Thanks. I know there secondary effects because I know that they get attributed to sunspots. I was just curious if they were using that knowledge to work backward through time and calculate/estimate sunspot activity of if they could do it back then.
Interesting. Good to know.
Look at the chart, and note that data starts around 1610. Lenses were being ground in the late 1500’s, and Galileo directed his telescopes to the Sun around 1610.