Natural light may be refracted, absorbed, or scattered by atmospheric constituents. The study of these effects constitutes atmospheric optics, the oldest branch of theoretical meteorology. Its early development was related to astronomy because of the limitations imposed by the atmosphere on distant seeing. Through the years, many outstanding figures contributed to the subject. In his quest for realism in painting, Leonardo da Vinci studied the phenomena of the rainbow and the reduction of visual contrast with range. Isaac Newton gave explanations based on his experiments on refraction for the rainbow, and later the halo. The celebrated scattering theory published in 1871 by John Strutt (later Lord Rayleigh) was developed to explain the color and polarization of light from the clear sky. This project continues the long tradition of attempting to explain why the sky looks the way it does by focusing on one of its most complex phenomena, twilight. Digital photography and spectroradiometry are used to measure the brightness, color, and polarization of essentially the entire twilight sky. The observations are compared with existing theoretical models based on scattering and absorption by atmospheric particles and gases as a way of testing the models and improving them. The objective of the study is to develop accurate models for twilight that include the effects of scattering by molecules and background aerosols, absorption by ozone and other gases, and scattering and absorption by volcanic particles or other large aerosols that may sometimes be present.
