Solar cycle 15 was the fifteenth solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 10 years, beginning in August 1913 and ending in August 1923. The maximum smoothed sunspot number (monthly number of sunspots averaged over a twelve-month period) observed during the solar cycle was 105.4 (August 1917), and the minimum was 5.6. There were a total of 534 days with no sunspots during this cycle. A major geomagnetic storm during 13–15 May 1921 caused damage in communication systems and aurora displays in much of the eastern United States. 
Geomagnetic storms in March 1918, August 1919, October 1919, and March 1920 affected telegraph lines, while a solar flare on May 13, 1921 also affected rail signal and switching equipment, in what was known as the "New York Railroad Storm."