Sky compass

Bees use sunlight polarisation patterns to navigate. We can train ourselves to detect light polarisation too.

Bees famously use the sun to identify the direction of food sources and communicate those directions to other bees. To do this, bees can sense the polarisation patterns of the sky. With a little practice and training, us humans can see those polarisation patterns too.

First, let’s talk polarisation. Light waves – like all waves – oscillate up and down. But each light wave may have its own idea of what’s up and down. If a bunch of light waves are oscillating in the same way, if they all have the same idea of what’s up and down, then that light beam is said to be polarised. If not, the light beam is unpolarised.

Sunlight is unpolarised. However, when it bounces through all of the crud in our atmosphere it gets banged into a state of partial polarisation. If you have some way of detecting light polarisation, you can see a distinctive pattern in the sky. This is called the Rayleigh sky model. It looks something like this:

Rayleigh sky model
Hallsw, CC BY 3.0, via Wikimedia Commons

This particular pattern is present in the sky during sunrise and sunset. The red line in the middle is the most polarised, the black parts are the least polarised. This pattern moves across the sky as the sun moves. Bees detect the polarisation patterns based on the sun and use those to give directions to other bees:

Flying bees are capable of obtaining and signalling compass information that is derived purely from polarized light. Furthermore, they deal with the directional ambiguity that is inherent in polarized light by signalling all of the possible locations of the food source in their dances, thus maximizing the chances of recruitment to it.

Honeybee navigation: critically examining the role of the polarization compass

For a bee, the whole sky is a compass.

Now, I recently discovered that human beings can also detect light polarisation. Stare at the sky with the sun at your back, or stare at a white space on a computer monitor (which is polarised). After a while, a small yellow dumbbell shape will emerge. It should look something like this:

Dpbsmith, CC BY-SA 3.0, via Wikimedia Commons

That blurry little image is Haidinger’s Brush. If the yellow shape is vertical, the light source is polarised horizontally. If the yellow shape is horizontal, the light source is polarised vertically. This shape is generated by specific cells in the eye that are sensitive to polarisation. It can take some time and practice to see them, but once you work it out you have the world’s most useless super-sense.

For more on how to view the brush, see here:

2 Replies to “Sky compass”

  1. Thanks for this – it’s amazing and I can’t wait to try it once the sky clears!
    I think you have a mistake in your description of the plot, though. It’s centered on the zenith (not the sun) and shows the sky as the sun is setting or rising, i.e. either at the extreme left or right. Polarisation is always the least when the light is directly coming at you straight from the sun.

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