This same system was used for over a century by Wolf and his successors in
Both these sunspot series show consistent features over centuries. The sun's activity varies over a roughly 11-year period, rising to a maximum before dropping off again to almost nothing. We're not altogether sure about the cause, but maximum activity always occurs just before a regular flip in the polarity of the sun's magnetic field; the next such flip is expected within months. Not all peaks and troughs are equal. There are extended periods of low activity, such as the Maunder minimum, as well as prolonged periods of high activity. Some solar physicists think we may be entering a long quiet period now.
But how sure can we be? In the 1980s, nearly 100 years after Wolf's death, the seemingly model historical continuity of the sunspot series was seriously called into question.
These earlier observations counted only groups of sunspots, so the new series, ending in 1995, was directly comparable with that based on Wolf's method only after careful cross-checking and calibration. That turned up a surprise. Although the two series agreed fairly well in some places, in others they differed radically. In some periods before 1885, the new group sunspot number was lower than Wolf's by as much as a half (see diagram, below). Between 1945 and 1995, the Wolf numbers near maximum were consistently higher than the numbers revealed by the group method.
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