People often ask why the Greenwich Meridian is where it is and which came first, the line or the Observatory. The short answer is that the Observatory came first and the line is where it is because of the Observatory. But it's a bit more complicated than that!

The Greenwich Meridian

A meridian is any imaginary line running from the North Pole to the South Pole. Astronomers who wanted to plot the positions of stars and planets set their telescopes up on a meridian, which they used as a reference line for their measurements and charts.

In 1850 the Astronomer Royal at that time, George Airy, ordered a new and much more accurate telescope, which he designed himself. It was much larger than previous telescopes so he had to find firms who were capable of making it.

The seventh Astronomer Royal, George Biddell Airy

For the metal castings he chose the engineers Ransomes and Sims of Ipswich. They specialised in agricultural machinery, but had some of the best manufacturing facilities in the country and were related to Airy through his mother. The lenses and other optical parts were made by a well-known firm of specialist opticians in London, Troughton and Simms. They had made other telescopes for the Observatory, so Airy was confident of their ability.

Airy's transit circle, completed in 1851

The new design was for a type of telescope called a transit circle. The name comes from the fact that it was fixed on a north-south line for the observation of stars as they crossed, or transited, the meridian. It also had a circular scale for measuring the height of the stars above the horizon. The line through the centre of this telescope became the Greenwich Meridian.