An index giving the daily number of spots on the Sun, based on the relative sunspot number system originated by R. Wolf; symbol R_i. The Solar Influences Data Analysis Center (SIDC, formerly the Sunspot Index Data Center), based at the Royal Observatory of Belgium in Brussels, calculates the sunspot number from observations made at over 25 sites around the world. The observatory at Locarno, Switzerland, is used as a reference station to ensure consistency with the Zürich sunspot number used until 1981. The method of calculation was changed in 2015 July, using the sunspot counts of a different observer for calibration. Previously, the numbers had been calculated with reference to the observations of Wolf himself, but the reference observer is now the Swiss astronomer Heinrich Alfred Wolfer (1854–1931) who succeeded Wolf as director of the Swiss Federal Observatory in Zürich in 1894. Wolfer used a different system of counting sunspot numbers from Wolf which gave higher values. Sunspot numbers issued before 2015 have since been recalculated to bring them into line with the new system.

http://www.sidc.oma.be/index.php Solar Influences Data Analysis Center.