1799-1801: The Sun's invisible radiation William Herschell and the infrared; J.W. Ritter and the ultraviolet


1838: The solar constant (first try)

The solar constant is a measure of the sun's luminosity, and is defined by convection as the amount of energy incident per second on one square meter of the Earth's atmosphere, when the later is at a distance of one astronomical unit (1,495,985 km) from the Sun.


Pouillet's pyrrhometer. Water is contained in the cylindrical container a, with the sun-facing side b painted black. The thermometer d is shielded from the Sun by the contained, and the circular plate e is used to align the instrument by ensuring that the container's shadow is entirely projected upon it. [Reproduced from A.C. Young's The Sun (revised edition, 1897). Although various scientists had attempted to calculate the Sun's energy output, the first attempts at a direct measurement were carried out independently and more or less simultaneously by the French physicist Claude Pouillet (1790-1868) and British astronomer
John Herschel (1792-1871). Although they each designed different apparatus, the underlying principles were the same: a known mass of water is exposed to sunlight for a fixed period of time, and the accompanying rise in temperature recorded with a thermometer. The energy input rate from sunlight is then readily calculated, knowing the heat capacity of water. Their inferred value for the solar constant was about half the accepted modern value of 1367 +/- 4 Watt per square meter, because they failed to account for of absorption by the Earth's atmosphere. 1861: The solar diffrential rotation


1881: The solar constant (second try)

By the second half of the nineteenth century, through various solar observing expedition to mountaintops, it was becoming increasingly clear that the Earth's atmosphere absorbs a significant portion of the sun's luminosity. Consequently, attempts at determining the solar constant were moved to the highest practical altitudes.


Langley's 1881 base camp on California's Mt Whitney. Some of Langley's instruments failed to arrive or arrived damaged on the mountain, with the crucial spectral bolometer back in working order only by the end of August. The expedition was cut short on 8 September due to worsening observing conditions caused by the breakout of a series of wildfires elsewhere in California a few days earlier. Nonetheless, valuable data were collected. Reproduced from: Eddy, J.A. 1990, J. Hist. Astron., 21, p. 115. The american scientist
Samuel Langley (1834-1906) carried out the most elaborate attempt at determining the solar constant at the time, during an expedition to Mt Whitney, California, in July 1881. Using his recently invented bolometer (an instrument based on the varying electrical resistivity of metals with temperature), as well as other instruments, Langley carried out measurements at different wavelengths and at different altitudes, demonstrating the strong variation with wavelength of the absorption by Earth's atmosphere. However, the solar constant value he calculated at the time, 2903 Watt per square meters, is nearly a factor of two larger than the modern value (1367 W/m^2), something apparently due to errors in the data reduction procedure, since Langley's later assistant Charles Abbot (1872-1973) obtained 1465 W/m^2 with the original Mt Whitney data.