sunlight is polarized. bees navigate by it. you can too.
When sunlight scatters through the atmosphere, it becomes polarized — the light waves
align in predictable patterns relative to the sun's position. Bees, ants, and migrating
birds use this "skylight compass" to navigate even when the sun is hidden. Modern phone
camera sensors are partially sensitive to polarization, and we can exploit this to
estimate sun bearing from a patch of blue sky.
The Viking "sunstone" is now confirmed to be a real navigation tool — a calcite crystal that detects sky polarization.
In 2013, researchers found a calcite crystal in a Tudor-era shipwreck that matched the optical
properties of legendary Norse sunstones. Tests showed the stone could locate the sun within
±1° of arc — accurate enough to navigate the North Atlantic — on overcast days when the sun
was completely invisible. An ancient GPS, working on the same physics your phone's camera sees today.
—
° · estimated sun bearing
point at a patch of blue sky, 90° from the sun
—sky brightness
—pol. contrast %
—phone heading
camera heading—
sky brightness—
blue channel ratio—
polarization contrast—
estimated sun bearing—
confidence—
▶ how does sky polarization navigation work?
Sunlight is initially unpolarized. When it scatters off air molecules (Rayleigh scattering),
it becomes partially polarized in a pattern that forms a great circle 90° away from the sun.
The maximum polarization always lies at 90° from the sun's azimuth.
Bees and ants have specialised ommatidia in their compound eyes that detect this pattern
across the whole sky simultaneously, giving them an accurate compass even when clouds cover
the sun — as long as a small patch of blue sky remains visible.
In this experiment, we use a cruder method: phone cameras have RGB Bayer sensors that
are partially sensitive to polarization angle. By analysing the brightness asymmetry and
blue channel intensity across different heading angles, we can estimate the polarization
direction — and from that, the sun bearing.
For best results: go outside on a clear day. Point the camera at blue sky
that is roughly 90° away from where you think the sun is. Slowly rotate and watch the
polarization contrast reading change.