1. The possible impact of long-term variations of solar activity on climate change remains a topic of controversy; the following three volumes are a good starting point for those interested in learning more about this: Haigh, J. D., Lockwood, M., & Giampapa, M. S.: 2005, Saas-Fee Advanced Course, Vol. 34, The Sun, Solar Analogs and the Climate, ed. I. Rüedi, M. Güdel, & W. Schmutz, Springer
2. Benestad, R. E.: 2006, Solar Activity and Earth’s Climate, 2nd edn., Springer
3. Schrijver, C. J., & Siscoe, G. L.: 2010, Heliophysics III: Evolving Solar Activity and the Climates of Space and Earth, Cambridge University Press
4. On pre- and early-telescopic observations of sunspots, begin with Mitchell, W. M.: 1916, The history of the discovery of the solar spots, Popular Astronomy, 24, 22-ff
5. Vaquero, J. M, & Vázquez, M.: 2009, Astrophysics and Space Science Library, Vol. 361, The Sun Recorded Through History: Scientific Data Extracted from Historical Documents, Springer