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Sunspot Number Data
The most commonly used index of solar activity is the relative number of sunspots or the Wolf number, for a given day by the equation:
W = k (10 g + s ), where
W – is the Wolf number
s – is the total number of sunspots, regardless of their size in the visible hemisphere,
g – is the number of observed spot groups,
k – is the normalization coefficient that brings the values observed by different observers and telescopes
to the standard selected by Wolf.
Since 1849, Wolf numbers were regularly determined at the Zurich Observatory (reliable series).
Since 1981, a summary of all sunspot
observations and determination of Wolf numbers has been done at the
World Data Center for the production, preservation and
dissemination of the international sunspot number (WDC-SILSO)
of the Royal Observatory of Belgium in Brussels.
The international values of the daily Wolf numbers are finally determined at the end of each month according to
the data of all solar observatories participating in the observations. Currently, it is usually 65 - 76 solar observatories
In addition, Rudolf Wolf restored the monthly average values of this index since 1749 (the restored, numbered series)
and the average annual values since 1700 according to rather fragmentary data from individual European observers.
Using scientific publications, modern researchers on single telescopic observations were able to qualitatively
extend this series until 1611.
Since July 1st 2015, the original Sunspot number data provided by the
WDC-SILSO, have been replaced by a new entirely revised data series.
Now, the data are presented in a new array of files, containing additional values that were not
present in the original series.
This version is assigned the number 2.0
The past version with the number 1.0 is available in the
Archive section of the WDC-SILSO.
On our site :
Data on the relative number of sunspots up to July 1, 2015 correspond to the original version 1.0.
Since July 1, 2015, the data correspond to the American version published on the website
National Centers for Environmental Information
NOAA according to "Solar Bulletin" of the American Association of Variable Star Observers - Solar Division
(AAVSO Bulletin).
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