¿ìè¶ÌÊÓƵ

PEOPLE can see the Earth’s magnetic field, albeit unconsciously. The effect is too small to be noticeable, but other animals may use their eyes in this way to get their bearings.

The Earth’s magnetic field lines vary depending where you are. They always run south to north, but are horizontal only at the equator, dipping in at a steeper angle the greater the latitude. Birds and some other migrating animals seem to use these angles to help them navigate, and one theory is that they do so using light-sensitive cells. So Franz Thoss from the University of Leipzig and his team investigated whether people’s eye are sensitive to these field lines.

They measured the lowest level of light that people could detect in a small spot straight ahead of them. They had people face south, west and south south-west, and used a magnetic coil to create a horizontal north-facing field. They then repeated the experiment with the Earth’s natural magnetic field, which in Leipzig is angled 70 degrees downwards towards the north (see Diagram).

When the field lines coincided with the direction of the spot, which only occurred when people faced south in a horizontal field, the threshold of brightness at which their eyes first detected a very dim light went up.

The effect was small but significant, say the researchers. Although we’re not conscious of it, the same probably happens when photoreceptors in the eye are aligned with the Earth’s magnetic field.

John Phillips and Thosten Ritz of Virginia Tech in Blacksburg, Virginia, who study the light compass in newts, say they probably use specialised photoreceptors to detect the magnetic field lines. Magnetic fields interact with spinning electrons, and could theoretically influence photoreceptor chemicals at the quantum level, altering their efficiency, they say.