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., nonvolcanic) 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 in <i>both</i> the solar and antisolar skies and (2) change in markedly different ways with sun elevation.