An Unidentified Flying Object, or UFO, is the popular term for any aerial phenomenon that cannot immediately be identified. Some definitions, such as the one used by the American Air Force, define a UFO as an object unable to be identified after detailed scrutiny, while other definitions define an object as being a UFO from the time it is first reported as being unidentified, even though most of them subsequently become IFOs, Identified Flying Objects.
Reports of unidentified aerial phenomena date back to ancient times, long before telescopes were invented, but modern reports and the first official investigations began during World War II. UFO reports became even more common after the first widely publicized United States UFO sighting, by private pilot Kenneth Arnold in mid 1947, at the beginning of the Cold War. Hundreds of thousands of UFO reports have since been made worldwide.
(www.wikipedia.com. Adaptado)
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