As a youth fascinated by the idea that some of these stories might just be true, I spent countless hours on cold, windy hilltops watching the UFO 'believers' and hoping to see what they saw. This effort was completely successful: I witnessed many 'sightings'.
Of all the witnesses, two west country policeman are perhaps the most memorable: they had become quite famous after spending most of an evening chasing a mystery object that hovered in front of their patrol car, darting from side to side and sometimes approaching very close as they hurtled along at 90mph. I remember them being distinctly disenchanted when shown astronomical tables that placed Venus right in front of them on the straight bits of road.
These fine officers, and most other witnesses had fallen foul of two properties of the human eye:
Firstly, it's extreme sensitivity - the ability to easily see a 3W torch bulb suspended on a hill top 6 mi away for instance. Unfortunately, the eye is not calibrated, and a pin point source has an apparent brightness linked to the surrounding environment. That is how Venus can be described by witnesses as 'dazzling' and so on.
Secondly, once denied normal visual cues in low light situations, the eye can begin rapid involuntary movements. This makes distant light sources (eg stars, planets, satellites and aircraft) appear to 'dart' about the sky.
Just note how many UFO stories involve 'dazzling' objects that 'dart' about!
I am not suggesting that all sightings can be explained in this way. But very many can. And there are important lessons for professional observers like Pilots and Policemen. The eye can certainly deceive and the more 'expert' the witness, the more extreme the mis-interpretation of visual cues can become. That's just because we all share the same physiology and no amount of training can change that, whatever any given individual might think.
PS
I never really gave up on the UFO idea. The astonishing and continuing discovery of hundreds of extra-solar planets in the last few years gives the whole subject new momentum. But 'investigating' the anecdotes of witnesses is a hard road that rapidly becomes impassible if any kind of objective test is applied to the data. The subject was IMHO best discussed many years ago by Prof RV Jones, the man who 'discovered' the V2 rocket. His account is
here.