In ufology, a close encounter is an event in which a person witnesses an unidentified flying object. This terminology and the system of classification behind it were first suggested in astronomer and UFO researcher J. Allen Hynek's 1972 book The UFO Experience: A Scientific Inquiry. Categories beyond Hynek's original three have been added by others but have not gained universal acceptance, mainly because they lack the scientific rigor that Hynek aimed to bring to ufology. Sightings more than 150 metres from the witness are classified as "Daylight Discs," "Nocturnal Lights," or "Radar/Visual Reports." Sightings within about 150 metres are subclassified as various types of "close encounters." Hynek and others argued that a claimed close encounter must occur within about 150 metres to greatly reduce or eliminate the possibility of misidentifying conventional aircraft or other known phenomena.