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Exploring Sunspots



The picture below is an image of an active sun with several sunspots.  These sunspots have strong magnetic fields and are cooler than the rest of the Sun's surface.

Astronomy Picture of the Day

Taken on March 22, 1998
National Solar Observatory
This image can be viewed at the following website: http://mirrors.inside.net/apod/apod
ap980322.html


Using one of the earliest telescopes, Galileo recorded dark spots on the Sun in the seventeenth century.  Chinese astronomers had also reported dark spots some two centuries earlier.  Since before 1700, astronomers world-wide have devised different ways of counting and reporting the number of sunspots.  The method for calculating sunspot numbers used today was developed by Johann Rudolph Wolf.

Johann Rudolph Wolf (1816-1893).
National Portrait Gallery, Smithsonian Institution, Washington D.C. In 1848 Rudolph Wolf devised a daily method of estimating solar activity by counting the number of individual spots and groups of spots on the face of the sun. Wolf chose to compute his sunspot number by adding 10 times the number of groups to the total count of individual spots, because neither quantity alone completely captured the level of activity. Today, Wolf sunspot counts continue, since no other index of the sun's activity reaches into the past as far and as continuously. An avid astronomical historian and an unrivaled expert on sunspot lore, Wolf confirmed the existence of a cycle in sunspot numbers. He also more accurately determined the cycle's length to be 11.1 years by using early historical records.



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Last modified on July 27, 2001.