clockwise? or counter-clockwise?

The Spinning Dancer, also known as the silhouette illusion, is a kinetic, bistable optical illusion resembling a pirouetting female dancer. The illusion, created by web designer Nobuyuki Kayahara, involves the apparent direction of motion of the figure. Some observers initially see the figure as spinning clockwise and some counterclockwise.

The illusion derives from the lack of visual cues for depth. For instance, her arms could be swinging either in front of her to the left or behind her to the left, and hence with her circling clockwise or counter-clockwise on either her left or right foot. She changes leg because she is facing either towards or away from the observer, there being no surface features on the silhouette to indicate at any point which side of her is presented: the least ambiguous positions are her profiles when she is on either side of her circle, though it’s still not known whether the foreground or background leg is on the floor, and from where she moves indeterminately either on the near or far arc across to the other profile.”

Bistable perception
“Depending on the perception of the observer, the apparent direction of spin may change any number of times, although some observers may have difficulty perceiving a change in motion at all. One way of changing the direction perceived is to use averted vision and mentally look for an arm going behind instead of in front, then carefully move the eyes back. Some may perceive a change in direction more easily by narrowing visual focus to a specific region of the image, such as the spinning foot or the shadow below the dancer and gradually looking upwards. One can also try to tilt one’s head to perceive a change in direction. ”

It is not the dancer that spins, it is you.