Energy and Thermal Physics

Measuring the Sun

Stories from Physics for 11-14 14-16 IOP RESOURCES

Perhaps the first quantitative measurement of solar radiation reaching the Earth was carried out by the French physicist Claude Pouillet. In the 1830s, Pouillet constructed a double-walled cylinder 61 cm long and of 10 cm radius. He filled the space between the walls with ice. At one end, he placed a piece of opaque material, with a pinhole drilled into it, that allowed a beam of sunlight to enter the interior of the double cylinder and to fall on the blackened bulb of a thermometer inserted through the opposite end of the tube. Using a calculated value of the heat capacity of the instrument, he could estimate the solar radiation falling on the device. Pouillet observed that, at noon on the summer solstice, solar radiation caused the thermometer reading to increase by 7.5°C. From his data, he calculated that the annual solar radiation would be sufficient to melt a layer of ice surrounding the globe 14 m thick.

Whilst on a tour of Europe, John Herschel, the son of the German British astronomer Frederick William Herschel, travelled over an Alpine pass and recorded in his journal the effects of solar radiation: “Vision quite scorched with the € [the Sun] & found sensation dreadful.” He noted that this experience sparked a curiosity to investigate the power output of the star:

…the scorching effect of the Sun’s rays upon every exposed part of the skin proved so severe as to excite in my mind a lively desire to subject to some precise means of measurement the cause of so disagreeable an effect.

Herschel invented a new instrument, the actinometer, to study solar radiation. The device consisted of a thermometer-like device but the thermometer tube, rather than terminating in the usual bulb, was attached to a cylinder with a moveable metal cap that could be adjusted by turning a screw. The cylinder was filled with copper sulphate solution (a dark colour to absorb radiation) and placed in a box shielded on three sides and with a glass wall on the fourth. A comparison of the average heating in direct sunlight with the mean cooling in the shade gave a measure of the effect of solar radiation. Whilst the device did not allow Herschel to make quantitative estimates of solar radiation, he concluded that the Sun’s surface must be hotter than a furnace.

In 1838, Herschel was working at the Cape of Good Hope, carrying out astronomical observations, including the production of a catalogue of nebulae. He returned to his interest in solar radiation by conducting an experiment using a tin cylinder, filled with inky water, within a larger iron container with a circular hole drilled in it that allowed light to fall on the tin vessel. From data collected using the instrument, Herschel proposed a unit of solar radiation, the actine, which was defined as the intensity of vertically incident radiation that will melt a layer of ice one micrometre thick in one minute. He calculated the yearly solar radiation intensity would be sufficient to melt a layer of ice 26.652 m deep across the surface of the planet.

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