It is thought there is a protein in their eyes that causes a shadow when they are in the correct alignment with the earth’s magnetic field.
When they align to the north, they successfully pounce on their prey 74% of the time. If they align in any other direction, they are only successful 18% of the time.
So while other animals use the earth’s magnetic field to judge distance, foxes use it to judge distance, depth and direction. Pretty darn cool. A true superpower.