parallax
Parallax is the apparent shift an object shows against its background when the observer's position changes. In astronomy it is the basis of distance measurement, and in UFO sightings it helps in judging distance and size.
Parallax is the apparent shift of a nearby object against a more distant background when the observer changes position. It is easy to try: if we look at an outstretched finger alternately with the left eye and then the right eye, the finger "jumps" in front of the background. The closer the object, the larger the shift.
In astronomy
Astronomers use parallax to measure the distances of stars. Over half a year, Earth's orbit around the Sun carries it to two points far apart from each other, and nearby stars shift slightly relative to the distant background stars. The star's distance can be calculated from the angle of this shift. The method rests on geometric foundations and is one of the fundamental rungs of cosmic distance measurement.
Significance for UFO/UAP sightings
Parallax also plays a role in an everyday sense when sightings are evaluated. It is difficult for a single observer to judge the distance and size of an object in the sky, because there is no point of reference. If the observer is moving (for example, sitting in a car), a distant celestial body such as the Moon or a bright planet seems to "follow" them, since the parallax is practically zero. A nearby object, by contrast, shifts quickly against the background.
According to skeptical researchers, the absence or misunderstanding of parallax is a common reason why observers mistake distant lights for nearby objects following them. By comparing several sightings or recordings made from different locations, however, the object's distance and true size can be estimated on the basis of parallax, which makes it a useful tool in identification.