heliograph / sunshine recorder

A meteorological instrument, also known as a heliophanograph, that allows a daily record to be obtained of the time intervals during which solar radiation strikes directly or, failing that, reaches the Earth's surface with sufficient intensity to produce well-defined shadows. This latter circumstance occurs when there is a thin layer of high cloud in the sky. The heliograph most commonly used in weather observatories is the so-called Campbell-Stokes type; named in honor of John Francis Campbell (1822-1885), who invented it in 1853, and George Gabriel Stokes (1819-1903), who perfected it in 1879. Its main element is a solid glass sphere, 96 mm in diameter, mounted on a metal support shaped like a spherical cap, on which is placed, fitted between two grooves, a graduated cardboard strip printed with the hours. The sphere acts as a converging lens, whatever the direction from which sunlight strikes. When the sun emerges over the horizon, the light strikes the sphere and it concentrates the rays onto a point of the card, burning it. As the sun rises, the point of convergence changes, forming a charred trace on the strip, which is only interrupted during the time the sun is covered by clouds. There are three different positions where the cardboard strip can be fixed, as well as three different strip models (a straight one, a short curved one, and a long curved one), since depending on the time of year, the sun reaches a greater or lesser height above the horizon, also varying the inclination of its path relative to the ground. Each day a strip is placed in the heliograph, on which the hours of sunshine are recorded.