Modified from Wikipedia.
The Miracle of the Sun is a miraculous event that was witnessed by as many as 100,000 people on 13 October 1917 in the Cova da Iria fields near Fátima, Portugal.
According to many witness statements, after a downfall of rain, the clouds broke and the sun appeared as an opaque, spinning disk in the sky. It was said to be significantly less bright than normal, and cast multicolored lights across the landscape, the shadows on the landscape, the people, and the surrounding clouds. The sun was then reported to have careened towards the earth in a zigzag pattern, frightening some of those present who thought it meant the end of the world. Witnesses reported that the ground and their previously wet clothes became completely dry.
Estimates of the number of witnesses range from "thirty to forty thousand" by Avelino de Almeida, writing for the Portuguese newspaper O Século, to one hundred thousand, estimated by Dr. Joseph Garrett, Professor of Natural Sciences at Coimbra University, both of whom were present that day.
The miracle was attributed by believers to Our Lady of Fatima, an apparition of the Blessed Virgin Mary to three young shepherd children in 1917, as having been predicted in advance by the three children on 13 July , 19 August , and 13 September  1917. The children reported that the Lady had promised them that at mid-day on 13 October in the Cova da Iria the Lady would reveal her identity to the children and provide a miracle "so that all may believe."
According to witness reports, the miracle of the sun lasted approximately ten minutes. The three shepherd children, in addition to reporting seeing the actions of the sun that day, also reported seeing a panorama of visions, including those of Jesus, the Blessed Virgin Mary, and of Saint Joseph blessing the people.
The most widely-cited descriptions of the events reported at Fatima are taken from the writings of John De Marchi, an Italian Catholic priest and researcher. De Marchi spent seven years in Fatima, from 1943 to 1950, conducting original research and interviewing the principals at undisturbed length. In The Immaculate Heart, published in 1952, De Marchi reports that, "[t]heir ranks (those present on 13 October) included believers and non-believers, pious old ladies and scoffing young men. Hundreds, from these mixed categories, have given formal testimony. Reports do vary; impressions are in minor details confused, but none to our knowledge has directly denied the visible prodigy of the sun."
Some of the witness statements follow below. They are taken from John De Marchi's several books on the matter.
- "Before the astonished eyes of the crowd, whose aspect was biblical as they stood bare-headed, eagerly searching the sky, the sun trembled, made sudden incredible movements outside all cosmic laws — the sun 'danced' according to the typical expression of the people." ― Avelino de Almeida, writing for O Século (Portugal's most widely-circulated and influential newspaper, which was pro-government and anti-clerical at the time Almeida's previous articles had been to satirize the previously reported events at Fatima). However, Almeida had not abandoned the Catholic faith.
- "The sun, at one moment surrounded with scarlet flame, at another aureoled in yellow and deep purple, seemed to be in an exceeding fast and whirling movement, at times appearing to be loosened from the sky and to be approaching the earth, strongly radiating heat." ― Dr. Domingos Pinto Coelho, writing for the newspaper Ordem.
- "…The silver sun, enveloped in the same gauzy grey light, was seen to whirl and turn in the circle of broken clouds… The light turned a beautiful blue, as if it had come through the stained-glass windows of a cathedral, and spread itself over the people who knelt with outstretched hands… people wept and prayed with uncovered heads, in the presence of a miracle they had awaited. The seconds seemed like hours, so vivid were they." ― Reporter for the Lisbon newspaper O Dia.
- "The sun's disc did not remain immobile. This was not the sparkling of a heavenly body, for it spun round on itself in a mad whirl, when suddenly a clamor was heard from all the people. The sun, whirling, seemed to loosen itself from the firmament and advance threateningly upon the earth as if to crush us with its huge fiery weight. The sensation during those moments was terrible." ― Dr. Almeida Garrett, Professor of Natural Sciences at Coimbra University.
- "As if like a bolt from the blue, the clouds were wrenched apart, and the sun at its zenith appeared in all its splendor. It began to revolve vertiginously on its axis, like the most magnificent firewheel that could be imagined, taking on all the colors of the rainbow and sending forth multi-colored flashes of light, producing the most astounding effect. This sublime and incomparable spectacle, which was repeated three distinct times, lasted for about ten minutes. The immense multitude, overcome by the evidence of such a tremendous prodigy, threw themselves on their knees." ― Dr. Formigão, a professor at the seminary at Santarem, and a priest.
- "I feel incapable of describing what I saw. I looked fixedly at the sun, which seemed pale and did not hurt my eyes. Looking like a ball of snow, revolving on itself, it suddenly seemed to come down in a zig-zag, menacing the earth. Terrified, I ran and hid myself among the people, who were weeping and expecting the end of the world at any moment." ― Rev. Joaquim Lourenco, describing his boyhood experience in Alburitel, eighteen kilometers from Fatima.
- "On that day of October 13, 1917, without remembering the predictions of the children, I was enchanted by a remarkable spectacle in the sky of a kind I had never seen before. I saw it from this veranda…” ― Portuguese poet Afonso Lopes Vieira.
While De Marchi's states that "Reports do vary; impressions are in minor details confused", Kevin McClure states that he had never seen such a collection of contradictory accounts of a case in any of the research he had done in the previous ten years.
No scientific accounts exist of any unusual solar or astronomic activity during the time the sun was reported to have "danced", and there are no witness reports of any unusual solar phenomenon further than forty miles out from Cova da Iria.
It has been alleged that the fact that an unspecified "miracle" had been predicted in advance, the abrupt beginning and end of the alleged miracle of the sun, the varied nature of the observers as including both skeptics and believers alike, the sheer numbers of people present, and the lack of any causative factor, all reasonably preclude the theory of a mass hallucination. That the activity of the sun was reported as visible by those up to 18 kilometers away, also precludes the theory of a collective hallucination or mass hysteria.
Visionaries claimed that the apparition now known as Our Lady of Fatima had promised in July, August, and September, that a miracle would occur on 13 October 1917 "so that all may believe". Pio Scatizzi, S.J. describes events of Fatima and concludes
This miraculous even was not confined in time and space as Pope Pius XII himself witnessed the miracle of the sun from the Vatican gardens in a heavenly confirmation of the momentous decision he had made in proclaiming a dogma ex cathedra..
The ... solar phenomena were not observed in any observatory. Impossible that they should escape notice of so many astronomers and indeed the other inhabitants of the hemisphere… there is no question of an astronomical or meteorological event phenomenon …Either all the observers in Fatima were collectively deceived and erred in their testimony, or we must suppose an extra-natural intervention.
Here's what Time magazine has to say:
Millions of Roman Catholics revere a spot near Fátima, in Portugal, as the scene of a miraculous appearance of the Virgin Mary. In Fátima last week stood Federico Cardinal Tedeschini, archpriest of St. Peter's Basilica in Rome. To gathered pilgrims he brought momentous news.
On three successive days, Oct. 30, Oct. 31 and Nov. 1, 1950, said the Cardinal, the miraculous vision of Fátima was repeated for Pope Pius XII: "The Holy Father turned his gaze from the Vatican gardens toward the sun, and there was renewed for his eyes the miracle of this valley." Nov. 1 was the day the Pope proclaimed the dogma of the bodily ascension of the Virgin Mary into Heaven.