Heliometer

Heliometer, an instrument due originally to Savery and Bouguer, and improved a century later by Dollond and Fraunhofer, its purpose being to measure the angular diameter of the sun, or any other heavenly body with an appreciable disc, or the angular distance between any two bodies near each other. One form consists of an object-glass cut into halves; each half gives a separate image of the heavenly body, but as the halves are made to slide apart the images draw nearer to each other and ultimately coincide. The distance apart of the two halves measures the required angular distance.