n 1908 George Ellery Hale discovered that some absorption lines in the spectra of sunspots were split into several narrow parallel lines, indicating the presence of a magnetic field. By measuring the distance between the split lines, Hale determined the magnetic fields to be as high as 4500 gauss, thousands of times more powerful than the Sun’s overall magnetic field [2]. He found the magnetic field of a sunspot group to be bipolar; the polarity of the leading spots in a group (spots on the side towards which the sun rotates) is opposite to that of the trailing spots in the same group. This holds true for all of the groups in a solar hemisphere. Moreover, the polarity of leading spots is identical to the polarity of the hemisphere in which the group appears. At the peak of the 11-year sunspot cycle the Sun’s magnetic field flips, reversing the polarity pattern. Thus, Hale determined the true solar cycle to be 22 years long.

Figure 3
A cluster of four sunspot groups taken from a solar drawing done at the 150-Foot Solar Tower at Mt. Wilson Observatory. The polarities (R=positive, V=negative) and field strengths are determined by directly measuring the Zeeman splitting of the 5250 line of iron. Note how the leading spots in all of the groups are of the same polarity. These drawings are still being done today in the same manner as was developed by Hale in 1917 [3]. Drawing done by the author on 99-10-13.