Mendoza's circle, 1812.
© Science Museum / Science & Society Picture Library
Print with diagrams of 'Mendoza's 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. 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.