21 March – 21 June 2016 in Florence, Rome, Naples, Palermo, Padua


Angelo Secchi (1818-1878), Rome, 1863

Brass, wood;
Height 60 cm

INAF-Rome Astronomical Observatory, [n.i. M0023/2493]

This instrument was invented in 1863 by Angelo Secchi, with the purpose of estimating the energy radiated by the Sun by means of the recorded differences in temperature. This was one of the first attempts to determine the solar constant, i.e. the energy radiated by the Sun in all wavelengths, including those out of the visible spectrum. Secchi presented the instrument as a modified version of a devise invented by the Scottish physicist John James Waterston (1811-1883): it consisted of an airtight chamber with dark walls which, after the insertion of a thermometer, was rendered airless; the solar radiation entered the chamber via a small hole. In Secchi’s thermos-heliometer the sunlight entering through a circular hole of 25 mm diameter, would warm the water placed inside a small boiler, which was thermally insulated from the environment by a wooden layer. By means of two internal thermometers (one being exposed to solar rays, the second one to water) the variation in temperature of the fluid in given time could be determined, thus allowing the amount of energies coming from the Sun, to be estimated. The instrument leans on a mounting similar to those in use for theodolites, and can be evened out by means of three adjusting screw and two levels resting on the base. The base also presents a small compass, to help azimuthal orientation. The measurements made with this instrument were also used in meteorology and were part of the research correlating solar phenomena with atmospheric ones, typical of Secchi’s scientific approach. The solar constant was eventually determined by Samuel Langley, who studied especially the infrared region of the solar spectrum. [a.a., m.f., f.p.]

Bibl.: Calisi in Chinnici (ed.) 2009, p. 200; Secchi 1863, p. 105; Altamore 2012, pp. 138 e ss.