Hynek’s first three categories are collectively known as “encounters” and generally refer to UFOs seen at a distance. The first of this trio is the “Nocturnal Light” which occurs when a witness sees a light in the sky at night behaving in a way that cannot be explained rationally. The second category is that of “Daylight Disc”, which includes all UFOs seen in the sky during daylight whether they are actually disc-shaped or not. The final agreed encounter category is the “Radar Contact”, which refers to UFOs picked up on radar. Some use a fourth encounter category: “Radar-Visual” which refers to UFOs that are tracked on radar at the same time as being seen, whether by day or night.
from "Alien Encounters" by Rupert Matthews.
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