Why Those White Dots in the Blue Sky Are Your Own Blood Cells

(Symbolbild) On clear days, many people notice tiny bright dots darting across the blue sky, following winding paths through their field of vision. Eye doctor Richard Scheerer described this phenomenon as early as 1924. The dots follow fixed paths and accelerate in time with the heartbeat. Their origin lies not in the air, but in the viewer's eye. (Photo: © Forschung und Wissen, AI image) When looking at the blue sky on a clear day, with a little patience, one can see tiny bright points flitting along twisted paths. These points are neither dust nor imagination. They follow defined paths, disappear within fractions of a second, and speed up with each heartbeat. A German ophthalmologist described this phenomenon in 1924, and it is caused just a fraction of a millimeter in front of the photoreceptor cells. The phenomenon is most distinctly observed on a cloudless day when gazing into a uniform blue sky away from the sun. After a few seconds, the points appear: tiny, equally sized light spots that jerkily dart along short, winding paths before disappearing. Some leave a brief dark tail behind them, while others seem to follow the same path as a predecessor just before. The dots appear in an area about 15 degrees around the point that the eye is currently fixating on, but never exactly at the center. Many people have seen them throughout their lives without ever thinking about them, while others only notice them when they specifically pay attention. They are harmless in both cases, and they provide one of the rare opportunities for a person to observe a process happening within their own body. In technical terms, this phenomenon is referred to as 'Blue Field Entoptics' or the 'Scheerer phenomenon,' named after its describer. 'Entoptic' means that the cause of a visual impression lies within the eye itself, not in the external world. In 1924, ophthalmologist Richard Scheerer published a detailed study in the 'Klinische Monatsblätter für Augenheilkunde' on the entoptic visibility of blood movement within the eye, highlighting its clinical importance. Others had previously observed the light dots, but Scheerer directed the medical community's attention to them. However, his hypothesis that the dots were related to blood flow in the retina could only be directly tested decades later. This necessitated microscopes, video technology, and ultimately methods to film individual blood cells in the live human eye. White Blood Cells in the Retina Create the Light Dots The bright points seen in the blue sky are white blood cells traveling through the tiniest capillaries in front of the retina. Red blood cells absorb blue light and cast shadows, which the brain filters out. White blood cells, however, allow blue light to pass through and thus appear as bright, moving light spots. Blue Light at 430 Nanometers Makes the Blood Vessels Visible The human retina is somewhat inverted; blood vessels and nerve fibers lie in front of the light-sensitive photoreceptor cells, and light must first pass through them. The red blood pigment hemoglobin strongly absorbs blue light with a wavelength of about 430 nanometers. The capillaries, densely filled with red blood cells, constantly cast shadows onto the photoreceptors beneath them in blue light. Since these shadows remain in the same position, the photoreceptors adapt, and the brain filters out the vessel pattern. White blood cells are larger than red blood cells, often wider than the capillary itself, and must deform to pass through. Before them, a gap forms in the flow of red blood cells, stacking up behind them. Through the gap, blue light suddenly falls unimpeded onto the photoreceptors, which are accustomed to shadows, and at that precise point, a bright dot with a dark tail appears. Points in the Blue Sky Follow the Heartbeat The blood flows through capillaries not uniformly but in spurts in rhythm with the heart. Consequently, the light spots momentarily accelerate with each heartbeat, an observation one can verify easily by feeling one's pulse. As they originate from fixed capillary paths, successive points often take the same route. Each eye sees different points, as each has its own vascular network; when viewed with both eyes, the patterns overlap. The exact center of the visual field remains calm because the point of sharpest vision in the fovea is free of blood vessels to ensure that no shadow interferes with the light. Thus, the points dance in a ring around the fixed point. The blue field entoptics appear most strongly against a uniformly blue background, but in everyday life, a clear sky is sufficient.