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SUNSPOT NUMBERS
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Sunspot Numbers (Click for access to all ftp files)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 sunpsot 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 stongly depends on observer interpretation and experience and on the stability of the Earth's atmosphere above the observing site. Moreover, 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. Today, much more sophisticated measurements of solar activity are made routinely, but none has the link with the past that sunspot numbers have. -- J.A. McKinnon 1. International Sunspot Number brief description Relative Sunspot Numbers -- The collection of sunspot numbers provided here contains several kinds of tables -- tables that give spot counts averaged over different time intervals. Professor M. Waldmeier, Director of the Swiss Federal Observatory in Zurich, Switzerland, published these observations in 1961 as a book entitled THE SUNSPOT-ACTIVITY IN THE YEARS 1610-1960 (Zurich Schulthess and Company AG) and updated more recently by the Royal Observatory of Belgium's Solar Influences Data Analysis Center. The relative sunspot number is an index of the activity of the entire visible disk of the Sun. It is determined each day without reference to preceding days. Each isolated cluster of sunspots is termed a sunspot group, and it may consist of one or a large number of distinct spots whose size can range from 10 or more square degrees of the solar surface down to the limit of resolution (e.g., 1/25 square degree). The relative sunspot number is defined as R = K (10g + s), where g is the number of sunspot groups and s is the total number of distinct spots. The scale factor K (usually less than unity) depends on the observer and is intended to effect the conversion to the scale originated by Wolf. The provisional daily Zurich relative sunspot numbers, Rz, were based upon observations made at Zurich and its two branch stations in Arosa and Locarno and communicated by M. Waldmeier of the Swiss Federal Observatory. Beginning January 1, 1981, the Zurich relative sunspot number program is replaced by the Solar Influences Data Analysis Center (formerly the Sunspot Index Data Center) (c/o Dr. R. Van der Linden, 3 av. Circulaire, B-118 Bruxelles, Belgium). The determination of the provisional International Sunspot Numbers Ri results from a statistical treatment of the data originating from more than twenty-five observing stations. These stations constitute an international network, with the Locarno (Switzerland) station as the reference station, to guarantee continuity with the past Zurich series of Rz. The definitive International Sunspot Numbers Ri are evaluated by a similar method based on a network of observing stations selected for their high number of observations, their continuity during the whole year and an existing series of observations during the last years. Also taken into account is the stability of the K monthly factors with reference to the Locarno station. These relative sunspot numbers are now designated Ri (International) instead of Rz (Zurich). The international sunspot number is produced by the Solar Influences Data Analysis Center (SIDC), World Data Center for the Sunspot Index, at the Royal Observatory of Belgium. NGDC kindly requests that you acknowledge the SIDC when using these data. Please refer to the SIDC site for additional information. The SIDC requests users of the data to credit: SIDC, RWC Belgium, World Data Center for the Sunspot Index, Royal Observatory of Belgium, `year(s)-of-data'.
2. American Relative Sunspot Numbers brief desciptive text
3. Ancient sunspot data 165 B.C. to 1684 A.D.
4. Group Sunspot Numbers (Doug Hoyt re-evaluation) 1610-1995
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