For many centuries Greenwich Observatory was the world's leading astronomical establishment, and it marks the 'zero for longitude', dividing the world into two hemispheres. More than 20 years ago, the telescopes were shifted to the clearer skies of Sussex; but now the largest of the old telescopes, the 28-inch refractor, has been brought back and is in full operation once more.

Patrick Moore and Dr Percy Seymour, of the National Maritime Museum, discuss this new development, and explain the modern role of the Greenwich refractor, which is still the largest telescope of its kind in the British Isles.