This paper reviews the science of sun photometry, tracing its
origins back to research by Newton. Modern sun photometry
originated through the works of the Angstrom family in Sweden
and the Smithsonian Astrophysical Observatory's program started
by Langley and energized over many years by Abbott. In the case
of the Smithsonian's program, the objective was to search for
evidence between fluctuations in climate and solar radiation.
Multiwavelength sun photometry as a science came about
primarily by turning this nuisance factor of atmospheric
corrections into useful scientific information. Modern sun
photometry uses solar radiometers calibrated to accuracies of
one part in a thousand and routinely assess atmospheric
spectral optical depths to high accuracy. There is presently a
global network of sun photometers providing information about
the spectral variation of aerosol optical depth and information
mapping atmospheric trace constituents.