What happened
On August 12, 1883, José Bonilla, director of the observatory in Zacatecas, was observing sunspots through a telescope. At around eight in the morning he noticed small, luminous objects crossing the solar disk one after another – against it they appeared dark, sometimes with what looked like a misty halo. By the following day he had counted more than three hundred of them. He captured several on photographic plates with an exposure time of about one hundredth of a second. His account, together with a photograph, was published in 1886 by the French monthly “L’Astronomie.” Bonilla’s photograph is sometimes called the oldest UFO picture, although in the 19th century no one yet spoke of spaceships.
Witnesses
Astronomer José Bonilla and his colleagues at the Zacatecas observatory.
Explanations and hypotheses
The editors of “L’Astronomie” already suggested that Bonilla had seen birds, insects, or dust floating near the telescope; at high magnification, out-of-focus silhouettes of geese or other migratory birds could have looked like mysterious bodies. In 2011 Mexican researchers proposed that they were fragments of a comet passing very close to Earth, but this hypothesis has few supporters. Most historians regard the observation as a curiosity involving birds.
Disputed A likely explanation exists, but some of the facts are still disputed.
More: Wikipedia (en)