Airy transit circle, Greenwich, London, 1914.
© Science Museum / Science & Society Picture Library
Photograph showing Airy's Transit Circle installed at the Royal Observatory, Greenwich in 1850. Designed by the Astronomer Royal, George Biddell Airy (1801-1892), the instrument was used make observations on the north-south meridian. Used in conjunction with very accurate timepieces, called regulator clocks, an observer would note the instance an object crossed the meridian. This was used to regulate and check timepieces, as well as accurately determine the position of the stars, planets, Moon and Sun in the Sky. The instrument also defines zero degrees longitude and is therefore the prime meridian of the world. Agreed in 1885 at a Washington conference, the matter was determined by the predominance of the world's shipping using the Greenwich meridian for their nautical maps.