Prime Meridian

The Royal Observatory in Greenwich is the home of Greenwich Mean Time (GMT) and the Prime Meridian of the world.

What is a meridian?

A meridian is a north-south line selected as the zero reference line for astronomical observations. By comparing thousands of observations taken from the same meridian it is possible to build up an accurate map of the sky.


The line in Greenwich represents the Prime Meridian of the World – Longitude 0º. Every place on Earth is measured in terms of its distance east or west from this line. The line itself divides the eastern and western hemispheres of the Earth – just as the Equator divides the northern and southern hemispheres.

Where is the meridian?

In 1884 the Prime Meridian was defined by the position of the large ‘Transit Circle’ telescope in the Observatory’s Meridian Observatory. The transit circle was built by Sir George Biddell Airy, the 7th Astronomer Royal, in 1850. The cross-hairs in the eyepiece of the Transit Circle precisely defined Longitude 0° for the world. As the earth’s crust is moving very slightly all the time the exact position of the Prime Meridian is now moving very slightly too, but the original reference for the prime meridian of the world remains the Airy Transit Circle in the Royal Observatory, even if the exact location of the line may move to either side of Airy’s meridian.

Observing the Line


8 Comments CherryPie on Sep 12th 2014

8 Responses to “The Prime Meridian”

  1. ....peter:) says:

    From this place that world sets their clocks and the stat of day Cherie… three good pictures….peter:)

  2. Sackerson says:

    I guess the screws in the sign in the first picture had better be removable!

  3. Bernard says:

    As an avid clock collector, I have always been interested in the different ways that ‘man’ has tried over the centuries, to measure time. The text above mentions ‘Greenwich Mean Time’. The word ‘mean’ here implies it is the mean and not the absolute. We just can’t pin it down. I personally love the ‘Sun-time’ as indicated on my garden Sun-dial, but of course this varies with the day, the week and the month – so no good for catching that train to London! Somewhere I read about a Flower Clock; twelve flowers that opened their petals at different hours of the day. Ha! Back to Mother Nature. It suits me fine. :)

  4. liz says:

    Wow! So you can stand with one foot each side of the line and be in two places at once!