The sunspot cycle, 649 B.C. to A.D. 2000

Annual sunspot numbers since 1700 and the known maxima and minima since 1610 show a similarity of pattern from century to century. This suggests that the mean cycle is approximately 11-1/9 years. The records of sunspots and aurorae enable magnitudes and dates of sunspot maxima since at least A.D. 300 to be estimated. The constancy of the mean cycle over long periods enables the number of missing maxima to be calculated, and, using certain general principles, a table of minima complete since at least 200 B.C. can be established. A 78-year cycle appears to exist in the length of the sunspot cycle and an irregular cycle of about 200 to 205 years may exist in auroral intensity. A characteristic pattern in even centuries enables some predictions to be made for the next 50 years. Intervals between intense maxima in the range 200 to 1,000 years apart show clusters at certain values; these values are close to multiples of 11.11. Intervals between well-dated maxima since A.D. 300 are often slightly less than such multiples, for example, 554 instead of 555; from B.C. 200 to A.D. 300, intervals are slightly greater. In classical and early medieval times the cycle was thus slightly less. The variability of the sunspot cycle is only apparent. The fundamental rhythm of 11.1 years (together with the 78-year cycle) is constant through the centuries; temporary aberrations are partly due to variations in sunspot intensity, inasmuch as active cycles tend to become “early” and weak cycles “late.”