In 1848, the Swiss astronomer, Johann Rudolph Wolf, introduced a daily measurement of sunspot number. His method, which is still used today, counts the total number of spots visible on the face of the Sun and the number of groups into which they cluster, because neither quantity alone satisfactorily measures sunspot activity.
An observer computes a daily sunspot number by multiplying the number of groups he sees by ten and then adding this product to his total count of individual spots. Results, however, vary greatly, since the measurement strongly depends on observer interpretation and experience and on the stability of the Earth's atmosphere above the observing site.
The use of Earth as a platform from which to record these numbers contributes to their variability, too, because the Sun rotates and the evolving spot groups are distributed unevenly across solar longitudes. To compensate for these limitations, each daily international number is computed as a weighted average of measurements made from a network of cooperating observatories.
Smoothed Sunspot Number
The daily sunspot number has little, if any, relationship to ionospheric variability. However, the most widely used Ionospheric Index, R12, is derived from the daily sunspot numbers. The R12 index is a twelve-month smoothed relative sunspot number. To calculate the R12 index for July 1980 add half of the Jan 1980 value plus the sum of the Feb through Dec 1980 values plus half of the Jan 1981 value and by divide the sum twelve:
[(n1/2)+(n2+n3+........n11+n12)+(n13/2)]/12 (where n1 = Jan 1980, n7 = July 1980 and n13 = Jan 1981)
Today, much more sophisticated measurements of solar activity are made routinely, but none has the link with the past that sunspot numbers have.