An index giving the number of spots on the Sun, based on the relative sunspot number system originated by R. Wolf; symbol Ri. 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 Zurich sunspot number used until 1981.
http://www.sidc.oma.be/index.php Solar Influences Data Analysis Center.
Subjects: Astronomy and Astrophysics.