The umbrella should have a pattern or make stripes on a solid color umbrella with tape
Eye movements (microsaccades) http://www.michaelbach.de/ot/mot_eyeJitter/index.html from Pinna B & Spillmann L (2002) A new illusion of floating motion in depth. Perception 31:1501–1502. Involuntary eye movements – This illusion makes us aware of the small movements our eyes make. Our eyes these movements even if we think we are focusing on an object. This is to prevent the photoreceptors in the retina from desensitizing. If photoreceptors desensitize they will no longer pass the signal to the brain of the object we are seeing. It would reduce the visibility of an object.
Optokinetic Response Vertebrates exhibit numerous reflex behaviors that are driven by visual stimuli. The OKR response encompasses smooth eye rotations which track moving patterns followed by sharp movements called saccades.