Sunspot visibility

This paper advances a detailed theory of sunspot visibility based on the physiology of the human eye. It is found that the average person under optimal conditions can detect a sunspot whose penumbral diameter is as small as 27 arcsec. The technique for filtering the sunlight, the observer's diet, age, and amount of close-up reading will not change sunspot visibility in practical cases, so the modern observations and theory are fully comparable with the ancient Chinese observations.