Rudolf Wolf (1816–1893)

Rudolf Wolf was born on July 7, 1816 in Fällanden, near Zürich. He studied astronomy in Zürich, Vienna and Berlin, and upon graduating moved to Bern to teach mathematics and physics. He was appointed director of the small astronomical observatory in Bern in 1847. In 1855 he moved back to Zürich, where he was appointed Professor of Astronomy at the University and at Polytechnic school (now the ETH), where he later became Director of the Observatory.

Wolf's interest in sunspots was inspired by his observation of a particularly large and spectacular sunspot group in December of 1847. He began his own telescopic observations and records of sunspots, which he carried out continuously for the following 46 years. Much impressed with Samuel Schwabe's discovery of the sunspot cycle, he embarked on a program of historical studies aimed at reconstructing the variation in the number of sunspots as far back in the past as possible, based on surviving notebooks and drawings of long gone astronomers. During this program Wolf defined his now famous relative sunspot number. By 1868, Wolf had a reliable sunspot number reconstruction back to 1745. He then pushed his reconstruction all the way back to 1610, although the paucity of data effectively rendered these older determinations far less reliable. Wolf was the first to note the possible existence in the sunspot record of a longer modulation period of about 55 years.

By 1852 Wolf was one of four people to independently and simultaneously notice the coincidence between the 11 year sunspot cycle and the cycle of geomagnetic activity. Wolf and others also noted a similar correspondence between sunspot cycle and frequency of auroral activity. Wolf sought a similar periodicity in various meteorological phenomena, but without conclusive results.

Wolf was a broad and prolific author. His "Mathematics, Physics, Geodesy, and Astronomy" saw six editions between 1852 and 1893. His "History of Recent Astronomy" and "Handbuch der Astronomie" were both extremely popular in the late nineteenth and early twentieth century. He contributed to the "Biographies of Swiss Men of Science" and the "Handbuch der Mathematik." Wolf reported the results of his historical research on sunspots in his "Astronomische Mittheilungen," a private research journal which appeared in 13 volumes between 1852 and 1893, and of which Wolf was the sole author. His sunspot number monitoring work continued at the Zürich Observatory until 1979, when it was transferred to Brussels. The Wolf sunspot number, as it is now called, remains the favored historical indicator of past solar activity.

Bibliography

Hoyt, D.V., and Schatten, K.H. 1997, The Role of the Sunin Climate Change, Oxford University Press.