The Second Purple Light

Abstract

BY photoelectric observation of the twilight sky, Bigg¹ has found evidence of scattering of the Sun's rays from an elevated atmospheric layer. Assuming primary scattering, he calculates its height to average some 81 km. and finds an association between the intensity of the effect and the incidence of meteor showers. He also observed scattering by a lower dust layer (10–25 km.), which is the cause of the purple light frequently observed above the western horizon after sunset and before sunrise. But after some great volcanic eruptions a striking feature of the brilliant sunsets has been the long-continued after-glows, with a second purple light appearing some time after the first had set and only fading away some 1½–2 hr. after sunset.