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And an angle from heaven did come down

And an angle from heaven did come down: imagine the sky as an enormous clock, with the moon its hand and a dial of stars. That was how the old lunar-distance method of measuring longitude worked, and that's exactly what this 16th-century engraving - from Peter Apian's Introductio Geographica, published in Ingolstadt in 1533 - depicts.

The race to measure longitude - the lines girdling the globe along a north-south, "Chocolate Orange" axis - was a great drama of the early navigational age. It was "a daunting, multifaceted technological challenge ... (which) continued to elude the greatest scientific minds for more than two centuries," as William J H Andrewes, Curator of the Collection of Scientific Instruments at Harvard, explains. Without an accurate method for plotting their courses, ships were constantly disappearing. The whole colonial adventure was floundering at sea. In 1598, Philip III of Spain offered a 6,000-ducat pension to whomever might come up with a solution to the problem. The pension was still unclaimed in 1714, when Queen Anne's government put up a new pounds 20,000 reward.

Dava Sobel's Longitude, the story of John Harrison, the self-taught Yorkshire carpenter who invented the maritime clock and eventually won the jackpot, was the surprise smash bestseller in Britain and the US last year. It was inspired by the scholarly researches of Andrewes and others, now collected in The Quest for Longitude (Harvard, pounds 49.95). All you could ever want to know about celestial geometry, meridian transits, maps, compasses and the innards of clocks, wonderfully illustrated in colour and black-and-white.