Is it a bird? Is it a plane? Is it an angel? These are some of the questions asked about a very peculiar trick of light on a misty mountaintop. As you climb above the fog, with the sun directly behind you, you sometimes see a giant shadowy figure encased in a giant halo. This is called a Brocken spectre.
Many legends and folklore surround the Brocken spectre. In China’s often misty Huangshan Mountains, it was referred to as Buddha’s Light and it indicated that someone was enlightened. When the Spanish settled in California in the 18th century, they called them Los Vigilantes Oscuros, or dark watchers. It became generally known as a Brocken spectre because of its frequent appearances on northern Germany’s highest peak, the Brocken (1,141m).
A Brocken spectre. Photo: Vitali Mamchuk/Shutterstock
In German legend, the apparition suggested the sinister presence of demons and witches. Allegedly, a climber once fell to his death when he was startled by the image, and so was “killed by his own shadow.” The superstitions around it prompted such writers as Johann Wolfgang von Goethe, Lewis Carroll, Carl Jung, and Charles Dickens to include it in their works as omens. It would be cool if these myths were true, but the Brocken spectre is just simple physics.
The Spectrum of Brocken, a vintage engraving. From the Universe and Humanity, 1910. Photo: Morphart Creation/Shutterstock
How does it form?
A Brocken spectre occurs when the sun shines behind a person standing on a peak above clouds or fog, casting a magnified shadow and a large halo onto the misty depths below. This is due to a process called diffraction. Diffraction occurs when sunlight penetrates water droplets. It reflects off the sides and backscatters in the person and the sun’s direction.
The centre of the spectre, the shadow’s head, is an abstract point directly opposite the sun, called the anti-solar point. A circular rainbow bends around it. The shadow’s increased size is an illusion that takes place when the brain has no reference points by which to judge size. It interprets the shadow as bigger than it actually is.
Over the last century, scientists have tried to recreate the conditions for this mysterious phenomenon. Scottish physicist Charles Thomson Rees Wilson conducted one such famous experiment. He was enthralled with glories and natural optical illusions caused by the weather.
Wilson became fascinated with the Brocken spectre after witnessing it in 1894 from the summit of Ben Nevis, Scotland’s highest peak. Years later, he built a cloud chamber in the Cavendish Laboratory at Cambridge University. While he didn’t recreate the Brocken spectre, the chamber’s ability to detect radiation earned him the Nobel Prize in 1927.
Glories and Brocken spectres
The spectre is categorized as a type of glory; an inverted rainbow with red rings in its outer sections and blue rings in the centre. Variations of brightness and darkness are from differences in the droplets’ interactions with sunlight.
Airline passengers occasionally witness Brocken spectre-like glories. When sunlight shines on water droplets in clouds, a circular rainbow forms around the plane’s silhouette as it passes in front of the sun.
A Brocker spectre even appeared from space in 2003, aboard the space shuttle Columbia. It was affectionately named Astronaut’s Glory, in honor of the astronauts who died on re-entry a few days later.
The Brocken spectre, also called Brocken bow or mountain spectre, is the magnified shadow of an observer cast on clouds opposite the sun. Photo: MemoryMan/Shutterstock
