The autokinetic effect is one of the hazards you’ll face when boating at night, and it’s also one that few people know about. It all boils down to this: don’t always trust your eyes on the ocean. Also known as ‘induced movement,’ the autokinetic effect is essentially a glitch in human visual perception which causes the extraocular muscles (the six muscles which control eye movement) to send the wrong messages to your brain.

Boating in the dark can play havoc with your eyes.

Boating in the dark can play havoc with your eyes.

This effect most commonly occurs when you stare at a stationary point of light in the darkness—such as another boat’s anchor light, a lighthouse, or a channel marker off in the distance. As you stare, tension in your eye muscles cause your brain to think that the light is moving, when it’s not. Often it will appear to dance on the horizon, move across the horizon, or change position, as long as you stare directly at it. In fact, you’ve probably experienced this before because most people report seeing induced movement when staring at stars for more than a moment or two—is that one a satellite, moving across the sky? No, it’s just your eyes “tricking” you. While star-gazing, of course, this isn’t a problem. But if you’re altering course to avoid a moving boat that’s actually a fixed lighthouse, the autokinetic effect can create a very real danger.

The severity of this phenomenon varies from one boater to the next, because the amplitude of movement that your brain interprets from the tensioned eye muscles is different from person to person. So one person might look at a light and think it’s moving quickly from right to left, while another sees the very same object at the same exact time, yet believes it to be moving slowly from left to right.

Luckily, there’s a simple solution: Move your eyes to a different point, and observe the light out of the corner of your eye. If it really is a stationary light, it’ll appear stationary once again. This happens because your peripheral vision is much more light-sensitive, and using it also allows your eyes to remain relaxed, as opposed to straining to focus on the object. Out of the corner of your eye, you’ll be able to detect any real movement and its direction of travel. You’ll be able to tell the difference between a fixed marker and a cruising boat, and enjoy your night on the water in full safety.

-Lenny Rudow