Seven inch heliometer, Cape Town, South Africa, 1909.
© Science Museum / Science & Society Picture Library
Photograph of an equatorially mounted heliometer, of 7 inch aperture, at the Cape Observatory, in South Africa. Made by Repsold and Sons, Hamburg in Germany, the instrument has a split main lens that is used to measure very precisely the angular separation of objects in the heavens. Sir David Gill (1843-1914), Scottish astronomer, used this heliometer to determine stellar and solar parallax to measure the distance to nearby stars and that of the Sun.