Solar cycle
Solar cycle 15 was the fifteenth solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 10.1 years, beginning in July 1913 and ending in August 1923. The maximum smoothed sunspot number (SIDC formula) observed during the solar cycle was 175.7 (August 1917), and the starting minimum was 2.5. During the minimum transit from solar cycle 15 to 16, there were a total of 534 days with no sunspots.