Wolf sunspot number

A measure of the sunspot activity on the solar disc, taking into account spot groups as well as individual spots. It was devised by Rudolf Wolf of the Zürich Observatory, and is also known as the Zürich sunspot number. The value, R, is calculated from the formula R=k(10g + f) where g is the number of sunspot groups, f the total number of spots and k a weighting factor depending on the instruments used and the observer. The value of k is about 1 for telescopes of 100 mm aperture.