At the heart of most misidentifications is that most people don't know the night sky.
Let's start with Venus. Apart from the Moon, it's the brightest object in the night sky. It's bright enough to be seen in light-polluted cities and shines even in the twilight. Sometimes you can see it during the day if you know where to look.
Venus is responsible for a vast number of UFO reports. Before people mistook it for a flying saucer, Venus caused scares about airships and later of enemy aircraft. Although I've never imagined Venus to be a flying saucer, I have mistaken it for an approaching aircraft. But if you keep watching, aircraft change course and reveal their navigation lights. Venus doesn't noticeably move over a short time period.
Nonetheless a planet (or a star) may appear to move in some circumstances. Here are some of them:
Our eyes focus on the bright object. If clouds are moving, it gives the startling impression that the object is moving around. Sometimes there is thin drifting cloud which you may not even see. What you will see is the object brightening and dimming. Our brains interpret this as movement towards and away from us.
- Because you are moving.
An astronomical object is very far away, and its direction doesn't change as we travel in a vehicle. So it seems to follow us. When you were a child, you probably saw the Moon doing this.
- Autokinetic effect
Autokinesis is an effect of the human visual system. It makes a small non-moving point of light seem to wander around if its background lacks reference points.
- The Earth turns on its axis.
During the night, as the Earth turns, stars and planets rise, move across the sky, and then set. Anything rising low in the sky may be hidden by buildings or hills and then seem to appear suddenly.
Mercury generates the fewest UFO reports because it's hardest to see, but Mars, Saturn and Jupiter often get reported as UFOs.
In October 2010, a reporter arrived with a cameraman at a New York City street, looking for UFOs that had supposedly been sighted earlier. She was excited to find one still hovering, burbling that we mustn't be fooled by its looking like a star.
Actually, it looked more like Jupiter than a star and when the camera was turned on it, you could see the planet with its four largest moons. But the reporter got particularly excited when the camera zoomed and she could see all these lights moving around. This is a good example of magnification as another cause of apparent movement. As magnification increases, the camera or other optical device has to be steadier. Every tiny tremor makes what you see jump around.
The distortion caused by our atmosphere can also make stars and planets twinkle, change color or seem to make jerky movements. The more air the light travels through, the greater the effect, so objects near the horizon may be strikingly affected. A famous "UFO" case included three "star-like" objects hovering for hours low on the horizon. A senior military man said they "moved rapidly in sharp, angular movements and displayed red, green and blue lights."
Considering the atmospheric effects and other reasons for seeing apparent motion, you may wonder if the star-like objects were perhaps stars. Since the witness gave a good account of their positions, a planetarium program identified them as the three bright stars, Deneb, Vega and Sirius.
So, in summary, how do you tell a planet (or star!) from a flying saucer? Although I haven't seen a definitive alien spacecraft identification guide, there are many excellent astronomy guides available. You can also check the positions of stars and planets for any time and location on Heavens-above.com. Finally, planetarium software such as Stellarium is available as a free open source program.
(1) Bernier, D & Horn, L "The Autokinetic Effect: Variables of Perception"
(2) Moore, R "Common IFO Phenomena" http://www.deltapro.co.uk/IFOguide.HTML (accessed 11.11.10)
(3) Printy, T "Venus: The queen of UFOs" http://home.comcast.net/~tprinty/UFO/Venusufo.htm (accessed 11.14.10)
(4) Ministry of Defence report on Devon "flying cross" incident, document released in 1998
(5) Ridpath, I "What were the other lights seen by Col Halt?"
http://ianridpath.com/ufo/rendlesham3.htm (accessed 11.10.10)
(6) Ridpath, I "Devon 'flying cross' of 1967 revisited" http://www.ianridpath.com/ufo/flyingcross.htm (accessed 11.10.10)
(7) Slight, D "PCs Play Tag with Light in the Sky" http://www.ianridpath.com/ufo/image/flyingcrossMirror.jpg