HELIOMETER, an instrument originally designed for measuring the variation of the sun's diameter at different seasons of the year (named from Gr. i X cos, sun, and Or a measure) . By subsequent improvements it became one of the most accurate astronomical instruments for measuring angular distances between stars, but its work is now done by photography and the heliom eter is rarely used.
In the standard heliometer the object-glass is cut into two semi-circular segments. In the zero position the two halves form practically a single object-glass; they can be displaced by sliding their straight edges over one another, the amount of displacement being measured by a screw; a double image of each star is thus formed, i.e., one by each half-lens. If the distance between two stars is being measured, the observation consists in turning the screw until the image from one star coincides with that from the other. For greater precision it is found desirable to move both segments an equal amount in opposite directions, so that the centre of the field of view where the observations of coincidence are made remains a centre of symmetry. Distances up to 2° are measured with these instruments.
The first use of a divided object-glass to give a double image is due to Servington Savary in 1743, but his segments were not movable. Pierre Bouguer (1748) invented an instrument used in the manner of a modern heliometer but employing two complete lenses instead of half-lenses to give the double image. In John Dollond combined Savary's idea of the divided object-glass with Bouguer's method and constructed the first really practical heliometer. The heliometer acquired a great reputation through the work of Bessel at Konigsberg, who used it to measure the parallax of 61 Cygni. Many instruments were made for the transit of Venus expeditions in 1874. The last great heliometer observer was Sir David Gill; his measurements of the parallax of southern stars remained for a long while unsurpassed ; he used it also for his important determinations of the solar parallax by observations of Mars and certain minor planets.
See Bessel, Astronomische Untersuchungen, vol. i. ; Gill, Annals of Cape Observatory, vol. vii., pp. 1-71.