太陽活動周期
Solar cycle 4 was the fourth solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 13.6 years, beginning in September 1784 and ending in April 1798 (thus overlapping the Dalton Minimum). The maximum smoothed sunspot number (SIDC formula) observed during the solar cycle was 235.3 (in February 1788), and the starting minimum was 15.9.