For David Morris, it was an ordinary stroll last month along the cliffs of England’s southwestern coast: his Terrier, a sunny morning, with ships passing on the horizon.
But one vessel seemed a little out of place. It appeared above the horizon, as if hovering over the sea, suspended in the air.
“I told myself, ‘It must be on water,’” said Mr. Morris, a 52-year-old property developer who lives near the Lizard, the most southerly point in mainland Britain. “My head doesn’t want to understand that, but it must be on water.”
Mr. Morris said he hadn’t expected to cause a stir on social media when he posted a photograph that appeared to show the floating ship on his Facebook account. “It’s just a boat picture,” he said in a telephone interview.
But, of course, it wasn’t. Now, many are trying to wrap their heads around a picture that seems to depict the impossible.
What Mr. Morris said he saw was an example of an optical illusion known as a superior mirage, which occurs when the temperature difference between the sea and the air causes a change in air density and forces light from the sun to bend around the horizon.
Cold air usually sits on top of warm air — the more one climbs, the colder it gets. But on that sunny morning in Cornwall last month, the situation was reversed: Cold air lay above the chilly sea, with warm air on top.
The temperature inversion produced a mirage. The light coming from the ship toward Mr. Morris was refracted, because meteorological conditions formed layers of air that had different temperatures, making light travel through them at a different speed.
The ship appeared higher than it should be, because the human brain —and, as it turns out, cameras — can’t process the effect that different temperatures have on how images are perceived.
(Hang in there.)
Light usually travels to the eyes through straight lines, which lets them see things in a straight way, said Dr. Claire Cisowski, a research fellow in optics at the University of Glasgow.
But, she said, “sometimes an image is deflected when the rays of light that reach us go through different layers.”
That is what happens when looking through water: A straw in a glass of water, or a hand immersed in the sea, might look out of alignment, because light travels through air and water at different speeds.
The same principle applied with the ship in Cornwall, except that instead of moving from water to air, light traveled from air to air, Dr. Cisowski said.
“Air is not always the same — it has different properties whether it’s cold or hot,” she said. “So as light travels differently through these different layers, our brain tries to make sense of that.”
In the case of Mr. Morris’ experience, since cold air is denser than warm air, light rays coming from the ship were bent downward. From the coast, it appeared to Mr. Morris that the ship was in a higher position than it may have really been.
“When light reaches our eyes, they can’t retrace the whole trajectory as if it was bent,” Dr. Cisowski said. “So we form an image as if it was coming from a straight line, because our eyes want to prolong what they see.”
And like an eye, a camera can’t reconstruct that bent trajectory either, according to Dr. Cisowski. “It’s as if the ray of light was coming from a straight line, too.”
It’s not the first time optical illusions have gone viral on the internet, and the floating ship has not achieved the same fame as a blue-and-black dress — or was it gold and white? — did in 2015. At least, not yet.
Mr. Morris said this was also not the first time he had seen what appeared to be a floating ship, although the BBC forecaster David Braine said in a short video that what happened was highly uncommon. “It’s quite unusual to see such an optical illusion in British waters, but it does happen very rarely,” he said.
Superior 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 with greater frequency.
But people may be more used to their opposite: inferior mirages. When a hot surface causes cool air to sit on top of warmer air, 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, Mr. Morris said he had not paid attention for too long to the levitating ship — the Maribel, which was off the coast of France as of Saturday and is scheduled to reach New York on Tuesday.
Instead, he marveled at the landscape around him as he resumed his walk.
He said, “I told myself, ‘How lucky we are to live in this part of the world.’”
Mike Ives and Shannon Hall contributed reporting.