A UFO or Unidentified Flying Object is any object or optical phenomenon observed in the sky which cannot be identified, even after being thoroughly investigated by qualified people.
Reports of strange apparitions in the sky have been recorded throughout history, some which may have been comets, bright meteors, or atmospheric optical phenomena such as parhelia; others remain unexplained. The army of Alexander the Great in 329 BC saw "two silver shields" in the sky. In 1235 the army of Oritsume in Japan saw mysterious lights in the sky. On April 14, 1561 the skies over Nuremberg were reportedly filled with a multitude of objects, including cylinders and spheres, seemingly engaged in an aerial battle. Previously treated as divine portents, angels, and other religious omens, such phenomena came to be regarded as a non-supernatural, suitable for scientific investigation, in the immediate aftermath of World War II, when the term "UFO" was coined.
There were several reports of unidentified aircraft in the Scandinavian nations in the 1930s. In both the European and Japanese aerial theatres during World War II, " Foo-fighters" (balls of light that followed aircraft) were reported by both Allied and Axis pilots. In 1946, there were numerous reports of unidentified aircraft in the Scandinavian nations, along with isolated reports from France and Portugal, broadly referred to as "Russian hail" because it was thought for a time that these mysterious objects were Russian tests of captured German V1 or V2 rockets. This was subsequently shown to not be the case, and the phenomenon remains unexplained to this day, although experts generally believe that a significant fraction of the reports were based on misperceptions of natural phenomena. Today, the "Russian Hail" is more generally referred to as "Ghost Rockets."
