Seasonal variation of oriental sunspot sightings

Abstract

An important, though inexact, index of past solar activity^1–3 is the frequency of unaided-eye sunspot sightings in the Orient^1,3–7. When the glare of the Sun is reduced by a natural atmospheric filter, a single sunspot or compact group of sunspots located within the central part of the solar disk can be seen with the unaided eye if its area exceeds 0.05‰ of the Sun's visible hemisphere⁸. The oriental sunspot records represent one of the few available sources of solar data before the invention of the telescope^4,5. The abundance of historical sunspot records from China and Korea must be attributed partly to the regular occurrence of atmospheric conditions suitable for the detection of sunspots^4–6. Considerably diminished solar brightness, caused by, for example, atmospheric haze, dust storms, severe atmospheric absorption (for example, near sunrise or sunset) or reflection off still waters, is necessary to render sunspots visible to the unaided eye. Indeed, many of the oriental sunspot records explicitly mention reduced solar brightness⁶ compatible with these conditions. We discuss here what information the dates of the oriental sunspot sightings provide on past atmospheric conditions in the Orient and hence on past solar activity.