SEEKING AN INCONSTANT CONSTANT: THE QUEST TO DISCOVER THE VARIABILITY OF THE SUN FROM WILLIAM HERSCHEL TO ANDREW ELLICOTT DOUGLASS

Abstract

The question of whether the ―solar constant,‖ the amount of radiation from the Sun that impinges on a square centimeter of the Earth's atmosphere each minute, is in fact inconstant—i.e., whether the Sun might be a variable star—has long been one of the great quests of solar physics, and began in earnest with William Herschel. With the invention of the bolometer, an instrument of great sensitivity, by S.P. Langley in 1880, the question finally became capable of quantitative investigation, and was the subject of a long series of observations by Langley's successor at the Smithsonian Astrophysical Observatory in Washington, D.C., C.G. Abbot. His analysis of observations between 1902 and 1957 not only indicated variability in the solar constant but sought to tie this variability in with the climate of the Earth. Similar findings were reported by A.E. Douglass, based on his pioneering studies of tree rings. Though their findings were later challenged, they had established the question of the variability of the solar constant and its potential effects on terrestrial climate as of the first importance, and established parameters to more sophisticated, but still contentious, approaches in the 1950s, to be considered in a subsequent paper.