A debunkers' guide to UFOs was today issued by a leading astronomer who believes people have been fooling themselves over sightings of alien spaceships for 50 years.
It all began in 1947 with a flock of geese glinting in the sun, said Dr Jacqueline Mitton from the Royal Astronomical Society.
American pilot Kenneth Arnold, puzzled when he saw the geese from his private plane, described them as flying "like a saucer would if you skipped it across the water". Thus the term "flying saucers" was born.
Since then interest in UFOs had grown to become a worldwide obsession, said Dr Mitton. But she is convinced every sighting can be explained by natural or man-made phenomena.
Speaking at the British Association Festival of Science at Sheffield University, she said so many people now believed in UFOs that respectable scientists were afraid to get involved in debunking them. It was time for real scientists to confront "pseudo science" she argued.
"I don't there is a single scrap of evidence that we have been visited by aliens, or that any strange lights people see in the sky or any other strange phenomena is anything to do with aliens coming to the earth," she said.
Dr Mitton, who acts as the Royal Astronomical Society's press officer, said she became interested in UFOs after meeting a number of "believers" while taking part in a TV show on the paranormal.
Investigating the phenomenon, she came up with a list of the most common reasons why people think they see UFOs. They include:
Meteors. Large meteors entering the atmosphere may burn up to produce brightly coloured lights in the sky.
Noctilucent clouds. High ice crystal clouds usually seen in northern latitudes which glow when the sun shines through them.
Lenticular clouds. Lens shaped clouds that look remarkably like a flying saucer. Aurora (northern lights). Appearing in areas where they are not normally seen, have been mistaken for "rays" projected from a space ship.
Crepuscular rays. Shafts of sunlight appearing between clouds which again have been mistaken for rays from a UFO.
Sun Dogs. A phenomenon where bright spots of light appear on clouds caused by illuminated ice crystals.
Birds. Geese flying at a high altitude in a V formation and reflecting the sun, like those which started the whole flying saucer craze.
Planets and stars. Venus and the star Sirius in particular can be exceptionally bright and "twinkle" in the sky, appearing to "come and go".
Other sightings could be explained by military aircraft, weather balloons and real space craft such as the Mir space station.
Dr Mitton said it was possible the UFO craze had undermined genuine investigations into the possibility of life beyond the earth. An example was the Seti (search for extra-terrestrial intelligence) programme which an initially enthusiastic NASA had stopped funding.
However Dr Mitton did not rule out the idea of extra-terrestrial life or even the possibility that aliens might one day visit the earth.
"I have to believe there probably is, was or will be life somewhere, but it could be so scattered in time or space that we wouldn't necessarily have had the good fortune to interact with it in any way," she said.