An unidentified flying object is an object observed in the sky that is not readily identified. Most UFOs are later identified as conventional objects or phenomena. The term is widely used for claimed observations of extraterrestrial spacecraft. — Terminology — The term "UFO" was …
Document on sighting of a UFO occurred on December 16, 1977, in the state of Bahia, Brazil.
Fata Morgana, a type of mirage in which objects located below the astronomical horizon appear to be hovering in the sky just above the horizon, may be responsible for some UFO sightings. (Here, the shape floating above the horizon is the reflected image of a boat.) Fata Morgana can also distort the appearance of distant objects, sometimes making them unrecognizable
Lenticular clouds have in some cases been reported as UFOs due to their peculiar shape