Distributions of purple-light colors and spectra in clear twilights

Clear civil twilights often exhibit a range of pastel purples, but the combined angular and temporal distributions of this purple light are unknown. Such distributions can now be measured by using a hyperspectral camera fitted with a fisheye lens, which gives new insights into the spread and evolution of purple-light spectra and colors during ordinary (i.e., non-volcanic) clear twilights. Because the purple light lacks clear-cut borders, a first step is to develop a perceptually based definition of its angular extent. Surprisingly, this leads to the discovery that colorimetrically distinct purples (1) sometimes appear simultaneously in both the solar and antisolar skies and (2) change in markedly different ways with sun elevation.