What You Never Knew About the Prime Meridian

Meridians and observatories go hand in hand. In fact, of the many ‘local’ prime meridians, no less than four ran through astronomical observatories (New and Old US Naval Observatory Meridians; Greenwich Meridian; and Paris Meridian). There is good reason for this: in the days before GPS (and even after) navigation was by the sky.

Photo showing detail of Johnson's comparative view of mountains and rivers by continent. The Greenwich and Paris Observatories are visible on the left.
Detail of Johnson’s comparative view of mountains and rivers by continent. The Greenwich and Paris Observatories are visible on the left. (Photo own work)

Our modern day prime meridian, through Greenwich Observatory in London, divides Earth north-to-south into two equal hemispheres. It serves as the basis for standard time. It is a recognized standard for describing locations east or west on the globe.

Finding the Meridian 

First, let’s establish that there are no natural laws that dictate placement of a prime meridian. Unlike the equator which naturally goes around the fattest part of the planet, there is no such circumferential line through the poles, nor are there any other properties making one ring better than another. With this in mind, it is obvious that the meridian would be placed where it was administratively and politically convenient.

Ptolemy, in his Geographia, was the first to use a prime meridian. Sea fares proposed a magnetic prime meridian, but in retrospect we see that this was not feasible. Magnetic north and true north are not one in the same; using a line through magnetic north would result in either meridians (besides than the prime meridian and antimeridian) running at other than  perpendicular to the equator or meridians that did not converge at the poles.

British Royal astronomers developed the lunar method of determining longitude. With this method, a ship’s navigator would ascertain the position of the moon in the sky and refer to tables to determine the time at a reference point. Comparing the time at the reference point with the local time (also determined astronomically), the navigator could determine longitude (each hour of difference was worth 15 degrees of longitude distance from the reference point). It was the astronomers at the Royal Observatory, Greenwich who compiled the necessary reference tables, all tied to Greenwich.

Photo of comparative map with detail from Comparative Heights of Mountains by Duncan showing Greenwich Observatory.
Detail from Comparative Heights of Mountains map by Duncan showing Greenwich Observatory. (Photo own work)

The Greenwich meridian took off, and near the end of the 19th century, well over half of all ships used it as their reference point of longitude. In 1884, delegates of 25 nations met for the International Meridian Conference, where they agreed to establish the Greenwich meridian as the prime meridian.

Implications of the Prime Meridian

Many living in the late Victorian period would have known of the importance of and debate over the prime meridian. With that in mind, in addition to astronomical discoveries of the day (Neptune), astronomical observatories would have been of interest. With that in mind, it’s not a surprise that many comparative maps showed the heights of observatories.

A widely agreed upon prime meridian meant that navigators and map makers could be certain that each was describing the same spot on the planet as each would be measuring longitude from the same spot.

Photo of diagram relating solar times in various cities to that in Washington, DC.
Johnson and Ward, A Diagram Exhibiting the Difference in Time… This diagram relates mean solar time in various cities to the time in Washington, DC. For example, at high noon in DC, clocks in New York, NY would read 12:12 PM. (Photo own work)

Establishment of a prime meridian would affect Victorians’ lives in a more day-to-day fashion: standard time. Prior to standard time & time zones, each town set their clocks by high noon. This was complicated by the success of the railroads. To prevent train wrecks, railroads set their clocks to consistent times across their areas of service. While effective within a railroad system, connecting to another railroad was complicated. Imagine that you were to board a train in town A, where local time was noon, but railroad time was 11:50 AM, traveling 1 hour to town B, where local time was 1:15 PM but railroad time was 12:50 PM, connecting to another railroad’s train that departed at 1:15 PM on that railroad’s time which is 15 min behind local time in town B. The solution: everyone in a geographic area agree to set their clocks to one time, minimizing the frequencies of changing their watches.

Want to learn more about comparative maps? Check out my free ebook!

© Peter Roehrich, 2016

2 thoughts on “What You Never Knew About the Prime Meridian

  1. Fantastic post Peter! Amazing that something we take so for granted today, (time), was once not the same everywhere! On a purely esthetic level, the last diagram in this post is beautiful, and could today be appreciated for it’s design alone!


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