The polarisation of the sky

When light from the Sun or the Moon strikes Earth’s atmo­sphere it is scattered, sent in all dir­ec­tions by the atoms and molecules that make up the air. During this scat­tering process some of the light is polar­ised — instead of the electric and magnetic fields oscil­lating in many planes sim­ul­tan­eously, they oscil­late in only one plane.

The polar­isa­tion of the light from the Sun or Moon is at right angles to the dir­ec­tion that the light is coming from; when the Sun or the Moon is very low in the sky (at sunset/sunrise or moonset/moonrise) the dir­ec­tion of polar­isa­tion is parallel to the horizon. The degree of polar­isa­tion depends on the angle between the light and the atmo­sphere, with the greatest degree of polar­isa­tion occuring when looking at 90° to the source.

Source: Chris­topher Kyba

The image on the left shows the degree of polar­isa­tion observed in a rural location. The image on the right shows the same section of the night sky, but observed from an urban location (the circular patterns in both images are caused by the movement of stars across the sky.) Because light pol­lu­tion from street­lamps is not polar­ised, the effect of the street­lamps is to destroy the polar­isa­tion “data” that some animals use to navigate.

Source: C. C. M. Kyba, T. Ruhtz, J. Fischer and F. Hölker “Lunar skylight polar­iz­a­tion signal polluted by urban lighting”, Journal of Geo­phys­ical Research 116 (2011). doi: 10.1029/2011JD016698.