Sunspot Numbers

Note: NGDC offers a Zurich Slide Set of sunspots and related phenomena for educational uses.

Johann Rudolf Wolf

Johann Rudolf Wolf (1816-1893).

National Portrait Gallery, Smithsonian Institution, Washington D.C.

In 1848 Rudolf 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. Wolf, who became director of the Zurich Observatory, discovered independently the coincidence of the sunspot cycle with disturbances in the earth's magnetic field.

How Sunspot Numbers are Computed Today

An observer computes a daily sunspot number by multiplying the number of groups he/she sees by ten and then adding this product to his total count of individual spots, same way that Wolf did. Many refer to the sunspot number as a Wolf number or count (or as a Zurich Sunspot Number). Results, however, vary greatly, since the measurement strongly 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.

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Annual Mean Sunspot Numbers 1700-2006. Sunspot counts rise and fall approximately every 11.1 years. The cycle, though, is not symmetrical, for the spot count takes on the average about 4.8 years to rise from a minimum to a maximum and another 6.2 years to fall to a minimum once again. The largest annual mean number (190.2) occurred in 1957.

Today, much more sophisticated measurements of solar activity are made routinely, but none has the link with the past that sunspot numbers have.

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