Angelo Secchi, Roma, 1863
Brass, wood
hight 60 cm
Museum of INAF-Astronomical Observatory of Rome, Monte Porzio Catone (Rome)

Among Angelo Secchi’s contributions to solar physics should be remembered the conception of the thermo-Heliometer, shown here (Figure 1). Made in 1863, it is a tool that allowed to estimate the energy irradiated by the sun through a differential measurement of temperature. It is one of the first attempts to measure the solar constant, i.e. the energy irradiated by the sun at all wavelengths, even those outside the visible spectrum.
The sunlight penetrated the instrument through a small circular 25 mm diamater hole, and it heated the water placed in the small boiler inside and thermally insulated from the surrounding environment (Figure 2a-b). Through the two internal thermometers (one exposed to sunlight, the other one in contact with water), the temperature variation of the liquid was measured over a period of time, allowing to retrieve the amount of solar radiation.
The instrument is supported by a mount of the type used for theodolites, and can be levelled by means of the three screws and the two levels placed at the base. On the base is inserted a small compass, useful for the orientation in azimuth.
The measurements carried out with this device were also aimed at meteorology studies and were part of the search for correlations between the solar and atmospheric phenomena that characterized the scientific approach of Secchi.
The first determination of the solar constant was obtained by Samuel Langley in 1881, thanks to the invention of the bolometer, a tool capable of detecting a temperature change of less than 1/100,000 degree Celsius.

Aldo Altamore - Università degli Studi “Roma Tre”
Marco Faccini - INAF-Osservatorio Astronomico di Roma