I believe the records used to study it today were collected in China.
But how were they observed? I remember more years ago than I like to admit my grade school class was allowed to look at a solar eclipse through smoke-blackened glass, which is probably verboten today. We later used exposed blank film negatives. I was just wondering how reliable solar observation techniques were at that time. I know the Chinese kept scrupulous astronomical records but how accurate would sunspot counts be?