Troughton's reflecting circle, 1817.
© Science Museum / Science & Society Picture Library
Print of a top and bottom view of 'Troughton's reflecting circle', an astronomical instrument. The engraving, taken from 'Rees Cyclopedia' published 1812-1820, illustrates the working of this hand-held instrument that is used to accurately measure angles between the stars, Sun and planets. Fitted with a mirror and working on the same principle as a sextant, it can be used for navigational purposes. Having a full circular scale it was well suited for measuring wide angles. These were often encountered when using the 'Lunar Method', for finding longitude at sea, where the angle between the Moon and a distant star had to be measured.