On the rainy morning of October 13, 1917, a large crowd stood waiting in the Cova da Iria, near the town of Fátima, north of Lisbon. They had come because three shepherd children said the Virgin Mary had promised a miracle on that date, "so that all may believe."

Estimates of the crowd run from about 50,000 to about 70,000. What those people saw has been argued over ever since by bishops, physicists and eye surgeons, and the argument is as much about how people look at the sky as about the sky itself.

Three children and a promise

The children were Lúcia dos Santos, born on March 22, 1907, in the hamlet of Aljustrel, and her younger cousins Francisco and Jacinta Marto. On May 13, 1917, while minding sheep, they reported meeting a lady by a holm oak, "shining white, brighter than the sun," as Lúcia later recorded. She asked them to return on the 13th of each month. In her 1941 memoir, Lúcia wrote that in July she asked the Lady for a miracle and was told it would come in October.

Newspapers reported the prophecy and pilgrims began to arrive. Portugal's First Republic, established in 1910, was officially secular, and the regional administrator suspected the prophecies were political. Shortly before the August apparition he took the children into custody. Lúcia's obituary in The Telegraph records that he threatened to boil them in oil unless they denied what they had seen, then locked them in the county jail overnight. They did not deny it.

What the crowd reported

The Telegraph's account has Lúcia telling the crowd to put away their umbrellas and then crying out, "Look at the sun!" Many witnesses said the clouds broke and the sun appeared as a dull, opaque disc that could be looked at directly. It spun, cast colored light across the people and the landscape, then seemed to plunge toward the earth before zigzagging back into place. The episode was said to last about ten minutes. Some said their soaked clothes were suddenly dry.

A reporter for the Lisbon paper O Dia described a silver sun "enveloped in the same gauzy grey light." Manuel Nunes Formigão, a priest who taught at the seminary in Santarém, compared it to "the most magnificent firewheel that could be imagined." Another witness had the sun advancing "as if to crush us with its huge fiery weight." That line is often credited to Gonçalo de Almeida Garrett, a professor of natural sciences at Coimbra, which made it sound like the verdict of a scientist. The priest and physicist Stanley Jaki concluded that it came from his son José, a young lawyer.

The reports were far from uniform. Some people saw only colors. Some said they saw the Virgin. A number saw nothing at all. The children described something else again: Lúcia later said that while the crowd watched the sun change color, the three of them saw Mary, Jesus and Joseph in the sky.

Distance is the believers' strongest card. Joaquim Lourenço, later a priest, recalled watching as a boy from Alburitel, about 18 kilometers (11 miles) away, and seeing a pale sun, in his words "like a ball of snow, revolving on itself."

A crowd that came ready

The same testimony shows what people expected. Joaquim Gregório Tavares, who was present, admitted that he arrived after talking with level-headed people who were "anticipating some changes of colour in the Sun." In Alburitel, according to a witness named Maria do Carmo, villagers spent that morning darkening bits of glass over candle smoke to watch the sun safely.

The most useful witness may be Domingos Pinto Coelho, a Catholic lawyer who had described the October event for the newspaper Ordem. On October 17 he wrote that a few days afterward, in Lisbon, he had looked at a half-overcast sun and seen "the same succession of colors, the same rotary movement." Without meaning to, he had repeated the experiment.

The photograph that isn't

Hundreds of reporters and photographers were reportedly on the field, among them Judah Ruah of O Século, yet no authentic photograph shows the sun doing anything unusual. The black and white image most often shared as the miracle itself, published by L'Osservatore Romano in 1951, is not from Fátima. One account says it was taken on another occasion in a different Portuguese town; the religion scholar Lisa Schwebel identified it as a solar eclipse photographed elsewhere before 1917. The sources disagree on what it shows but agree on where it was not taken. As the skeptic Brian Dunning put it, whatever the crowd saw was "not interesting enough to be noticeable in a photograph."

Explanations, sacred and natural

The Church moved slowly. The local bishop opened a canonical investigation in November 1917, and Bishop José da Silva declared the events "worthy of belief" on October 13, 1930. In 2000 the Vatican's Congregation for the Doctrine of the Faith classed Fátima as a private revelation and quoted Cardinal Prospero Lambertini, the future Benedict XIV: such revelations are put before the faithful as "probable and credible to piety." Catholics may accept them but are not obliged to.

Belief reached the top. In handwritten notes discovered in 2008, Pope Pius XII recorded seeing the solar miracle from the Vatican gardens on four days in 1950. Jaki proposed a divinely timed lens of ice crystals in the air, and in 2012 the physicist Artur Wirowski published a mathematical model of light passing through a cloud of vibrating, charged ice crystals.

The physicist Auguste Meessen, speaking at a 2003 symposium, argued that afterimages from sun gazing produce the dancing, and that bleaching of light-sensitive retinal cells produces the colors. He pointed to Heroldsbach, Germany, in 1949, where many in a crowd of over 10,000 reported similar sights. Others have proposed stratospheric or Saharan dust, thin cloud or a sundog. The science writer Benjamin Radford made the simplest objection: "everyone on Earth is under the same sun," and nobody elsewhere saw it move.

Modern pilgrimages supply a control group. In October 2009 a clairvoyant predicted that the Virgin would appear at Knock, in County Mayo, Ireland, and pilgrims staring upward reported the sun "dancing in the sky." By December Eamonn O'Donoghue, an eye surgeon at University Hospital Galway, had seen five cases of solar retinopathy linked to Knock, against one at most in a normal year, and called the colored effects "sort of a cheap trick." Writing in Slate, Phil Plait explained that a saturated retina produces afterimages with illusions of color and movement.

What the evidence supports

Something happened at the Cova da Iria. Tens of thousands of people, believers and doubters among them, reported an unusual sun, and even Radford says no one suggests they were hoaxing. What the record does not support is the popular version in which the sun itself moved. No distant observatory recorded it, no photograph shows it, the witnesses disagree about what they saw, and other crowds told to look up at an appointed hour have reported the same effects.

Francisco and Jacinta died in the influenza pandemic of 1918 to 1919. They were canonized at Fátima on May 13, 2017, before a crowd the Vatican put at 500,000. Lúcia became a nun and died in 2005, aged 97. In Alburitel on the morning of the miracle, before anyone had seen anything, people were already blackening their glass.