An instrument formerly used for measuring the diameter of the Sun or the angular separation between close stars. A heliometer is a refracting telescope whose objective lens is split in half, so that the two halves can be moved along the dividing line. This allows two star images or opposite limbs of the Sun to be superimposed. The separation of the lenses and their position angle could be converted into angular measure. Heliometers were regularly used in the nineteenth century to measure the parallax of stars and angular diameters of planets, but have now been superseded.