47 Fun Facts About Prime Meridian

Where does the prime meridian meet every other line of longitude?

At the two poles: every meridian is half of a great circle running from the North Pole to the South Pole, so they all converge there; the prime meridian is simply the one chosen to count as zero.

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

A brass strip marked the line once the old observatory became a museum; it has since been replaced by stainless steel, and visitors stand astride it to put one foot in each hemisphere.

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

Eastern and Western: unlike the equator, which the Earth's spin fixes in place, this dividing line is pure convention, so the two halves are simply whatever the chosen meridian makes them.

In which city was the International Meridian Conference held?

Washington, D.C. hosted because the call came from the United States: Congress authorised the president in 1882 to invite the world's governments, and their delegates gathered in October 1884.

Which US president called the 1884 International Meridian Conference?

Chester A. Arthur issued the invitations under an 1882 Act of Congress; yet his own country did not write standard time into law until 1918.

Which industry put North America on Greenwich-based time zones in November 1883, a year before the Meridian Conference?

The railways: tired of juggling scores of local times, the companies switched to a handful of standard zones at noon on 18 November 1883, decades before Congress made the system law.

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

France abstained, kept the Paris meridian on its maps for decades, and when it finally adopted Greenwich time in 1911 the law called it 'Paris mean time, retarded by 9 minutes and 21 seconds'.

Which time standard replaced Greenwich Mean Time as the world's civil time reference in 1972?

Coordinated Universal Time, or UTC, is kept by atomic clocks but nudged with leap seconds so that it stays within about a second of mean solar time on the prime meridian.

Which queen approved a £20,000 reward in 1714 for a practical way to find longitude at sea?

Queen Anne approved the Longitude Act at the very end of her reign; the commissioners it named had so little to judge that they did not hold a meeting until 1737.

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

George Airy's new transit circle set the line from January 1851, a few metres east of the one before; Britain's own Ordnance Survey never switched, and its maps are still drawn from the older meridian.

How far east of Greenwich's historic meridian line do satellite navigation devices place zero longitude?

About 100 metres: satellite zero lies 102 m east of the courtyard line, because the old telescope was levelled against local gravity, which pulls slightly off true there, while satellites orbit the Earth's centre of mass.

Since 1924 the BBC has broadcast the Greenwich Time Signal as six short tones known as what?

The pips went on air on 5 February 1924, timed by the Royal Observatory; the last one is now longer than the others so that listeners can tell which tone marks the hour, which begins the instant it starts.

After leaving Africa, the prime meridian heads south across which ocean?

The Atlantic: the line leaves land in West Africa and meets none again until Antarctica, one reason that most of the prime meridian's length lies over the sea.

What carries Greenwich's historic meridian telescope north-east by about 2.5 cm a year relative to modern zero longitude?

Plate tectonics: the observatory rides on the Eurasian plate, which creeps north-east at roughly 2.5 cm a year, while today's zero meridian is tied to a satellite-era frame that averages out the motion of all the plates.

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

A green laser has marked the line across the night sky since 16 December 1999; it shines north from a point above the telescope that defined the meridian and can be seen more than 20 km away in clear air.

Which of these European countries does the prime meridian pass through?

Spain sits on the Greenwich line yet keeps Central European Time, an hour ahead: Franco moved the clocks forward in 1940 to align with Germany, and they have never been moved back.

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

Ghana: the line clips the far north of Togo on its way south, but it is Ghana's coast, at the port city of Tema, that it finally leaves.

What name do mapmakers give the spot in the ocean where the prime meridian meets the equator?

Null Island is no island at all: so much badly geocoded data defaults to zero latitude and zero longitude that mapmakers gave the empty patch of sea a name; a weather buoy was moored there until 2021.

Which ancient geographer's Geographia gave coordinates for places across the known world 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 prime meridian's 'Fortunate Isles' usually identified?

The Canaries are the usual identification, though other island groups off north-west Africa have their backers; either way the isles were the westernmost land the ancients recorded, a natural place to start counting.

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

Ujjain, then called Avanti, was the zero point of Indian astronomy; it is still nicknamed the Greenwich of India, and Jai Singh II built one of his five stone observatories there.

Which 1494 treaty divided newly found lands between Castile and Portugal along a meridian west of Cape Verde?

Tordesillas moved the dividing line from the Pope's 100 leagues to 370 leagues west of the Cape Verde islands, a shift that later gave Portugal its claim to the eastern bulge of Brazil.

Which island, then seen as the western edge of the Old World, did Cardinal Richelieu's 1634 conference choose as prime meridian?

El Hierro, then known as Ferro, stayed on continental maps into the 19th century; geographers later simply fixed its line at exactly 20 degrees west of Paris, so it became the Paris meridian in disguise.

Before sea clocks, who first proposed finding longitude by timing the eclipses of Jupiter's moons?

Galileo Galilei, who had found the four moons in 1610, pitched them to two governments as a celestial clock; the method proved hopeless on a rolling deck, but on land it later helped surveyors redraw national maps.

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 almanac tabulated the Moon's position against Greenwich time, so any navigator using its lunar-distance tables was reckoning longitude from Greenwich, whatever flag his ship flew.

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

John Harrison's fourth timekeeper, the watch-sized H4, kept such good time on a 1761-62 voyage to Jamaica that it showed a clock could carry home-port time across an ocean and so fix longitude at sea.

Which king founded the Royal Observatory at Greenwich in 1675?

Charles II ordered 'a small observatory within our park at Greenwich, upon the highest ground' in June 1675, persuaded that astronomy held the key to finding longitude at sea.

Greenwich's time ball, first dropped in 1833, was set to fall at what time instead of noon?

1 pm: the ball climbs halfway up its mast at 12:55 as a warning, reaches the top at 12:58 and drops exactly on the hour, a signal set up so that ships below could check their chronometers.

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

New Zealand set a single national time in 1868, eleven and a half hours ahead of Greenwich, so that telegraph offices up and down the country could keep the same opening hours.

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

Sandford Fleming, a Scottish-born Canadian engineer, is said to have taken up the cause after a misprinted timetable left him stranded in Ireland; his 1876 paper proposed one 24-hour clock for the whole world.

In 1707 a British fleet was wrecked on which islands, a disaster often linked to the Longitude Act seven years later?

The Isles of Scilly: four warships of Admiral Sir Cloudesley Shovell's fleet struck the rocks on the night of 22 October 1707, and well over a thousand men drowned, the admiral among them.

To where did the working Royal Greenwich Observatory move after the Second World War to escape London's light pollution?

Herstmonceux: the astronomers took a decade to settle into the Sussex castle, finishing in 1957, then moved on again to Cambridge before the institution was closed in 1998.

Which planet's prime meridian was first fixed in the 1830s, using a small dark spot seen through a telescope?

Mars: the German astronomers Wilhelm Beer and Johann Mädler took the spot as their zero when they began mapping the planet in 1830; the region, later named Meridiani, is where the rover Opportunity landed.

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 hottest spots; the reference mark is a tiny crater at 20 degrees west, named Hun Kal, which is Mayan for twenty.

Jupiter's 'System III' longitudes, used because the planet shows no solid surface, follow the rotation of what?

Its magnetic field: the cloud bands turn at different rates at different latitudes, so astronomers track the field instead, which spins once every 9 hours 55 minutes with the planet's deep interior.

Where does the Moon's prime meridian lie?

Through the middle of the face visible from Earth: because the Moon is tidally locked, the same hemisphere always points our way, which gives its map a natural centre line that Earth's own lacks.

Which English astronomer, who saw the great solar flare of 1859, gives his name to the numbered rotations of the Sun?

Richard Carrington began his sunspot record in 1853, and because the Sun has no fixed surface, its rotations are simply counted from 9 November of that year, when the meridian facing Earth was declared zero.

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

180 degrees: the Date Line follows the antimeridian of Greenwich down the Pacific but zigzags around island groups, swinging far to the east so that all of Kiribati keeps the same calendar day.

Which architect, a former astronomy professor, designed the Royal Observatory at Greenwich in 1675?

Christopher Wren, best known for St Paul's Cathedral, had been a professor of astronomy at Oxford; he suggested building on the ruins of Greenwich Castle, whose solid foundations could be reused.

One hour of time difference corresponds to how many degrees of longitude?

15 degrees: the Earth turns through 360 degrees in 24 hours, so each hour is 15 degrees and each degree four minutes, which is why knowing the time at a reference meridian tells a navigator how far east or west he is.

Which Astronomer Royal, the first to hold the post, compiled the Greenwich star catalogue that underpinned lunar-distance longitude?

John Flamsteed guarded his star positions so jealously that Newton and Halley printed them without his consent in 1712; he got hold of 300 of the 400 copies and burned them.

Besides the Azores, which waterway was proposed at the 1884 conference as a 'neutral' prime meridian?

The Bering Strait: backers of a 'neutral' meridian wanted a line chosen on purely geographical grounds; opponents replied that no observatory or telegraph cable could be set up there to fix it.

Which handheld instrument let navigators measure the Moon's angle from a star for the lunar-distance method?

The sextant, named for its arc of one-sixth of a circle, measured the angle between the Moon and a star; printed tables then turned that angle into the time at Greenwich, and so into longitude.

What did Greenwich's 1851 transit circle use to align its telescope with the local vertical?

A basin of mercury: pointing the telescope straight down at the liquid mirror let the observer line up the crosswires with their own reflection, which fixed the true vertical along the pull of gravity.

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

Queen Maud Land runs from 20 degrees west to 45 degrees east, so zero longitude reaches the coast inside it; Norway annexed it in January 1939, five days before a German expedition arrived to stake a rival claim.

The modern prime meridian is fixed partly by radio telescopes on different continents timing signals from which distant objects?

Quasars: radio dishes thousands of kilometres apart compare the arrival times of a quasar's signal; with precise ranging to satellites and the Moon, the technique took over from the telescope at Greenwich.