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.
Hemispheres
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.
From this place that world sets their clocks and the stat of day Cherie… three good pictures….peter:)
Thanks Peter
I guess the screws in the sign in the first picture had better be removable!
I think they probably are which is more than can be said about the line on the ground outside the building.
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.
The flower clock sounds fascinating it would be lovely to see that one.
Time is very illusive and it always seems to be speeding up!
Wow! So you can stand with one foot each side of the line and be in two places at once!
An interesting thought isn’t it?