I am away this week filming the pilot for The Skeptologists. For NeuroLogica this week I am updating and editing some previous essays that I have written. This one was originally published in my Weird Science column in February 2005.

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I once saw a UFO. That is, I saw an object in the sky I couldn’t identify. Chances are you have too, probably more than once. What I saw were lights in a large “V” shape, moving silently, too slow to be a plane, moving out of view after about 10 minutes. Was it a flying saucer, an alien spacecraft, a time-traveling psychic Bigfoot, or perhaps something more prosaic-something boring?

There are thousands of reported UFO sightings each year, and in this digital age you can easily find numerous pictures and video clips on the internet. Does this mean we are being visited by alien spacecraft? Probably not.

After more than half a century of fascination with flying saucers, there has yet to emerge a single piece of credible evidence that we are being visited by aliens. There isn’t one unambiguous photograph or video that holds up to scientific scrutiny, not one piece of physical evidence. No smoking saucer.

Any reasonable person should ask believers why that is. Believers will often counter that the aliens don’t want us to know they are here (in which case they are doing a pretty bad job of hiding their presence, what with all the crashed saucers and anal probing), but that is just special pleading. No evidence is still no evidence.

Skeptics also point out that the very concept of a “flying saucer” was born of nothing more than a reporter’s liberties. In 1947, pilot Kenneth Arnold started the modern flying saucer craze when he reported seeing several UFOs. He described them as boomerang-shaped, but also noted that they were hopping, like a saucer skipping on the water. A reporter then coined the phrase “flying saucer” and the image stuck. And the fact that most UFO witnesses report seeing saucer-shaped objects demonstrates how suggestible we are.

There are numerous known stimuli for unusual or unexplained sightings. Astronomical objects seem to be the most commonly mistaken for UFOs; Venus is often a bright and unexpected addition to the early evening or early morning sky, for example. On rare occasions Venus may also sport a halo, giving it an even more unusual appearance. (This is likely the source of President Jimmy Carter’s UFO sighting.) The crescent moon can seem eerie peeking through the clouds, and it can seem to follow a traveling observer. In addition to natural objects, countless man-made artifacts now clutter the sky: satellites, planes, rockets, weather balloons, experimental aircraft and more. Then, too, there are outright hoaxes.

Proponents of the “extra-terrestrial hypothesis” (ETH) often point out that there is a residue of unexplained sightings, occurrences that can only be due to real flying saucers. These partisans are committing, however, a suite of logical fallacies. First, “currently unexplained” does not equal “unexplainable”-a good explanation may be just around the bend. Second, “unexplained” does not mean alien spacecraft (a bit of illogic called the argument from ignorance)-unexplained just means unexplained. Third, the fact that there remain unexplained cases does not necessarily point to the ETH. Given the millions of such sightings, isn’t it reasonable to propose that there should by necessity be a small percentage of unexplained cases, even in a world without alien visitors? Sometimes we just can’t explain things. That doesn’t mean a specific improbable theory must be right.

That there are no flying saucers does, of course, mean we should try to explain how so many observers could be mistaken. Well, this is not as difficult as it may seem. First, most sightings are of points or shapeless blobs of light-those could be any number of mundane things. Other sightings are of shiny or metallic-seeming objects, but without clear detail to suggest a spacecraft.

Sometimes people do report details, like windows or fins. They also report objects moving at fantastic speeds or carrying out seemingly impossible maneuvers. However, when viewing an object against the sky, without a clear background for reference, it is impossible to estimate size, distance, and speed, and we are subject to optical illusions. Such details are therefore not reliable, and there are numerous cases when they are demonstrably wrong.

Also, human beings have an innate tendency to perceive details that are not present, often triggered by expectation or suggestion. And our memories are not reliable; they are malleable and subject to contamination. Even the so called “reliable” witness can be unreliable: Air Force pilots mistake common objects for UFOs all the time.

It is admirable to look up into the sky with awe and wonder. Astronomy is awesome, and true scientific mysteries invite our wonder. But curiosity must be coupled with intellectual discipline. We should be aware of the limitations of our own observations and memory, the human tendency toward suggestibility and wishful thinking, and the dictates of logic.

So what was that object I saw in the sky? Turns out it was five mischievous ultralight-airplane pilots, flying in formation. But if I had never discovered the truth, it wouldn’t mean we were being visited by alien spacecraft.