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Main Page   >   Data   >   Solar Activity   >   Sunspot Number Metadata Catalogue

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Since July 1st 2015, the original Sunspot number data provided by the WDC-SILSO, have been replaced by a new entirely revised data series.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.
Daily total sunspot number, 1818-5.2015. Reference:
SILSO data/image,
Royal Observatory of Belgium, Brussels.
Monthly mean total sunspot number, 1749-5.2015.
Yearly mean total sunspot number, 1700-2014.

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).
Daily sunspot numbers from 1944. Reference:
National Centers for Environmental Information,
NOAA U.S. Department of Commerce.
Monthly mean sunspot numbers from 12.1944.
Yearly sunspot numbers from 1945.

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