Gregorian reflecting telescope, 1763-65.
© Science Museum / Science & Society Picture Library
This reflecting telescope was made by the famous London instrument maker, James Short (1710-1768). It is one of several 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. The telescope was supplied with a heliometer, a split-lens micrometer (shown here attached to the front of the telescope) which was originally used to measure the angular size of the Sun.