#16H.   The Sun -- History

    Large sunspots may be occasionally seen by the unaided eye, with the sun near the horizon and dimmed by thick haze. Early observations of this sort exist, but not much was remembered of them, and when around 1610 they were seen more clearly with the help of the telescope, the discovery came as a great surprise.

    Three observers claimed the discovery--Galileo, who risked blindness by looking at the Sun through a telescope, the priest Christopher Scheiner in Germany, who invented the safe observing method of projecting the Sun's image onto a screen, and Johann Fabricius in Holland. After the initial studies, however, interest in sunspots declined, in part because (for reasons still not known) they almost disappeared between 1645 and 1715.

Heinrich Schwabe

    The modern period of sunspot research began with Heinrich Schwabe, a pharmacist in the German town of Dessau who was also an amateur astronomer. Schwabe's original interest was the possibility of an unknown planet close to the Sun, which he believed might be detected as a dark spot when passing in front of the Sun. Year after year, beginning in 1826, on every clear day, Schwabe would scan the sun and record its spots, until 17 years later he began to suspect a regular variation. He published his findings in a short article "Solar Observations during 1843" which at first attracted little attention. One of the few who noted and was impresed was Rudolf Wolf of Bern, Switzerland (later Zürich), who from 1847 began observing sunspots himself. In 1848 Wolf devised the "Zürich sunspot number" to gauge the number of sunspots (giving extra weight to groupings), which is still used (in the figure below as well).

Sunspot Cycle (1850 - 1975)

    Carrington had seen a flare, of the rare variety that is visible in white light (only about 50 are known). No more than 17 hours later a great magnetic storm erupted, with aurora seen as far as Cuba. Carrington noted the connection, but cautioned that "one swallow does not make a summer."

    In 1892 George Ellery Hale--founder of the great astronomical observatories on Mt. Wilson and later on Mt. Palomar--devised the spectroheliograph, an instrument scanning the Sun in a narrow range of color emitted by a certain atom ("spectral line"). In July of that year he used it to observe two large flares, followed (19.5 and 30 hours later, respectively) by big magnetic storms. Hale also tuned his instrument to spectral lines known to change when produced in strong magnetic fields, and concluded in 1908 that sunspots were intensely magnetic. That fit nicely with observations during total eclipses of the Sun, when streaks above sunspot areas were often seen to arch above the Sun's edge like field lines of a bar magnet.

    After World War II scientists began monitoring the cosmic radiation, the flow of high-energy ions arriving from distant space, using arrays of Geiger counters and other instruments. Starting in 1942 they occasionally saw peaks of intensity after strong flares. Flares also emit x-rays, believed to be produced by great numbers of high-energy electrons accelerated in the flare region and trapped in it. The GOES spacecraft regularly monitors such emissions, and x-ray cameras aboard satellites--e.g the Solar Maximum Mission, launched in 1980, and more recently Yohkoh, launched in 1991--have taken some remarkable flare pictures.

Coronal Mass Ejections

Further Explorations:

• Biography of Heinrich Schwabe
• "Explorer of the Universe: A biography of George Ellery Hale" by Helen Wright, Dutton & Co., 1966 (republished by Amer. Inst. of Physics, 1994, 487 pp, $29.95 hard cover)
• "Early Solar Physics" by A.J. Meadows, Pergamon Press, 1970.
• "The New Solar Physics" by John A. Eddy, Westview Press, Boulder, 1978