Derived from the Greek word meaning “opposite the sun”, an anthelion is an atmospheric optical and rare occurrence. It appears as a luminous halo visible under certain atmospheric conditions, mainly around or close to the sun.
This natural spectacle appears as a circle of light or a circular rainbow surrounding the sun at a certain angular distance.
How does it happen? It happens when the sun’s rays interact with ice crystals in high clouds, such as cirrus clouds.
These crystals act as prisms, refracting and reflecting sunlight in a process that disperses the different colours of the spectrum. Each colour is refracted at a slightly different angle, creating the mesmerising rainbow effect.
Back in 1947, Hendrik Christoffel van de Hulst was researching the aviator’s halo and theorised that it could be caused by surface waves. So how do these rings form?
Van de Hulst suggested that one explanation might be the deflection of light rays by surface waves on water droplets. But this remains a hypothesis. The exact process behind these auroras is still unknown.
