For David Morris, it was a normal ride last month along the cliffs of England’s southwest coast: his Terrier, a sunny morning, with ships passing over the horizon.
But a ship seemed a little out of place. Appeared above the horizon, as if hovering over the sea, suspended in the air.
“I said to myself, ‘It must be in the water,'” said Morris, a 52-year-old real estate developer who lives near Lizard, the southernmost point of mainland Britain. “My head doesn’t want to understand this, but it must be in the water.”
Morris said he did not expect to cause a stir on social media when he posted a photograph that appeared to show the ship floating on his Facebook account. “It’s just a picture of a boat,” he said in a telephone interview.
But, of course, it was not. Now, many are trying to wrap their heads in an image that seems to portray the impossible.
What Morris said he saw was an example of an optical illusion known as the upper mirage, which occurs when the temperature difference between the sea and the air causes a change in the density of the air and forces sunlight to bend over the horizon.
Cold air is usually over hot air – the more you climb, the colder it gets. But on that sunny morning in Cornwall last month, the situation was reversed: the cold air hung over the cold sea, with the hot air at the top.
The inversion of temperature produced a mirage. The light coming from the ship towards Mr. Morris was refracted, because the weather conditions formed layers of air with different temperatures, causing the light to travel through them at a different speed.
The ship looked taller than it should have, because the human brain – and, apparently, the cameras – cannot process the effect that different temperatures have on how the images are perceived.
(Hold on.)
The light usually reaches the eyes through straight lines, which allows them to see things directly, said Dr. Claire Cisowski, an optics researcher at the University of Glasgow.
But, she said, “sometimes an image is deflected when the rays of light that hit us pass through different layers”.
This is what happens when you look through water: a straw in a glass of water, or a hand immersed in the sea, can appear out of alignment, because light travels through the air and water at different speeds.
The same principle applies to the ship in Cornwall, except that instead of passing from water to air, light travels from air to air, said Cisowski.
“The air is not always the same – it has different properties, be it cold or hot,” she said. “So, because light travels differently through these different layers, our brain tries to make sense of it.”
In the case of Mr. Morris’s experiment, since cold air is denser than hot air, the rays of light from the ship were directed downwards. From the coast, it seemed to Mr. Morris that the ship was in a higher position than it actually was.
“When the light hits our eyes, they cannot retrace the entire trajectory as if it were bent,” said Cisowski. “So we form an image as if it came from a straight line, because our eyes want to prolong what they see.”
And, like an eye, a camera cannot reconstruct this curved path either, according to Dr. Cisowski. “It is as if the ray of light also came from a straight line.”
Isn’t it the first time that optical illusions have gone viral on the internet, and hasn’t the floating ship achieved the same fame as a blue and black dress – or was it gold and white? – did in 2015. At least, not yet.
Morris said this was not the first time that he had seen what appeared to be a floating ship, although BBC analyst David Braine said in a short video that what happened was highly unusual. “It is quite unusual to see such an optical illusion in British waters, but it happens very rarely,” he said.
Higher mirages are more common in the Arctic, where they occur because temperature differences between the sea and the air cause a similar change in air density more frequently.
But people may be more used to its opposite: inferior mirages. When a hot surface causes cold air to overlap with warmer air, the rays of light are bent upward, leading the viewer to see a patch of blue sky appear in the desert like a pool of water or a mirage on a road .
In Cornwall, Morris said he had not paid much attention to the levitating ship – the Maribel, which was off the coast of France on Saturday and is due to arrive in New York on Tuesday.
Instead, he marveled at the landscape around him as he resumed his walk.
He said, “I said to myself, ‘How lucky we are to live in this part of the world.’
Mike Ives and Shannon Hall contributed reporting.