dark, usually irregularly shaped spots on the sun's surface that are actually solar magnetic storms. The Chinese recorded dark features on the sun seen with the naked eye in 28 B.C.
Other observers including Kepler suspected that these events might be transits of Mercury or Venus. Galileo observed them systematically for several weeks before concluding that they had to be events taking place on the solar surface. The temperature of the spots is lower than that of the surrounding photosphere; thus the spots are darker. All but the smallest show a dark central portion (the umbra) with a lighter outer area (the penumbra). Studies of the spectra of sunspots show evidence of the Zeeman effect
, indicating the presence of a large magnetic field. In addition, measurements of the Doppler effect
in the spectral lines show that there is a vortex motion in sunspots similar to that of a tornado on earth. The lower temperature of the gases constituting a sunspot results from the lower pressure due to the strong magnetic field. Sunspots appear usually only between latitudes from 5° to 35° north and south of the sun's equator. Sunspots are not permanent since the sun's surface is gaseous. Because the sun rotates on its axis, a sunspot cannot be observed continuously for more than about two weeks. In 1826 amateur astronomer Heinrich Schwabe began a series of solar observations (in hopes of finding planet Vulcan). By 1843 he had collected enough data to announce the existence of the sunspot cycle. An 11-year cycle from one period of maximum activity to the next is usually observed. However, a period during which most sunspots have one magnetic polarity is followed by another period during which most have the opposite magnetic polarity; thus, the cycle actually covers 22 years. During each 11-year period sunspots appear first at higher latitudes and later at latitudes closer to the solar equator as the period progresses. The spots often form in pairs or groups, with a large, long-lived leader spot matched with one or more smaller spots of opposite magnetic polarity. A number of phenomena are associated with sunspots. Sunspot activity produces various disturbances on earth—these include magnetic storms which manifest themselves as aurorae, interference with radio reception and electric power grids, and disturbances of the magnetic compass. Periods in which an increase in sunspots is observed are called active periods. Reviewing historical records in 1890, E. Maunder noticed that sunspot counts fell drastically between 1645 and 1715. In 1976 J. Eddy correlated Maunder's data with a low frequency of aurorae and the reduced sizes of annual tree rings. This "Maunder Minimum" may have played a role in the unusually low temperatures in the northern hemisphere during this period, which is known as the Little Ice Age.
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