In 1917, three children in Fátima, Portugal, claimed that they had encountered an apparition of the Virgin Mary while returning home from tending sheep.

According to the trio, Mary told them that she would reappear on the 13th day of every month for six months (like Urkel) before she disappeared. Word of this miracle spread, and visitors started showing up to the town. But Mary would only appear to Lucia, the eldest of the children, who would have to describe the vision she was seeing to others. According to accounts of the children, Mary promised that on the final visit, on October 13, 1917, she would offer proof to others in the way of a miracle.

Word got around, and an estimated crowd of 70,000 showed up to witness the promised apparition. A lot of the crowd was not disappointed, as they looked up and saw what has become known as the "miracle of the Sun". Various witnesses reported the Sun, usually known for being a stable blob in the sky, hurtling towards Earth before returning to its original position, whirling around on itself, and producing flashes of colored light.

“Looking at the sun, I noticed that everything was becoming darkened. I looked first at the nearest objects and then extended my glance further afield as far as the horizon. I saw everything had assumed an amethyst color. Objects around me, the sky and the atmosphere, were of the same color. Everything both near and far had changed, taking on the color of old yellow damask," José Maria de Almeida Garrett, a science professor who witnessed the event said, though some claim this account was actually given by his son.

"People looked as if they were suffering from jaundice, and I recall a sensation of amusement at seeing them look so ugly and unattractive. My own hand was the same color," he continued. "All the phenomena which I have described were observed by me in a calm and serene state of mind, and without any emotional disturbance. It is for others to interpret and explain them."

So, how do we explain them? Well, one thing we know for certain is that nothing out of the ordinary happened to the Sun on that day, given that the phenomenon was not witnessed anywhere else on Earth. Whatever happened, it was a local phenomenon.

One explanation, proposed by Artur Wirowski of the Lodz University of Technology, is that witnesses could have seen a "sun dog", which are caused by clouds of ice crystals high in the atmosphere. These clouds can also scatter color across the sky, perhaps accounting for what witnesses saw.

However, it was also a highly variable phenomenon, with witnesses reporting very different accounts of what they saw, from spinning Catherine wheels to the Sun changing color like a disco light. There are also no photos of the Sun doing its tricks, despite an expectant crowd of 70,000, and the attendance of the press.

A much more obvious explanation is that the crowd had a heady mix of mass hysteria with a helping of solar retinopathy, or burning of the retinas, from staring up at the Sun for too long. It's a condition that has been documented in later religious individuals who stared too long at the Sun during religious rituals. 

The symptoms, including changing of color and a Sun that appears to zip across the sky (perhaps an afterimage of the Sun that stayed with witnesses as they turned their heads) explain events a lot better than a sun dog, given that not everybody witnessed the phenomenon. Perhaps some were just too sensible, even when gathered to witness a miracle on the word of three children, to stare directly at the Sun.