Meeting Abstract
Although the dramatic changes in illumination level, and to a lesser extent the associated changes in color, that accompany the rising and setting of the sun are well known, the effects of the presence of the moon and its phase on general illumination during twilight have received less attention. This is unfortunate, given the importance of crepuscular periods to animal activity, the importance of lunar phase during crepuscular periods to animal reproduction, and the ability of certain species to see color under dim light. We measured downwelling spectral irradiance (from 350 to 800 nm) during evening civil and nautical twilight (solar elevation down to -12 deg) at a remote site chosen to minimize light pollution. Nine sets of measurements were taken to cover the first half of the lunar cycle (from new to full moon) and were used to calculate human-based chromaticity and to generate images of the average color of the sky and the appearance of objects. Lunar phase had no consistent effect on downwelling irradiance until solar elevation was less than -8 deg. For solar elevations lower than -8 deg, the effect of the moon increased with the fraction of the lunar disk illuminated until the fraction was approximately 50%. For fractions greater than 50%, the brightness and chromaticity of the downwelling irradiance were approximately independent of the fraction illuminated, likely because the greater brightness of a fuller moon was offset by its lower elevation during twilight. In summary, the phase of the moon can profoundly affect both the brightness of the illumination and the appearance of objects during late twilight, affecting vision and providing ample cues for lunar timing, even for animals that lack spatial vision.
