Edit post Follow this blog Administration + Create my blog

Published by jack elliot

The courtyard of the Royal Observatory in Greenwich, England is home to a strip,

first made of brass and now replaced by stainless steel,

that historically marked zero degrees longitude.

This meant that on one side of it lay the Eastern hemisphere, with the Western hemisphere on the other.

Whereas the parallels of latitude are defined by their distances from the Equator and the Poles, which are mathematically defined regions, longitudinal lines are imaginary north-south lines that pass through both geographical Poles. The choice of the prime meridian among these was purely arbitrary, but not before they were agreed upon by the scientific community.



Before we look at how Greenwich was chosen as the home of the prime meridian, we will have to turn our attention to a transit circle. Just like any position on the Earth can be given using its latitude and longitude, the position of a star can be plotted using its declination and right ascension – two parameters that can be measured using a transit circle.

A mathematician and astronomer, Sir George Biddell Airy was the Astronomer Royal of the Royal Greenwich Observatory from 1835 to 1881. A stickler for perfection, Airy believed that it was the Observatory’s duty to chart stars and planets, which could then be used for navigation. The Airy Transit Circle was built to address this need in 1851.


It is interesting to note that navigation at sea and the charting of stars during this era was a largely local affair, with preferences based on religion and nationality. Maps did exist, but these were often based on longitude east or west of a number of popular cities. In fact, the reference meridians that were employed nearly spanned a third of the entire globe.

The need for a prime meridian

With nations and continents linked like never before, the need for standardisation became paramount, not just from a scientific perspective, but also because it made commercial sense. It was under these circumstances that the U.S. President Chester Alan Arthur convened the International Meridian Conference in Washington in 1884.

The conference was attended by delegates from 25 countries but the choice of Greenwich meridian as the prime meridian was in fact a foregone conclusion. The vote to choose Greenwich was passed, 22 to 1, on October 13, 1884. Whereas France and Brazil abstained, diplomatically, San Domingo (now the Dominican Republic) voted against it. The conference also decided that Greenwich Mean Time (GMT) will be the standard for astronomy

and setting time zones.


There were two main reasons why the conference was merely a formality in selecting the prime meridian. One was the fact that the U.S., the rising power in the Western hemisphere, threw its weight behind the proposal as it had already adopted the Greenwich meridian, both for navigation and as the basis for its own national time zone system. Secondly, 72% of the world’s commerce in late 19th Century already depended on nautical charts based on Greenwich.

As the Greenwich meridian was defined based on the position of the Airy Transit Circle, the prime meridian was also now officially situated there. The precise definition of longitude zero degrees in the world therefore corresponded to the cross-hairs in the eyepiece of the Airy Transit Circle.


We refer to this prime meridian as the historic prime meridian now as the true prime meridian of the world no longer resides along that line. Based on satellite data and many other measurements, a prime meridian that defined a plane passing through the centre of the Earth was required and chosen late in the 20th Century. The IERS Reference Meridian (IRM), also known as the International Reference Meridian, is maintained by the International Earth Rotation and Reference Systems Service (IERS), a body, as the name suggests, responsible for maintaining global time and other reference frame standards.

The IRM is the prime meridian of our world now, meaning that it defines 0° longitude according to international agreement. The historic prime meridian is actually in the Western hemisphere now as the IRM passes 102.5 m to the east of the historic prime meridian at the latitude of the Airy Transit Circle. This, however, hasn’t stopped visitors to the Royal Observatory clicking selfies with the strip marking the historic prime meridian or having photos taken with their legs spread across the strip, in the assumption that they have one foot in each hemisphere.

We wouldn’t want to spoil their fun,

would we?

To be informed of the latest articles, subscribe:
Comment on this post