The picture shows the ring-shaped spectrum against a backdrop of cumulocirrus clouds.
The aeroplane's shadow can be faintly seen in the centre of the ring, with the colours fading from blue to red around it.
Rainbows are formed when sunlight strikes the curved inside of a raindrop at a specific angle and is reflected back through the water, creating a prism effect.
The apparent semicircle of a normal rainbow is only limited by the horizon. The full circle could be seen if the viewer were standing on a sufficiently high cliff, although it is more easily seen from aircraft.
Rainbows are long said to have had a profound religious and mythological significance. Before they were explained scientifically, they were described in the Bible as a symbol of God's covenant with Noah, promising that mankind would never again be destroyed by flood.
In Irish folklore, the leprechaun hides his pot of gold at the end of a rainbow. This would, however, be harder to do in the case of a circular one.