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.