1.6 Sunspots

Sunspots are dark areas of irregular shape on the surface of the Sun. Their short-term and long-term cyclical naturehas been established in the past century. Spots are often big enough to be seen with the naked eye. While direct observation ofthe Sun in a clear sky is painful and dangerous, it is feasible when the Sun is close to the horizon or when it is covered by athin veil of clouds or mist. Records of naked-eye sunspot observations in China go back to at least 28 BCE. In the West, therecord is much more problematical. It is possible that the Greek philosopher Anaxagoras observed a spot in 467 BCE, and it appearsthat there are a few scattered mentions in the ancient literature as well. However, in the dominant Aristotelian cosmology, theheavens were thought to be perfect and unchanging. A spot that comes and goes on the Sun would mean that there is change in theheavens. Given this theoretical predisposition, the difficulty of observing the Sun, and the cyclic nature of spots, it is littlewonder that records of sunspots are almost non-existent in Europe before the seventeenth century. A very large spot seen for no lessthan eight days in 807 was simply interpreted as a passage of Mercury in front of the Sun. Other mentions of spots seen on theSun were ignored by the astronomers and philosophers. In 1607 Johannes Kepler wished to observe a predicted transit of Mercury across the Sun's disk, and on theappointed day he projected the Sun's image through a small hole in the roof of his house (a camera obscura    ) and did indeed observe a black spot that he interpreted to beMercury. Had he been able to follow up on his observation the next day, he would still have seen the spot. Since he knew that Mercurytakes only a few hours to cross the Sun's disk during one of its infrequent transits, he would have known that what he observedcould not have been Mercury.

The scientific study of sunspots in the West began after the telescope had been brought into astronomy in1609. Although there is still some controversy about when and by whom sunspots were first observed through the telescope , we can say that Galileo and Thomas Harriot were the first, around the end of 1610; that Johannes and David Fabricius and Christoph Scheiner first observed them in March 1611, and that Johannes Fabricius was the first to publish on them. His book, De Maculis in Sole Observatis ("On the Spots Observed in the Sun") appeared in the autumn of 1611, but it remainedunknown to the other observers for some time.

In the meantime, Galileo had shown sunspots to a number of people in Rome during his triumphant visit there in thespring of 1611. But although some of his corespondents began making regular observations a few months later, Galileo himselfdid not undertake a study of sunspots until April 1612. Scheiner began his serious study of spots in October 1611 and his firsttract on the subject, Tres Epistolae de Maculis Solaribus Scriptae ad Marcum Welserum ("Three Letters on Solar Spots written to Marc Welser ") appeared in January 1612 under the pseudonym "Apelles latens post tabulam,"or "Apelles waiting behind the painting."