helioseographs

A helioseograph is an instrument used to record the duration of sunshine. It works by focusing the sun's rays onto a specially designed paper or card, causing it to scorch or burn. The path traced by the focused sunlight over the course of a day creates a visible record of the sun's presence. Early helioseographs typically used a spherical lens or a parabolic mirror to concentrate the sunlight. As the sun moves across the sky, the point of focus shifts, and the scorch mark lengthens. The length of the scorch mark is then directly proportional to the amount of time the sun was visible.

Different types of helioseographs exist, with the Campbell-Stokes recorder being one of the most common and widely used historical designs. This instrument features a glass sphere that magnifies the sun's rays and burns a line onto a card inserted into a grooved frame. Another type is the Jordan sunshine recorder, which uses a different optical principle. Modern meteorological services often employ electronic sunshine recorders that use photodetectors to measure solar radiation intensity, providing a more precise and continuous record of sunshine duration. While older helioseographs provided a direct physical record, modern instruments offer digital data that can be easily analyzed and integrated into weather forecasting systems. The data collected by helioseographs is valuable for climatological studies, agricultural planning, and understanding solar energy potential.