In past centuries, sailors were sometimes surprised to see upside-down ships sailing in the sky, or elaborate castles built on the ocean. When they turned towards them, these apparitions vanished. The illusions, called The Fata Morgana, can still be seen today if
the witches are in a mood the weather's right.
Sailors going along the Strait of Messina were often stunned, and scared, to see sprawling islands with towering castles appear on the horizon. Sometimes the islands deployed boats that sailed upside down in the air above the ocean. The boats may have become the basis for the legend of the Flying Dutchman, but the islands, with their odd castles, came to be known as Fata Morgana. They were named for Morgana Le Fey the villainous witch of Arthurian legend. She put up illusions to lure men to their death.
Fata Morgana are mirages of a particular type. When hot air hangs in a cloud, above air chilled by the ocean, light from objects on the ocean can be bent back down towards the Earth. People watching from the shore, or from boats, see objects that seem to be floating in the sky. To understand how the illusion works, imagine tracing the path of the wave of light in a car, the right side of the car facing the sky, and the left side facing the ground. To begin with, you are angled upwards, about to drive off into the sky like a jet taking off from a runway. Once you reach a certain height, though, your right wheels turn faster than your left wheels. The increased speed of the right wheels, and drag of the left wheels, slowly turns the car back downwards, until you're heading back towards the ground.
This is what happens to the light. Light moves faster in hotter, less-dense air. As light waves bounce off a boat and head up towards the sky, they start out in air cooled by the ocean. As they move further upwards, they encounter hotter air. This air turns the wave slightly, a bit at a time. With enough time in a temperature gradient, hotter air above and cooler air below, the light wave will curve back down to the ground. To someone on the ground, who can't see the entire path of the light, it looks like the light, and the boat or island it bounced off of, is floating in mid-air.
Image: Brocken Inaglory
Where the light starts out affects where it ends up. Light from just above the surface of the ocean can make a more dramatic curve than light that's suspended far above the ocean, at the top of the mast of a ship. Get the angles right, and images can appear to be inverted - so entire islands and boats can look like they're floating upside down. With the right combination of heat and light, we can see sprawling cities in the sky, doubled boats, and sometimes multiple suns.
[Via NASA APOD.]