If you've every looked out the window of an airplane and spotted a series of bright colorful rings against the clouds, you've seen a glory.
You probably thought it was an oddly shaped little rainbow, an easy enough mistake to make given that a glory does look like a very compact floating circle of a rainbow, with bright red lines along the outer edge and blue ones at the center of the circle.
However, circular rainbows are entirely phenomenon different from the glory, which is its own unique and special occurrence.
A glorious history
Glories were first scientifically reported in the mid-1730s when a group of European explorers gathered along the Peruvian Andes. The leader of the expedition, French explorer Pierre Bouguer, wrote this about the glory each of the men saw:
"A phenomenon which must be as old as the world, but which no one seems to have observed so far ... A cloud that covered us dissolved itself and let through the rays of the rising sun ... Then each of us saw his shadow projected upon the cloud… The closeness of the shadow allowed all its parts to be distinguished: arms, legs, the head. What seemed most remarkable to us was the appearance of a halo or glory around the head, consisting of three or four small concentric circles, very brightly colored, each of them with the same colors as the primary rainbow, with red outermost ..."
What Bouguer reports, with the shadow of each man on the clouds and their head surrounded by the glory like the halo of a saint, is called a Brocken spectre, and it's a phenomenon that often accompanies a glory.
At this time, the only way to see a glory was to hike to these incredible heights or be near a geyser or hot spring, according to NASA. As we took the skies by other means, including hot air balloons and airplanes, spotting glories became much more common. Even astronauts reported seeing glories from their space shuttle flights.
How does a glory form?
Glories are always located directly opposite from the sun. The come about as the result of backscattering, or the deflection of sunlight hitting tiny water droplets. If the droplets are uniform in size, a glory will be brighter and have higher color purity, according to the Hong Kong Observatory.
For a glory to be spotted, the sun and the observer must be in a sort of alignment with one another — that's the antisolar point, or the spot that is directly opposite sun from where the observer is. Antisolar points are relative to the observer, which is why, when those Europeans explorers experienced glories in the Andes, they noticed that their fellow team members couldn't see their glories.
"The most surprising thing was that," Spanish Capt. Antonio de Ulloa wrote. "Of the six or seven people that were present, each one saw the phenomenon only around the shadow of his own head, and saw nothing around other people's heads ..."
While the explanation for glories — sunlight and water droplets — sounds simple, the actual physics behind it remains something of a mystery to us. The current prevailing theory, put forth by physicist Moysés Nussenzveig, is that a glory is the result of wave tunneling. As described by Nature, wave tunneling is when the reflected sunlight doesn't directly hit the water droplet, like in the case of rainbows, but actually just passes near the droplet. This close contact "stirs up the electromagnetic waves within the droplet." Those waves eventually tunnel their way out of the droplet and send the light waves back to their source direction.
Their enigmatic physics only makes glories even more appealing. So the next time you spot a glory, appreciate not only its beauty but also its mysterious presence in nature.