A heliometer (from

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

ἥλιος ''hḗlios'' "sun" and ''measure'') is an instrument originally designed for measuring the variation of the

sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

's diameter at different

season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and ...

s of the year, but applied now to the modern form of the instrument which is capable of much wider use. The basic concept is to introduce a split element into a telescope's optical path so as to produce a double image. If one element is moved using a screw

micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...

, precise angle measurements can be made. The simplest arrangement is to split the

object lens In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be a single lens or mirror, or combinations of several optical eleme ...

in half, with one half fixed and the other attached to the micrometer screw and slid along the cut diameter. To measure the diameter of the sun, for example, the micrometer is first adjusted so that the two images of the solar disk coincide (the "zero" position where the split elements form essentially a single element). The micrometer is then adjusted so that diametrically opposite sides of the two images of the solar disk just touch each other. The difference in the two micrometer readings so obtained is the (angular) diameter of the sun. Similarly, a precise measurement of the apparent separation between two nearby stars, ''A'' and ''B'', is made by first superimposing the two images of the stars and then adjusting the double image so that star ''A'' in one image coincides with star ''B'' in the other. The difference in the two micrometer readings so obtained is the apparent separation or angular distance between the two stars. Bamberg_Heliometer_No_3253

The first application of the divided object-glass and the employment of double images in

astronomical Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxi ...

measures is due to Servington Savery of Shilstone in 1743.

Pierre Bouguer Pierre Bouguer () (16 February 1698, Croisic – 15 August 1758, Paris) was a French mathematician, geophysicist, geodesist, and astronomer. He is also known as "the father of naval architecture". Career Bouguer's father, Jean Bouguer, one ...

, in 1748, originated the true conception of measurement by double image without the auxiliary aid of a

filar micrometer A filar micrometer is a specialized eyepiece used in astronomical telescopes for astrometry measurements, in microscopes for specimen measurements, and in alignment and surveying telescopes for measuring angles and distances on nearby objects. Th ...

, that is by changing the distance between two object-glasses of equal focus.

John Dollond John Dollond FRS (10 June O.S. (21 June N.S.) 170630 November 1761) was an English optician, known for his successful optics business and his patenting and commercialization of achromatic doublets. Biography Dollond was the son of a Hugue ...

, in 1754, combined Savary's idea of the divided object-glass with Bouguer's method of measurement, resulting in the construction of the first really practical heliometers. As far as we can ascertain,

Joseph von Fraunhofer Joseph Ritter von Fraunhofer (; ; 6 March 1787 – 7 June 1826) was a German physicist and optical lens manufacturer. He made optical glass, an achromatic telescope, and objective lenses. He also invented the spectroscope and developed diffr ...

, some time not long before 1820, constructed the first heliometer with an achromatic divided object-glass, i.e. the first heliometer of the modern type. The first successful measurements of

stellar parallax Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant objects, and a basis for determining (through trigonometry) the distance of the object. Created by the different orbital p ...

(to determine the distance to a star) were made by

Friedrich Bessel Friedrich Wilhelm Bessel (; 22 July 1784 – 17 March 1846) was a German astronomer, mathematician, physicist, and geodesist. He was the first astronomer who determined reliable values for the distance from the sun to another star by the method ...

in 1838 for the star 61 Cygni using a Fraunhofer heliometer.. This was the aperture Fraunhofer heliometer at Königsberg Observatory built by Joseph von Fraunhofer's firm, though he did not live to see it delivered to Bessel. Although the heliometer was difficult to use, it had certain advantages for Bessel including a wider field of view compared to other

great refractors Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an equatorial mount. The preeminence and success of this style in observational astronomy defines an era in modern telescopy in the 19t ...

of the period, and overcame

atmospheric turbulence An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

in measurements compared to a

Notes

References

* * . *Willach, Rolf. "The Heliometer: Instrument for Gauging Distances in Space." Journal of the Antique Telescope Society, number 26, pp. 5–16 (2004).

External links

{{wiktionary
Photos from the largest heliometer in the world (Kuffner-Observatory, Vienna)
Telescope types Measuring instruments Sun