Editing Solar constant (section)

==Historical measurements==
In 1838, [[Claude Pouillet]] made the first estimate of the solar constant. Using a very simple [[pyrheliometer]] he developed, he obtained a value of 1.228&nbsp;kW/m<sup>2</sup>,<ref>[http://documents.irevues.inist.fr/bitstream/handle/2042/16943/meteo_2008_60_36.pdf'' The measurement of the solar constant by Claude Pouillet''], by J-L Dufresne, [http://www.smf.asso.fr/lameteo.html ''La Météorologie''] {{webarchive|url=https://web.archive.org/web/20100305140149/http://www.smf.asso.fr/lameteo.html |date=2010-03-05}}, No. 60, pp. 36–43, Feb. 2008.</ref> close to the current estimate.

In 1875, [[Jules Violle]] resumed the work of Pouillet and offered a somewhat larger estimate of 1.7&nbsp;kW/m<sup>2</sup> based, in part, on a measurement that he made from [[Mont Blanc]] in France.

In 1884, [[Samuel Pierpont Langley]] attempted to estimate the solar constant from [[Mount Whitney]] in California. By taking readings at different times of day, he tried to correct for effects due to atmospheric absorption. However, the final value he proposed, 2.903&nbsp;kW/m<sup>2</sup>, was much too large.

[[File:Solar radiation Langley 1903.png|thumb|A 1903 Langley bolograph with an erroneous solar constant of 2.54 calories/minute/square centimeter.]] Between 1902 and 1957, measurements by [[Charles Greeley Abbot]] and others at various high-altitude sites found values between 1.322 and 1.465&nbsp;kW/m<sup>2</sup>. Abbot showed that one of Langley's corrections was erroneously applied. Abbot's results varied between 1.89 and 2.22 calories (1.318 to 1.548&nbsp;&nbsp;kW/m<sup>2</sup>), a variation that appeared to be due to the Sun and not the Earth's atmosphere.<ref>{{EB1911|wstitle=Sun |volume=26 |page=87 |inline=1 |first=Ralph Allen |last=Sampson |author-link=Ralph Allan Sampson}}</ref>

In 1954 the solar constant was evaluated as 2.00 cal/min/cm<sup>2</sup> ± 2%.<ref>{{cite journal |title=The Solar Constant |author=Francis S. Johnson |journal= Journal of Meteorology |volume=11 |number=6 |date=December 1954 |doi=10.1175/1520-0469(1954)011<0431:TSC>2.0.CO;2 |pages=431–439|bibcode= 1954JAtS...11..432J|doi-access=free }}</ref> Current results are about 2.5 percent lower.