Many insects detect polarized skylight and use this cue as a compass. When used together with one other parameter, such as a memory of the distance travelled (path integration), this compass is sufficient to enable return journeys over considerable distances. However, as the sky’s polarization pattern originates from scattering of light in the atmosphere, it is subject to changes in the azimuth of the sun and the moon, as well as to degradation by cloud cover. Recent results are starting to demonstrate that polarization patterns can be more degraded than previously thought, and, most importantly, that the dim polarization patterns present in the night sky are used for insect navigation.
We now continue this line of research and wish to provide the first detailed analysis of the functions, limitations and benefits of this nocturnal compass. The polarization pattern formed around the full moon is a million times dimmer than that formed around the sun. What are the adaptations for high sensitivity in the nocturnal compass system? Within the first and last quarter of the lunar month, the polarization pattern becomes ten times dimmer again. Do nocturnal navigators trade precision for increased sensitivity? At which phase of the moon is the celestial polarization pattern bright enough to support reliable orientation?