than a local plumb line.