The
Wolf number (also known as the
International sunspot number,
relative sunspot number, or
Zürich number) is a quantity which measures the number of sunspots and groups of sunspots present on the surface of the sun.
The idea of computing sunspot numbers was originated by
Rudolf WolfJohann 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
ZürichZürich or Zurich is the largest city in Switzerland and the capital of the canton of Zürich. The city is Switzerland's main commercial and cultural centre and sometimes called the Cultural Capital of Switzerland, the political capital of Switzerland being Berne...
,
SwitzerlandSwitzerland , officially the Swiss Confederation , is a federal republic consisting of 26 states named cantons, with Bern as the seat of the federal authorities...
and, thus, the procedure he initiated bears his name (or place).
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The
Wolf number (also known as the
International sunspot number,
relative sunspot number, or
Zürich number) is a quantity which measures the number of sunspots and groups of sunspots present on the surface of the sun.
The idea of computing sunspot numbers was originated by
Rudolf WolfJohann 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
ZürichZürich or Zurich is the largest city in Switzerland and the capital of the canton of Zürich. The city is Switzerland's main commercial and cultural centre and sometimes called the Cultural Capital of Switzerland, the political capital of Switzerland being Berne...
,
SwitzerlandSwitzerland , officially the Swiss Confederation , is a federal republic consisting of 26 states named cantons, with Bern as the seat of the federal authorities...
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
FFTIn mathematics, the discrete Fourier transform is a specific kind of Fourier 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 SchwabeSamuel 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
R is the relative sunspot number,
s is the number of individual spots,
g is the number of sunspot groups, and
k is a factor that varies with location and instrumentation (also known as the
observatory factor).
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