Detail showing a heliometer, a split-lens micrometer originally used to measure the angular size of the Sun. Made by the famous London instrument maker Peter Dollond (1730-1820), it is fitted to a large reflecting telescope. It is one of several with matching telescopes ordered by the Royal Society of London to be taken on expeditions to observe and time the transit of Venus in 1769. It may have been used on Captain Cook's (1728-1779) first voyage when he was sent to Tahiti to view this rare event. By timing the transit from far-flung parts of the world it was posible for the first time to determine an accurate distance to the Sun, and hence the size of the Solar System.
© Science Museum / Science & Society Picture Library