Wolf number
Encyclopedia
The idea of computing sunspot numbers was originated by Rudolf Wolf
Rudolf Wolf
Johann Rudolf Wolf was a Swiss astronomer and mathematician best known for his research on sunspots.Wolf was born in Fällanden, near Zurich. He studied at the universities of Zurich, Vienna, and Berlin. Encke was one of his teachers. Wolf became professor of astronomy at the University of Bern in...
in 1849 in Zürich
Zürich
Zurich is the largest city in Switzerland and the capital of the canton of Zurich. It is located in central Switzerland at the northwestern tip of Lake Zurich...
, Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
and, thus, the procedure he initiated bears his name (or place). The combination of sunspots and their grouping is used because it compensates for variations in observing small sunspots.
This number has been collected and tabulated by researchers for around 300 years. They have found that sunspot activity is cyclical and reaches its maximum around every 9.5 to 11 years (note: Using data from SIDC for the last 300 years and running a FFT
Discrete Fourier transform
In mathematics, the discrete Fourier transform is a specific kind of discrete transform, used in Fourier analysis. It transforms one function into another, which is called the frequency domain representation, or simply the DFT, of the original function...
function on the data gives an average maximum at 10.4883 years/cycle). This cycle was first noted by Heinrich Schwabe
Heinrich Schwabe
Samuel Heinrich Schwabe a German astronomer remembered for his work on sunspots.Schwabe was born at Dessau. At first an apothecary, he turned his attention to astronomy, and in 1826 commenced his observations on sunspots. Schwabe was trying to discover a new planet inside the orbit of Mercury...
in 1843.
The relative sunspot number
is computed using the formula (collected as a daily index of sunspot activity):
where
-
is the number of individual spots, -
is the number of sunspot groups, and -
is a factor that varies with location and instrumentation (also known as the observatory factor or the personal reduction coefficient 
).