APOD: 2020 September 17 - Solar Cycle 25 Begins

Good morning, Rallypoint. Today's Astronomy Picture of the Day (APOD) is titled "Solar Cycle 25 Begins." How do we know when a solar cycle begins or ends? By the number of sunspots. Based on data going back to February 1755, sunspots appear and disappear in a periodic cycle of about 11 years.

Galileo Galilei, Christoph Scheiner and their contemporaries conducted the first systematic study of sunspots in the early 1600s. In 1775, Christian Horrebow first suggested a 'solar cycle' based on observations he conducted beginning in 1761 at the observatory Rundetaarn in Copenhagen. Discovery of the solar cycle is credited to Samuel Heinrich Schwabe, who after 17 years of observations noticed a periodic variation in the average number of sunspots and published his results in 1843. Rudolf Wolf compiled and studied these and other observations, reconstructing the cycle back to 1745, eventually pushing these reconstructions to the earliest observations of sunspots by Galileo and contemporaries in the early seventeenth century.

Wolf also created a numbering scheme (the "Wolf Index") to identify solar cycles and their annual sunspot count. Following this approach, the 1755–1766 solar cycle is traditionally numbered "1". Based on the Wolf Index, the general trend of monthly sunspot data now confirms that the minimum of the approximately 11 year cycle of solar activity occurred in December 2019. This marks the start of Solar Cycle 25. Today's APOD shows a quiet Sun (at minimum activity) on the right of this composite hemispherical view. In contrast, the left side shows the active Sun in April 2014 at the recognized maximum of Solar Cycle 24.