This instrument, produced by the Società Italiana Apparecchi di Precisione (SIAP), is based on the system developed in 1853 by Irish meteorologist John Francis Campbell and perfected in 1879 by Sir George Gabriel Stokes. The Campbell–Stokes model became the international standard for measuring the duration of sunshine. This heliophanometer was purchased to replace an older model, supplied by the Central Bureau of Meteorology in the late 19th century.
Its operation is simple and ingenious: a glass sphere acts as a converging lens, focusing the sun's rays onto a strip of photosensitive cardboard or paper. When the intensity of the radiation exceeds a minimum threshold the paper blackens or burns, leaving a visible trace. The length of this trace corresponds to the time the Sun was actually visible, unobstructed by clouds.
Used worldwide for its ability to record solar energy reaching the Earth's surface, the heliophanometer has allowed for the construction of long time series of data, which is fundamental to climatology. These measurements are essential for studying regional and long-term variations in insolation, crucial for understanding the Earth's energy balance, heat distribution, and the influence of cloud cover on contemporary climate patterns. Data collected with instruments like this still contribute to global climate models, such as those used for reports by the Intergovernmental Panel on Climate Change.
Empirical records of cloud cover obtained by historical sunshine recorders offer valuable insights into one of the most debated mechanisms of interaction between the cosmos and climate: the influence of galactic cosmic rays (GCRs) on cloud formation. The GCR flux, modulated by solar activity, increases during solar minima and decreases during solar maxima. According to one of the most widely accepted hypotheses, GCRs ionize the air in the lower atmosphere, promoting the creation of condensation nuclei and thus low clouds, which reflect solar radiation with a cooling effect. In this context, historical sunshine series recorded by heliophanometers become a fundamental resource for verifying and backdating these theories on the link between the solar cycle, cosmic rays, and Earth's climate.
But the study of the Sun is not limited to climatology alone. Variations in irradiance are closely linked to solar magnetic activity, responsible for so-called space weather, which investigates the effects of extreme solar variations, such as flares and coronal mass ejections, on the space environment and on planet Earth.
In this context, the heliograph represents a historical starting point: a tool which inaugurated the long tradition of measuring solar energy, now at the center of research that unites climatology, solar physics, and planetary security.

___Clementina Sasso