Those imbued with a less romantic spirit, though, sought to provide a rational explanation of the phenomenon of The Flying Dutchman. An early attempt appeared in Frank Stockton’s Round-About Rambles (1870) in which he tells of a captain’s reaction when some of his crew reported a sighting. “This strange appearance”, he explained, “was caused by the reflection of some ship that was sailing on the water below this image, but at such a distance they could not see it. There were certain conditions of the atmosphere…when the sun’s rays could form a perfect picture in the air of objects on the earth…this appearance in the air is called a mirage”.

A mirage is an optical illusion caused by the refraction of light. When light hits a boundary between two layers in the atmosphere which are at different temperatures, it bends and travels through the new layer at a different angle, the degree of change dependent upon the difference in density between the two layers.

Refracted light poses an almost insoluble problem for the human brain. When we see light, our brain assumes that it has travelled in a straight line from the object that has emitted it. Even if the light has refracted along the way, it simply places the object where it would have been had the light travelled in a straight line. Just think of what happens when looking at a fish in a clear stream; it is not where your brain tells you it is.

There are two basic forms of mirage; inferior, where the image appears to be lower than it really is, and superior, where it is higher. A superior mirage can make things appear bigger, closer, distorted so that the object appears stretched or elevated, an effect known as towering, and as if they are suspended or floating in air. It can even make an object below the horizon become visible.

The phenomenon of The Flying Dutchman is thought to be a complex form of superior mirage, a Fata Morgana, which occurs when there is thermal inversion, where cold, dense air near the Earth’s surface is trapped beneath a layer of significantly warmer air. If the thermal inversion is greater than 10 degrees Celsius in 100 meters, the curvature of the light rays within the inversion layer is stronger than the Earth’s curvature, effectively trapping them in an atmospheric duct. The duct will then concentrate the rays rather than allowing them to disperse into space. Mindbogglingly, if our eyes were strong enough, a duct would allow you to see around the whole of the earth and your own back and shoulders turned towards you.

While a thermal inversion does not necessarily create an atmospheric duct, an atmospheric duct cannot occur without a thermal inversion, and a Fata Morgana requires both. A Fata Morgana can be seen anywhere and at any altitude, once the conditions have been met, and it will display any form of distant object such as boats, islands, and the coastline. Often it will change rapidly, showing inverted and upright images that are stacked on top of each other or alternating between compressed and elongated images. However, to see one the observer must be within or below an atmospheric duct.

And the Arthurian connection? Fata Morgana is Italian for Morgan the fairy, a reference to the sorceress Morgan le Fay who in some Arthurian legends is his half-sister. Her first recorded appearance was in Geoffrey of Monmouth’s Vita Merlini (1150) and she was said to have used witchcraft to conjure up mirages of fairy castles or false coastlines to lure sailors to heir death, especially in the Strait of Messina. We may no longer imbue the sighting of a Fata Morgana with superstitious dread, but the power of the image still remains.