Cabinet of curiosities: the heliograph, a device that “records” sunlight without electricity

Today, perhaps more than ever, there is no doubt that the weather and on a smaller scale, the weather plays a major role in our lives. Capable of promoting the proliferation of life or, on the contrary, of stopping its progression, weather variations were surely one of the first signals that humans learned to use to interpret and predict their environment. And over the course of our evolution, new means of measurement have started to emerge to better quantify them. Half a millennium before the common erathe ancient Greeks traced the first sketches of what we would nowadays call rainfall. In -400, the Indians installed containers intended to collect rain around their crops to obtain a better appreciation of the conditions and constraints to which they would have to adapt.

The first means of thermometry were imagined at the beginning of the Common Era, then developed between the XVI^e and the XVIII^e century to finally give birth to thermometers modern in 1724. In 1802, the nephologist Luke Howard proposed a nomenclature for classify clouds. Snow and hail episodes are also scrupulously observed, hailstones measured and results reported. But the Sunfor its part, remains recalcitrant to these detailed examinations. How to quantify the daily duration sunshine without keeping the eyes constantly riveted to a sky punctuated by clouds ? How to estimate its intensity without losing its retina ? The solution was provided by an extravagant Celtist by the name of John Francis Campbell, in 1853.

The Highlander and the Sun

Although descended from a wealthy family of landowners, John Campbell knows a very different youth of that usually experienced by children from his social background. His father Democrat indeed places him under the tutelage of a bagpiper alongside whom he is trained in the culture and mentality of the Highlands, a heritage he will defend with fervor throughout his life. Insatiable curious and a great adventurer, John Campbell learned to speak eight languages, traveled the world and explored the field of culture as well as that of science. If he acquired his fame by becoming a brilliant collector of Gaelic tales (thus giving the oral tradition a chance to be preserved in writing), he was also interested in the geology and, you guessed it, at the meteorology.

However, in 1853 therefore, John Campbell has an idea : record sunshine by exploiting theenergy even the sun. His invention was simple – it consisted of a simple glass sphere placed in a glass bowl. drink – but its elegance lies in its ingenuity. As it moves across the sky, the sun projects its rays onto the sphere which concentrates them like a magnifying glass. The beam of light The resulting intense fire burns the wood more or less markedly depending on the amount of sunlight, thus creating an arc (a heliogram) in the bowl as the day progresses. Placed in a sufficiently clear place so that the light is not obstructed, the device thus makes it possible to record the duration and the amount of sunshine over the weeks and months, gradually calcining the inside of the bowl.

Campbell-Stokes heliograph

Campbell takes hundreds of measurements with his “recording sundial” (now more readily responding to the name ofheliograph). He tested it, perfected it, took it to Egypt to put it to the test of the scorching sun and then compiled its results in a report commissioned by the House of Commons in 1857. Despite the success of his invention however, he was somewhat frustrated by its lack of precision. A derivative of his device in operation at Greenwich Observatory uses a bowl of metal in which a strip of waterproofed fabric is placed and then changed daily, thus providing a much clearer and more detailed result. Campbell, inspired by this iteration is experimenting with different types of materials, but he is a mathematician and physicist which will give the heliograph the shape we know it today.

You may have already heard his name. It is found, among others, in the number of Stokes, the law of Stokes or the equations by Navier-Stokes. Another brilliant researcher is Sir George Gabriel (Stokes), this time in the fields of fluid mechanics, optics and geodesy. In 1879, he proposed his own improvement of Campbell’s heliograph. The glass sphere is mounted on a semi-circular axis allowing its orientation to be adjusted according to the latitude of the user. Between the sphere and the axis, a metal crescent with three notches can accommodate the strips of paper that will be used to record solar radiation. Each notch allows the paper to be placed according to the distance to theequator user, and three types of bands can be used depending on the season (and therefore the position of the sun in the sky), thus providing a heliogram of great precision.

The heliograph still shines

Note all the same that despite these improvements, the device remains imperfect. At thedawn and at duskthe sun’s rays pass through a larger portion of theatmosphere and meet so much attenuated that they fail to leave their mark on paper. The sunshine measurement can also be greatly degraded by a very changeable sky and in polar regions, the usefulness of the heliograph is quickly compromised when the sphere is covered with a thick layer of gel.

Nevertheless, despite all these disadvantages (for which more or less satisfactory solutions have been found over time), the Campbell-Stokes heliograph remains widely used today and provides valuable data to the fields of meteorology. , energetics or ecology. In 1964, the World Meteorological Organization (WMO) made it the interim reference heliograph for all member countries and provided a list of indications to standardize its use and reading. At the poles, the solar recorder is now accompanied by its own heating system to fight against the icy winds. So let’s make sure we keep it, too, in a sunny place on the shelves of our Cabinet of curiosities.