HELIOMETER (ἥλιος, sun, and μέτρον, measure), the name given by Bouguer, to a kind of double-image micrometer for measuring the diameters of the stars, and especially those of the sun and moon, or any small apparent distance between the heavenly bodies. Mr Savary of Exeter communicated to the Royal Society, in the year 1743, an account of a double-image micrometer, from which the heliometer proposed by Bouguer, five years afterwards (1748), does not differ in construction. This instrument is described under the head MICROMETER, in which the various improvements it has received are given in detail.