Prime Meridian: Establishing Global Time and Space

History and Overview of the Zero Degree Longitude Line

Prime Meridian line
FUTURE LIGHT/Photolibrary/Getty Images

The Prime Meridian is the universally decided zero longitude, an imaginary north/south line which bisects the world into two and begins the universal day. The line starts at the north pole, passes across the Royal Observatory in Greenwich, England, and ends at the south pole. Its existence is purely abstract, but it is a globally-unifying line that makes the measurement of time (clocks) and space (maps) consistent across our planet.

The Greenwich line was established in 1884 at the International Meridian Conference, held in Washington DC. That conference's main resolutions were: there was to be a single meridian; it was to cross at Greenwich; there was to be a universal day, and that day would start at mean midnight at the initial meridian. From that moment, the space and time on our globe have been universally coordinated.

Having a single prime meridian brings to the world's cartographers a universal map language allowing them to join their maps together, facilitating international trade and maritime navigation. At the same time, the world now had one matching chronology, a reference by which today you can tell what time of day it is anywhere in the world simply by knowing its longitude.

Latitudes and Longitudes

Mapping the entire globe is an ambitious task for people without satellites. In the case of latitude, the choice was easy.

Sailors and scientists set the zero latitude plane of the earth through its circumference at the equator and then divided the world from the equator to the north and south poles into ninety degrees. All other degrees of latitude are actual degrees between zero and ninety based on the arc from the plane along the equator.

Imagine a protractor with the equator at zero degrees and the north pole at ninety degrees.

However, for longitude, which could just as easily use the same measuring methodology, there is no logical starting plane or place. The 1884 conference essentially picked that starting place. Naturally, this ambitious (and highly politicized) stroke had its roots in antiquity, with the creation of domestic meridians, which first allowed local mapmakers a way to order their own known worlds.

Ptolemy and the Greeks

The classical Greeks were the first to attempt to create domestic meridians. Although there is some uncertainty, the most likely inventor was the Greek mathematician and geographer Eratosthenes (276–194 BCE). Unfortunately, his original works are lost, but they are quoted in the Greco-Roman historian Strabo's (63 BCE–23 CE) Geography. Eratosthenes chose a line on his maps marking the zero longitude as one that intersected with Alexandria (his birthplace) to act as his starting place.

The Greeks were not the only ones to invent the meridian concept of course. Sixth-century Islamic authorities used several meridians; the ancient Indians picked Sri Lanka; beginning in the mid-second century CE, south Asia used the observatory at Ujjain in Madhya Pradesh, India.

The Arabs picked a locality called Jamagird or Kangdiz; in China, it was at Beijing; in Japan at Kyoto. Each country picked a domestic meridian that made sense of their own maps.

Setting West and East

The invention of the first comprehensive use of geographic coordinates—joining an expanding world into one map—belongs to the Roman scholar Ptolemy (CE 100-170). Ptolemy set his zero longitude on the chain of the Canary Islands, the land he was aware of that was the furthest west of his known world. All of Ptolemy's world he mapped would be east of that point.

The majority of later mapmakers, including the Islamic scientists, followed Ptolemy's lead. But it was the voyages of discovery of the 15th and 16th centuries—not just Europe's of course—which established the importance and difficulties of having a unified map for navigation, eventually leading to the 1884 conference.

On most maps which plot the entire world today, the mid-point center marking the face of the world is still the Canary Islands, even if the zero longitude is in the UK, and even if the definition of the "west" includes the Americas today.

Seeing the World as a Unified Globe

By the mid 19th century there were at least 29 different domestic meridians in place, and international trade and politics were global, and the need for a coherent global map became acute. A prime meridian isn't just a line drawn on a map as 0 degrees longitude; it is also one that uses a specific astronomical observatory to publish a celestial calendar that sailors could use to identify where they were on the planet's surface by using the predicted positions of the stars and planets.

Each developing state had its own astronomers and own their own fixed points, but if the world was to progress in science and international trade, there needed to be a single meridian, an absolute astronomical mapping shared by the whole planet.

Establishing a Prime Mapping System

During the late 19th century, the United Kingdom was both the major colonial power and a major navigational power in the world. Their maps and navigational charts with the prime meridian passing through Greenwich were promulgated and many other countries adopted Greenwich as their prime meridians.

By 1884, international travel was commonplace and the need for a standardized prime meridian became readily apparent. Forty-one delegates from twenty-five "nations" met in Washington for a conference to establish zero degrees longitude and the prime meridian.

Why Greenwich?

Even though the most commonly used meridian at the time was Greenwich, not everyone was happy with the decision. The Americas, in particular, referred to Greenwich as a "dingy London suburb" and Berlin, Parsi, Washington DC, Jerusalem, Rome, Oslo, New Orleans, Mecca, Madrid, Kyoto, St. Paul's Cathedral in London, and the Pyramid of Giza, were all proposed as potential starting places by 1884.

Greenwich was selected as the prime meridian by a vote of twenty-two in favor, one against (Haiti), and two abstentions (France and Brazil).

Time Zones

With the establishment of the prime meridian and zero degrees longitude at Greenwich, the conference also established time zones. By establishing the prime meridian and zero degrees longitude in Greenwich, the world was then divided into 24 time zones (since the earth takes 24 hours to revolve on its axis) and thus each time zone was established every fifteen degrees of longitude, for a total of 360 degrees in a circle.

The establishment of the prime meridian in Greenwich in 1884 permanently established the system of latitude and longitude and time zones that we use to this day. Latitude and longitude are used in GPS and is the primary coordinate system for navigation on the planet.

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Rosenberg, Matt. "Prime Meridian: Establishing Global Time and Space." ThoughtCo, Dec. 14, 2017, thoughtco.com/what-is-the-prime-meridian-1435653. Rosenberg, Matt. (2017, December 14). Prime Meridian: Establishing Global Time and Space. Retrieved from https://www.thoughtco.com/what-is-the-prime-meridian-1435653 Rosenberg, Matt. "Prime Meridian: Establishing Global Time and Space." ThoughtCo. https://www.thoughtco.com/what-is-the-prime-meridian-1435653 (accessed February 25, 2018).