relative sunspot number
relative sunspot number
Symbol: R . A somewhat arbitrary index of the level of sunspot activity, derived from the formula R = k (f + 10g ), where g represents the number of sunspot groups, f the total number of their component spots, and k is a constant dependent on the estimated efficiency of, and also the equipment used by, a particular observer. Though a more objective assessment of the level of sunspot activity relies on the measurement of sunspot areas, relative sunspot numbers provide an almost continuous record back to the mid-18th century and yield graphs similar (at least so far as the yearly means are concerned) to those based on other indices.The relative sunspot number was originally called the Wolf number, after Rudolf Wolf who introduced it at Zürich in 1848 and extended it back using old observations. It later became known as the Zürich relative sunspot number and then, when the Zürich tradition came to an end with the transfer of responsibility to the Sunspot Index Data Center in Brussels in Jan. 1981, the International Sunspot Number (symbol: RI ). See also sunspot cycle.