Monthly Mean Sunspot Areas in the Last 275 Years

A reconstruction of the average monthly values of sunspot areas—the physical index of solar activity—is proposed. The uncertainty of the obtained values for a significance level $$\alpha = 0.05$$ is estimated. The duration of the number of monthly means has become 275 years. It is shown that the AR index, when used in reconstructions of solar activity of historical observations with small telescopes, is more advantageous than the indices of the number of spots or groups thereof. This index is not sensitive to the possible loss of small groups, which on average make up about a third of the overall number.