Gregorian reflecting telescope with detachable heliometer, 1763-1765

Split-lens micrometer attached to a Gregorian reflecting telescope, 1763-65.
Dated to the mid-eighteenth century this reflecting telescope was made by the famous London instrument maker, James Short. 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 First Voyage when he was sent to Tahiti to view this rare event. By timing the transit from far-flung parts on the world, it was possible 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 originally used to measure the angular size of the Sun.
Year made :
Inventory number :