What is the prime meridian?

What is the prime meridian?

The prime meridian is the meridian — a line of longitude running from the North Pole to the South Pole — that is defined to have longitude 0°. Every other position on Earth carries a longitude measured as an angle east or west of it, from 0° at the meridian itself to 180° on the far side of the globe. The choice of which meridian counts as zero is a pure convention: unlike the equator, which the Earth's spin fixes as the natural zero of latitude, no physical feature singles out one meridian over another, so the world simply had to agree on one.¹⁴

The prime meridian is an imaginary line, but it is anchored to a real place. By international agreement it passes through a particular telescope at the Royal Observatory in Greenwich, on the south bank of the Thames in London, and from there runs the full way around the globe through both poles. The line directly opposite it, at 180° longitude, is the antimeridian; it runs mostly through the open Pacific and is closely followed by the International Date Line.¹

Why is the prime meridian at Greenwich?

The prime meridian runs through Greenwich because an international conference chose it there in 1884. Delegates from twenty-six nations met in Washington, D.C., in October of that year and resolved, by a vote of 22 to 1, that "the meridian passing through the centre of the transit instrument at the Observatory of Greenwich" should be adopted as "the initial meridian for longitude." The Dominican Republic voted against; France and Brazil abstained.²

The choice formalised what mariners were already doing. By the 1880s the great majority of the world's commerce sailed under charts that reckoned longitude from Greenwich — nearly two-thirds of the world's shipping, by the Royal Observatory's own account — because Britain's naval and merchant fleets dominated the seas and the observatory's published navigational tables were the most widely used. Adopting Greenwich ratified existing practice rather than imposing a new convention; choosing any other meridian would have forced the majority of the world's ships to redraw their charts.¹²

How is the prime meridian marked at Greenwich?

The historic prime meridian is defined by the Airy Transit Circle, a telescope built into the Royal Observatory and named for George Biddell Airy, the seventh Astronomer Royal, who designed it. The instrument was completed in 1850, and from January 1851 it became the instrument that defined the Greenwich meridian; over the following century it was used to make around 600,000 observations of stars crossing the meridian. The cross-hairs in its eyepiece mark the exact north–south plane that is 0° longitude.¹

A transit telescope can define a meridian because of how it is mounted: the Airy Transit Circle swings only in the north–south plane, sweeping the sky along a single line of longitude as the Earth turns beneath it. The moment a known star crosses that line fixes both the local time and the orientation of the meridian. In the observatory courtyard the line is now marked by a strip of metal set into the stone, and at night by a green laser beam projected northward that can be seen from many kilometres away. The marked line is the spot where visitors stand with one foot in the Earth's eastern hemisphere and one in the western.¹

Why doesn't GPS read zero longitude at the Greenwich line?

A visitor standing on the marked line at Greenwich with a satellite-navigation receiver sees a longitude a small fraction of a degree west of zero, and must walk about 102 metres east before the receiver reads 0°. The historic meridian set into the courtyard and the meridian that modern satellite systems treat as zero are not the same line; the satellite line lies about 102 metres to the east.³¹

The gap is not an error in either system. It comes from the difference between two ways of defining "straight down," a difference geodesists call the deflection of the vertical. The Airy Transit Circle was levelled against the local direction of gravity — set with a plumb line and a trough of mercury, both of which settle along the pull of gravity at that exact spot. But the Earth's mass is not evenly distributed, so the local pull of gravity does not point precisely at the planet's centre; at Greenwich it tilts slightly. Modern satellite systems instead reference a smooth mathematical model of the Earth centred on its centre of mass, in which "down" means the straight line to that centre. The east–west part of the tilt at Greenwich is just large enough to account for the entire 102-metre shift.³

The modern zero is called the IERS Reference Meridian, maintained by the international body that monitors the Earth's rotation and defines its global coordinate frame. It is the line of zero longitude built into the World Geodetic System — the coordinate model used by the Global Positioning System (GPS) and essentially all satellite navigation. When this geocentric frame was established in the late twentieth century, its longitude origin was set to keep astronomical time continuous with the past rather than to sit exactly on the Greenwich telescope, so the roughly 102-metre offset was inherited deliberately rather than introduced by mistake.³¹

How does the prime meridian relate to time and time zones?

The prime meridian is the origin of civil time as well as of longitude, because the two are the same measurement seen from different angles. The Earth turns 360° in 24 hours, so 15° of longitude corresponds to one hour of time; local solar time runs ahead of the prime meridian to the east and behind it to the west. Mean solar time on the prime meridian — the time the Sun keeps at Greenwich, averaged over the year — is Greenwich Mean Time, which served as the world's civil time standard from 1884 until 1972.²

The 1884 conference made this link explicit. Alongside fixing the meridian, it resolved that a "universal day" for the whole world should begin "at the moment of mean midnight of the initial meridian" — midnight at Greenwich — the same instant marked today as 00:00 Coordinated Universal Time. The modern system of time zones grew out of this principle: each zone keeps its standard time set a whole or part number of hours ahead of or behind the time at the prime meridian, and the standard offsets in use today run from twelve hours behind it to fourteen hours ahead. The line where the calendar date changes, near the meridian opposite Greenwich, is the International Date Line.²

What prime meridians were used before Greenwich?

For most of the history of mapmaking there was no single prime meridian. In the seventeenth and eighteenth centuries each country tended to measure longitude from a meridian through its own capital or principal observatory: France used a meridian through Paris, and the German states used ones through Berlin and other cities. The result was that the same place could carry a different longitude on a French chart than on a British or German one, and a navigator had to know which meridian a chart was drawn from before the numbers meant anything.⁵

The 1884 agreement did not end the alternatives overnight. France, which had abstained from the Greenwich vote, continued to reckon longitude from the Paris meridian until 1911, when it adopted a national time that was Greenwich time in all but name.⁵² Over the following decades the Greenwich meridian displaced the national meridians for navigation and timekeeping alike, and it is now the single reference from which longitude and civil time are measured worldwide.⁵

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