| The diagram above shows the sunspot number for the years around 2008/2009. The horizontal axis is a day counter known as the Julian Date (here minus 2,450,000 days). The sunspot counts on the vertical axis are calculated as the total number of spots plus ten times the number of sunspot groups. The red curve shows the International Sunspot Data, which are the continuation of the Zürich sunspot numbers, which go back to the middle of the 19th century. The green curve shows my own counts, which have been reported to the British Astronomical Association, and the blue curve shows the collated data from all BAA observers. For all three curves the daily spot counts have been averaged first over 30 days and then over 9 such 30-day periods.
Since the 18th century, solar activity has been going up and down in cycles that are on average about 11 years long. A minimum of sunspot numbers is overdue, but even the 9x30-day averages are not smooth enough to be sure of the minimum until quite some time after it has happened. The graphs include data up to November 2009, and only around that time can we have at least some confidence that the minimum has occurred. The time of minimum seems to have been in November or December 2008.
During the second half of the 17th century there was a lengthy period of low solar activity. With the current minimum occurring rather late, there have been worries that another long period of hardly any sunspots might be starting. It is still too soon to be certain, but the data are still consistent with a normal rise of sunspot counts between now and another maximum in 2013.
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