Perhaps the most important precedent to animation and film, in 1612 during the summer months, Galileo made a series of sunspot observations which were published in Istoria e Dimostrazioni Intorno Alle Macchie Solari e Loro Accidenti Rome (History and Demonstrations Concerning Sunspots and their Properties, published 1613). Because these observations were made at appoximately the same time of day, the motion of the spots across the Sun can easily be seen.
[Video - format 3GP - 74Kb] galileogalilei-sunspotmovie.mov
Sunspots are dark areas of irregular shape on the surface of the Sun. Their short-term and long-term cyclical nature has been established in the past century. Spots are often big enough to be seen with the naked eye. While direct observation of the 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 a thin veil of clouds or mist. Records of naked-eye sunspot observations in China go back to at least 28 BCE. In the West, the record is much more problematical. It is possible that the Greek philosopher Anaxagoras observed a spot in 467 BCE, and it appears that there are a few scattered mentions in the ancient literature as well. However, in the dominant Aristotelian cosmology, the heavens were thought to be perfect and unchanging. A spot that comes and goes on the Sun would mean that there is change in the heavens. Given this theoretical predisposition, the difficulty of observing the Sun, and the cyclic nature of spots, it is little wonder that records of sunspots are almost non-existent in Europe before the seventeenth century. A very large spot seen for no less than eight days in 807 was simply interpreted as a passage of Mercury in front of the Sun. Other mentions of spots seen on the Sun 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 the appointed 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 be Mercury.
Galileo had shown sunspots to a number of people in Rome during his triumphant visit there in the spring of 1611. But although some of his corespondents began making regular observations a few months later, Galileo himself did not undertake a study of sunspots until April 1612. Scheiner began his serious study of spots in October 1611 and his first tract 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.» Welser was a scholar and banker in Augsburg, who was a patron of local scholars.
Sunspot plate from Scheiner's Tres Epistolae.
Scheiner, a Jesuit mathematician at the university of Ingolstadt (near Augsburg), wished to preserve the perfection of the Sun and the heavens and therefore argued that sunspots were satellites of the Sun. They appeared as black spots when they passed in front of the Sun but were invisible at other points in their orbits. Their orbits had to be very close to the Sun for their shapes were foreshortened as they approached its edge. Scheiner observed sunspots through a telescope equipped with colored glasses.
In the winter of 1611-12, when Galileo received a copy of Scheiner's tract from Welser along with a request for his comments, he was ill, and what little energy he had he was devoting to the publication of his Discourse on Bodies in Water. When, however, that book was at the printer's, in April 1612, he turned his attention to sunspots with the help of his protégé Benedetto Castelli, who was in Florence at the time. It was Castelli who developed the method of projecting the Sun's image through the telescope, a technique that made it possible to study the Sun in detail even when it was high in the sky. Galileo wrote his first letter to Welser on sunspots, in which he argued that spots were, in fact, on the surface of the Sun or in its atmosphere, and although he could not say for certain what they were, they appeared to him most like clouds.
Source: galileo.rice.edu