A measure of the number of spots on the visible disk of the Sun each day. It was introduced in 1848 by R. Wolf, and is calculated by taking into account both the total number of individual spots (f) and the number of sunspot groups (g), plus a factor k that represents the efficiency of the observer and telescope: R = k(10g + f). The Wolf sunspot number refers to Wolf’s initial observations at Bern Observatory, Switzerland, while the Zürich relative sunspot number, R_z, refers to the index produced at the Swiss Federal Observatory in Zürich, where Wolf moved in 1855. The Zürich sunspot number included observations from several locations, thus leading to the introduction of the k factor for observer efficiency. In 1981 responsibility for generating the relative sunspot number was transferred to the Solar Influences Data Analysis Center (SIDC, known before 2000 as the Sunspot Index Data Center) at the Royal Observatory of Belgium in Brussels. The index was renamed the International Sunspot Number and is designated R_i. Other sunspot indices include the Boulder sunspot number, published by the Space Environment Center of the US National Oceanic and Atmospheric Administration (NOAA), and the American relative sunspot number, published by the solar physics division of the American Association of Variable Star Observers.