Fig. 2. (A) Normalized quantal irradiance during sunset and twilight. The thick line denotes a spectrum taken at sunset (solar elevation –0.6°). The three lines with long-wavelength irradiance greater than at sunset denote solar elevations of 11°, 5.9° and 1.0° (in order of decreasing long-wavelength values). The three lines with 450 nm peak values greater than at sunset denote solar elevations of –3.6°, –6.5° and –9.3° (in order of increasing 450 nm peak values). (B) Normalized quantal irradiance due to three common sources of nocturnal illumination. Spectra in A and B are normalized so that their irradiances integrated from 350 to 700 nm are all equal. (C) Un-normalized spectra. All spectra presented in this study, with the exception of those taken within forests, are freely available from the authors.