Charles Greeley Abbot was an American astrophysicist, inventor and administrator, and a man of considerable technical ability. He studied at the Massachusetts Institute of Technology, at first in chemical engineering, but after two years changed to physics. After a year of post-graduate studies he was awarded the degree Master of Science (MS) in 1895 and was immediately appointed as assistant to Samuel Pierpont Langley, the first director of the Astrophysical Observatory of the Smithsonian Institution. Langley believed that careful observations of variations in the sun's energy output might by used to forecast the weather some time in advance and he began regular measurements for this purpose in 1902. Abbot assisted Langley in mapping the infrared spectrum of the sun and in determining the solar constant, that is, the total energy per square meter per second received by the earth from the sun above the earth's atmosphere. Several of the instruments they used were designed or adapted by Abbot. After Langley's death in 1906 Abbot was appointed director of the Astrophysical Observatory in 1907, a post he held to his retirement in 1944. During these years the observatory was dedicated mainly to the study of possible variations in the sun's output. He was soon convinced that there were significant variations in the solar constant - up to a few percent - and that these variations had an effect on the earth's weather. To prove the solar nature of the variations, atmospheric losses had to be accurately compensated for. This required several solar radiation monitoring stations, situated on desert mountain tops in various parts of the earth. Such stations were established in Chile and in Arizona in 1920. By 1925 Abbot felt a need for an observatory outside the Americas and with a grant from the National Geographic Society visited Algeria, Egypt, Baluchistan and South West Africa (now Namibia) to select a suitable site.
He arrived in South Africa early in March 1926, obtained the cooperation of the South African government (which administered South West Africa under a mandate at the time) and consulted Dr R.T.A. Innes* and other local experts before going to Namibia. After considering the region around Aus and the isolated peak of the Spitzkoppe and consulting local experts he decided that the observatory should be erected on Brukkaros Mountain, about 100 km north of Keetmanshoop. He spent the day of 17 March exploring the mountain in the company of Mr. A. Dryden, inspector of works at Keetmanshoop, who played a major role in planning and building the station. A road was first built from the Nama settlement of Berseba into the foothills of the mountain, for a distance of some 15 km. From its end a footpath of about three kilometers was built to provide access to the site. The site provisionally selected by Abbot proved unsuitable and Dryden chose a site higher up, just below the rim of the ring mountain. The solar radiation station was completed in September 1926 and observations were made there by field director William H. Hoover*, assisted by Frederick A. Greeley, until September 1929 and thereafter by field director Louis O. Sordahl* and assistants until December 1931, when the station was closed down and replaced by a new station in Egypt. [For the instruments and observations see Hoover, W.H.]
The daily values of the solar constant obtained at Brukkaros from December 1926 to December 1930 were published in Volume 5 of the Annals of the Astrophysical Observatory and turned out to be somewhat unstable. It appears that meteorological conditions made the compensation for atmospheric absorption inaccurate. First, the solar station was at a relatively low elevation, with the result that atmospheric absorption was both substantial and variable. Second, the site proved to be more windy than anticipated. The strong winds often carried dust over the mountain and also affected the stability of the instruments. Some years later, when reporting recomputed daily solar constant values for the period 1923 to 1939, Abbot had distanced himself from the Brukkaros results, describing them as inferior and subject to sources of error that were later eliminated. However, the variations he derived from the observations at the other solar stations did not convince most meteorologists and were ascribed to uncorrected variations in atmospheric absorption. This view was largely confirmed by later satellite observations above the earth's atmosphere.
Abbot's most important contribution to astrophysics was his accurate determination of the previously poorly established solar constant. His work also stimulated interest in possible variations in the solar constant. Small variations were in fact later found, caused by changes in the number and intensity of sunspots and faculae on the sun's surface. The 3000 measurements of solar irradiance made at Brukkaros were made available to the South African Weather Bureau when it initiated a country-wide radiation survey in 1952. At that time they constituted the longest series of measurements of the solar radiation incident on the central plateau of southern Africa. The measurements were used to calculate the solar irradiance available at various times during the day and year for possible industrial applications, and to estimate the amount of total (sun plus sky) radiation for climatological purposes.
Abbot was awarded a doctoral degree in 1914. He served as secretary of the Smithsonian Institution from 1928 until his retirement. He was interested in the practical use of solar energy and was a pioneer in this field with several patents to his name. Among others he designed a solar oven, solar cooker and solar still and while none of these proved commercially successful they showed his remarkable foresight in recognizing the importance of renewable energy sources.
