Scheiner, Christoph. On Sunspots. University of Chicago Press. 2010: 83.
Schwabe (1843) 太陽觀測載於Google圖書 (Observations of the sun in the year 1843), Astronomische Nachrichten, 21 : 233-236. From page 235: "Vergleicht man nun die Zahl der Gruppen und der flecken-freien Tage mit einander, so findet man, dass die Sonnenflecken eine Periode von ungefähr 10 Jahren hatten … " (If one compares the number of groups [of sunspots observed on the sun] and the sunspot-free days with one another, then one finds that the sunspots had a period of about 10 years … )
Williams GE. Precambrian Cyclic Rhythmites: Solar-Climatic or Tidal Signatures?. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. 1990, 330 (1615): 445. Bibcode:1990RSPTA.330..445W. S2CID 123165017. doi:10.1098/rsta.1990.0025.
Williams GE. Precambrian Cyclic Rhythmites: Solar-Climatic or Tidal Signatures?. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. 1990, 330 (1615): 445. Bibcode:1990RSPTA.330..445W. S2CID 123165017. doi:10.1098/rsta.1990.0025.
Williams GE. Precambrian Cyclic Rhythmites: Solar-Climatic or Tidal Signatures?. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. 1990, 330 (1615): 445. Bibcode:1990RSPTA.330..445W. S2CID 123165017. doi:10.1098/rsta.1990.0025.