Introduction

Abstract

The daily alternation between day and night is one process of nature whose regular repetition exerts a striking impact on the structure of organisms and the way of life of every inhabitant of our planet. Although the effects are often indirect ones, their ultimate source is always the remarkable change in illumination — by a billion times twice a day. The character of the energy balance between the earth’s surface and the surrounding atmosphere is qualitatively rearranged, the atmosphere moderating the transition because of its ability to scatter light. The change does not occur instantaneously but over the rather protracted period known as twilight, when many transient and surprisingly varied phenomena take place.