'Taking a Lunar Distance', 1891.
© Science Museum / Science & Society Picture Library
Engraving showing a naval officer measuring the position of the Moon with respect to the fixed stars from the deck of a ship at sea. Using a sextant, a navigational instrument, the sailor would be able to accurately determine the angle between certain stars and the lunar limb. With this measurement it was posible using the Nautical Almanac (navigational ephemeris) to calculate one's longitude at sea. Known as the 'Lunar Method' this technique required hours of mathematical calculations to be succesful. From 'The Midnight Sky: Familiar Notes on the Stars and Planets' by Edwin Dunkin (1821-1898), a popular account of the stars and planets published in 1891. Dunkin was chief asistant at the Royal Observatory, Greenwich.