ASTRONOMY TID-BITs 
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      Public interest, in addition to the attention given by ast-
ronomers to sunspots, continued to grow as more telescopes were built.  To see the sun without damaging the eye, it was common to project the telescope image onto a wall or screen.  In this way the spots could be drawn on paper and measured for size.     

        It was known the number of sunspots varied from time to time, but it was not until 1844 when the amateur astronomer Heinrich Schwab /  ³ published a  paper, that it was clear they come and go in cycles.  He found the time from one minimum number of spots to the next minimum to be about 10 years.

        This information prompted Rudolf Wolf, the director of astronomical observation at Zurich (Switzerland), to devise a consistent method of counting and reporting sunspots numbers.  His method was to count the total number of spots regardless of size or grouping, then add 10 to the count for each group of sunspots.

        He applied his method to the existing historical records of spots for the years 1700 to 1848 and confirmed that cycles did appear and that the average cycle length was 11.1 years.  His method of 'counting', called the WOLF number, /4 , was widely accepted and is used today for most counts made by the human eye, even though more accurate and useful records of the Sun's variable nature are recorded automatically by instrument.
       
        Sunspots are one of the visual results of changes in magnetic force fields generated by the dynamo effect of rotating fluids within the sun.  The magnetic field in a sunspot is at least 1000 times greater than the nearby regions and these differences result in variations in the nature and content of solar winds that continually flow outward in all directions to the Earth and far beyond.  The magnetic nature of sunspots may be the subject of another ASTRONOMY TID-BIT.

                         REFERENCES AND NOTES
/ ¹  Galileo Galilei, De Revolutionibus,
/ ²  E. H. Burritt, The Geography of the Heavens,  Pub. F. J. Huntington, Hartford, Conn., 1833, page 211.
/ ³   Herman and Goldberg, Sun, Weather, and Climate, National Aeronautics and Space Admin. Washington, 1978, page 12.
/ ⁴   ibid, page 13

Figure 2.  A sunspot group magnified about 100 times.  The central black area is the umbra;  lighter areas around the umbra is called the penumbra.  Undisturbed granules surround the spots.
(James B. Kaler, Astronomy, Harper Collins, New York, NY 1994, page 319)

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