Melvyn Bragg and guests discuss the relationship between astronomy and the British Empire. The 18th century explorer and astronomer James Cook wrote: 'Ambition leads me not only farther than any other man has been before me, but as far as I think it possible for man to go'. Cook's ambition took him to the far reaches of the Pacific and led to astronomical observations which measured the distance of Venus to the Sun with unprecedented accuracy. Cook's ambition was not just personal and astronomical. It represented the colonial ambition of the British Empire which was linked inextricably with science and trade. The discoveries about the Transit of Venus, made on Cook's voyage to Tahiti, marked the beginning of a period of expansion by the British which relied on maritime navigation based on astronomical knowledge. With Simon Schaffer, Professor in History and Philosophy of Science at the University of Cambridge; Kristen Lippincott, former Director of the Royal Observatory, Greenwich; Allan Chapman, Historian of Science at the History Faculty at Oxford University.