Chronostasis

How it works

Your eyes do not glide from one place to another. They jump. Each jump is called a saccade, and it takes between 20 and 200 milliseconds². During the jump the image smears across the retina. The brain switches vision off for that time, so you never see the smear. This is called saccadic masking².

That leaves a hole in your visual record. The brain fills it in a simple way. It takes the first stable image after the jump and stretches it backwards in time to cover the whole saccade¹. You experience the new scene as if it had been there since before the eyes moved.

Usually this is harmless. But if the first thing you land on is a clock, the second hand you land on gets stretched too. It seems to sit still for longer than a second. Then it ticks normally. That is the stopped clock¹.

The effect was measured in 2001 by Kielan Yarrow and colleagues at University College London¹. They found that the first image after a saccade was judged to last longer than it really did, and that the stretch grew with the length of the eye movement¹. Similar effects have since been found for hearing, when the head turns towards a sound, and for touch, when a hand reaches for an object¹.

Try it

1. Find an analogue clock with a second hand that ticks once a second. A digital clock showing seconds also works.
2. Look away from it, somewhere off to one side.
3. Snap your eyes back to the clock in one quick movement.
4. Watch the first tick. It seems to take longer than the ones that follow.
5. Repeat with a larger eye movement. The freeze is longer.

The effect is easier to notice than to test, because you cannot look at the clock without moving your eyes to it.

History and culture

The stopped clock is one of the few perception illusions most people have noticed on their own. The XKCD comic “Visual Field” mentions it in its notes on the eye³.