Rudolf Wolf (1816–1893)

Rudolf Wolf was born on 7 July 1816 in Fällanden, near Zürich. He studied astronomy in Zürich, Vienna and Berlin, and upon graduating moved to Bern to teach mathematics and physics. In 1847 he was appointed director of the small astronomical observatory in Bern. 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 inaugurated there in 1864. He died in Zürichon 6 December 1893.

Rudolf Wolf image
Portrait of Rudolf Wolf (1816–1893).
Credit: Wikipedia.

Wolf's interest in sunspots was fired by his observation of a particularly large and spectacular sunspot group in December 1847. Starting then 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 and secure in his position at the Bern (and later Zürich) Observatory, 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. It is in the course of carrying out this program that Wolf defined his now famous "relative sunspot number". By 1868 Wolf had a more or less reliable sunspot number reconstruction back to 1745, and 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.

In 1852 Wolf was one of four people to independently and, more or less, 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 then sought a similar peridiocity 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", published in 1877, and his "Handbuch der Astronomie", published 1893, were both extremely popular in the late nineteenth and early twentieth century. He contributed four volumes to the "Biographies of Swiss Men of Science" and two to the "Handbuch der Mathematik". Wolf reported the results of his historical researches on sunspots in his "Astronomische Mittheilungen", a kind of 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.

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