Fig. 4.

Global annual totals of biologically useful daylight, twilight, and moonlight, and global distribution of artificial light used in this analysis. Biologically useful light is expressed as the ratio of the illuminated duration when the estimated temperature is between 0 and 35 °C, and the total duration between these thermal limits (daylight, twilight, moonlight, and darkness). Twilight is defined as the duration when the sun is less than 12° below the horizon; moonlight is the duration when the lunar disk is more than 75% illuminated, unobscured by cloud cover and the sun is more than 12° below the horizon. Artificial light derived from DMSP satellite data for the year 2009 from National Oceanic and Atmospheric Administration National Geophysical Data Center.