Jul 4, 2007
*1980, Rendlesham Forest: US airbase staff see strange lights in woods
*1957, RB-47 encounter: US jet followed by UFO for 700 miles
1979, Livingston: Forestry worker sees dome-shaped object
*1950, McMinnville: Farm couple photograph saucer
*1961, Betty and Barney Hill incident: Couple see UFO and under hypnosis describe abduction
Sundogs: Refracted image of the sun
Space debris: Burning satellites or rocket fragments
Meteors: Such as bolides or fireballs
Clouds: Lenticular or disc-like
Mirages: Hot or cold-air induced images
Stars/planets: Such as Venus
Planes: Such as experimental aircraft
Ball lightning: Unpredictable brilliant spheres
Weather balloons: Classic explanation
Hallucination: Viewer under stress
Mass hysteria: Early explanation
Earthlights: Caused by electromagnetic fields in seismic activity areas
Sixty years ago Kenneth Arnold saw something which changed his own life and the lives of millions of others, and impacted on popular culture like a shockwave.
Flying his plane near Mount Rainier in the US state of Washington, he observed a line of strange objects either crescent-shaped or disc-like, flying with the motion of a saucer skimming on water.
Arnold's sighting, quickly picked up by the press, was followed a fortnight later by the revelation of perhaps the most notorious episode in the history of UFOs, at Roswell in New Mexico.
Having announced it had recovered a "flying disk", the Army airfield backtracked and referred only to a weather balloon.
What followed was perhaps one of the greatest conspiracy theories of all time, involving post-mortem examinations of swollen-bellied grey aliens, the cloning of sophisticated extraterrestrial technology and an epic cover-up. Or not, as the case may be.
In the 60 years since 1947's first major wave of sightings, thousands of ordinary people have claimed to have seen inexplicable objects in the sky.
When the Ministry of Defence released papers on its own investigations into the phenomenon in 2006, it was revealed more than 10,000 eyewitness accounts had been collected.
And for every sceptic who prefers explanations of weather balloons and freak atmospheric conditions there is someone who genuinely believes intelligent life is visiting the planet.
A national newspaper survey in 1998 suggested 33% of men and 24% of women thought aliens had already visited the earth.
Such polls are testament to the powerful impact of six decades of media coverage, disputed science, heated mythology and Hollywood films. We have now completed six decades of projecting our hopes and fears onto the UFO phenomenon.
David Clarke, a lecturer in journalism, has spent 30 years studying UFOs and the sociology of the flying saucer sighting.
He is no believer in little extraterrestrial men, but believes mainstream scientists should recognise the rational explanations for sightings are themselves worthy of study.
"They wouldn't touch it," he says. "It's got such a bad press. Anything that people don't have an immediate explanation for - it must be little green men."
The "ufologists" who study the phenomenon comprise both sceptics and believers. They seek to "resolve" each incident, explaining away each aspect. And there is a wealth of explanations for most sightings that is as fascinating for sceptical enthusiasts as the notion of space visitors.
Sundogs, or strange refractions of the sun in another part of the sky, burning space debris, weather balloons, ball lightning, meteors, disc-shaped or lenticular clouds, mirages, even the planet Venus low in the sky, are all classic methods of resolving UFO sightings.
But underlying them is a need also to explain people's desire to believe that a UFO sighting can be explained by alien activity. The timing of the start of the golden age of the UFO, in a Western world recovering from World War II and gearing up for the start of the Cold War, is significant.
"We were projecting things to reflect our fears and concerns about the Cold War," Mr Clarke says
"Organised religion was in decline but when worried or concerned it is comforting to feel there is a greater power looking after us. It is quite nice to think there is another civilisation that has been able to overcome the things destroying our civilisation."
The UFO phenomenon is also linked with the modern reliance on conspiracy theories, a mixture of a need to believe in something more than the mundane in an increasingly rational world and an all-pervading distrust of authority.
As the Fortean Times, which this month dedicates an entire issue to the UFO anniversary, puts it: "UFOs fill a niche in the human spirit that thrives on wondrous ideas."
Earlier generations had also seen UFOs but without the term flying saucer in existence, they were labelled as other things.
UFO students say there are peaks and troughs in sightings that are probably based on cultural, social and political trends.
Expert Paul Devereux says a new golden age during the 1990s, particularly after the broadcast of the cult television series the X-Files, has given way to a current wave of indifference.
Mr Clarke concurs, suggesting: "It could be the case that post-9/11 people are more concerned about the threat from terrorism or the environment."
US military said a crashed weather balloon explained Roswell
Mr Devereux has drawn on the work of controversial Canadian academic Michael Persinger and believes many unresolved UFO sightings can be explained by "earthlights", clouds of plasma being charged by strong electromagnetic fields occurring in areas of seismic activity.
Having witnessed a UFO that could not initially be explained, Mr Devereux has dedicated his life to research.
"It bugged the hell out of me, almost gave me a mental breakdown. I couldn't make it fit into the everyday mundane world view."
Pilot Ray Bowyer was the principal witness to the most recent publicised UFO sighting in the UK.
Flying a commercial plane from Southampton to Alderney in the Channel Islands in April this year, Mr Bowyer saw two objects up to a mile across in the sky over Guernsey.
"I saw a bright yellow object, a light in the sky some miles ahead. I could see this specific shape of a flattened disc, like a CD on its edge, slightly tilted."
He says some of his passengers, as well as another pilot, saw the objects and he has been told they were picked up on radar.
Mr Bowyer's sighting may be a prime candidate for the "earthlights" theory, coming just days before the Dover earthquake. He accepts this as a possible explanation.
"I'm open-minded about everything. It would be a fairly perverse universe if we were the only inhabitants."
Despite the drop-off in interest in UFOs, the ufologists and their acolytes carry on their work, and the UFO-loving public continues to believe in conspiracies.
"No matter how much material the authorities produce and release the people who want to believe a conspiracy to hide aliens will never be satisfied," Mr Clarke says.
"It is such an emotional thing. They are convinced they are here, that they are walking among us."