Tag Archives: sky

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.

Optical phenomena

Environmental Graffiti has a great post about optical (light) phenomena; I’ve picked out a couple of my favourites:

Crepuscular Rays

Crepuscular rays are caused by the scattering of beams of light; beams created by an object such as a tree or a cloud.

Star Trails

Star trails are created when the shutter of a camera is left open for a long time (a matter of hours) and the Earth’s rotation causes the star to move through the sky, leaving a trail behind on the image. The trails are formed in circles around the North Star Polaris, (or the South Star Sigma Octanis in the southern hemisphere), the only star that appears stationary from Earth because it is directly above the axis of rotation.

Circumhorizon Arc

A circumhorizon arc is caused by the refraction of light through ice crystals in cirrus clouds; it’s a bit like a rainbow stuck in the sky.