2.3 Wolf’s reconstruction of earlier data

Wolf extended the data back to 1749 using the primary observers along with many secondary observers but much of that earlier data is incomplete. Wolf often filled in gaps in the sunspot observations using geomagnetic activity measurements as proxies for the sunspot number. It is well recognized that the sunspot numbers are quite reliable since Wolf’s time but that earlier numbers are far less reliable. The monthly averages of the daily numbers are shown in Figure 2View Image.
View Image

Figure 2: Monthly averages of the daily International Sunspot Number. This illustrates the solar cycle and shows that it varies in amplitude, shape, and length. Months with observations from every day are shown in black. Months with 1 – 10 days of observation missing are shown in green. Months with 11 – 20 days of observation missing are shown in yellow. Months with more than 20 days of observation missing are shown in red. [Missing days from 1818 to the present were obtained from the International daily sunspot numbers. Missing days from 1750 to 1818 were obtained from the Group Sunspot Numbers and probably represent an over estimate.]

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