Sunspots
SunspotsThe scientific study of sunspots in the West began after the telescope had been brought into astronomy in 1609. 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 remained unknown to the other observers for some time.
Harriot's sunspot drawings.
High-resolution image available
In the meantime, 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."[1] Welser was a scholar and banker in Augsburg, who was a patron of local scholars.
Sunspot plate from Scheiner's Tres Epistolae.
High-resolution image also available.
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.
While Scheiner wrote in Latin, Galileo wrote his letter in Italian, and Welser
had to have it translated before Scheiner could read it. Scheiner had continued his
solar observations, and by the time he had mastered Galileo's letter he had
already finished two more letters of his own to Welser. He now added a third,
in which he commented that his observations agreed precisely with those of
Galileo and defended his judgment that sunspots were solar satellites. This
second series of letters was published by Welser in October 1612 under the
title De Maculis Solaribus . . . Accuratior Disquisitio ("A More
Accurate Disquisition . . . on Sunspots"). Scheiner maintained his pseudonym
of Apelles "or, if you prefer, Odysseus under the shield of Ajax." In the
meantime, Galileo had written a second letter to Welser in August 1612. In
this letter he showed a large number of sunspot observations, made at roughly
the same time of the day, so that the Sun's orientation was the same and the
motion of the spots across its disk could be easily followed,
as is shown in the sequence here.
Upon receiving Scheiner's second tract he wrote yet a third,
dated December 1612, attacking Apelles's opinions once again. At the end of
his last letter Galileo mentioned the
Copernican System favorably in a
way that some scholars have interpreted as his first endorsement of
that theory.
Galileo's three letters were published in Rome by the Lyncean Academy in the summer of 1613. About a third of the copies had reprints of the two tracts by Apelles (whose identity had in the meantime become known) in their original Latin. There was little doubt about the winner of this contest. Scheiner's language was convoluted, and not only did Galileo demolish his argument, he also criticized Scheiner's a priori method of argument: the Sun is perfect, therefore it cannot have spots on its surface.
Up to this point, relations between Galileo and Scheiner were not strained. Scheiner had treated Galileo with great respect, and Galileo had been courteous in his language. Ten years later, in his Assayer, Galileo complained about those who would steal his priority of discovery, mentioning the case of sunspots but not mentioning Scheiner. It is almost certain that Galileo was complaining about several others who had published on sunspots but who had not recognized his priority. Scheiner, who at this time was settling in Rome, took Galileo's complaint to be directed at him and became Galileo's sworn enemy.
Scheiner had in the meantime published several important books on optics, and he had continued his study of the Sun. He published his results in a massive tome, Rosa Ursina, ("The Rose of Orsini"),[2] which became the standard treatise on sunspots for over a century. Scheiner had abandoned his opinion that spots were solar satellites, and he indeed came out in favor of the system of Tycho Brahe and abandoned the perfection of the heavens. His method of illustrating the motion of individual spots across the face of the Sun became the standard way of rendering this motion and the changing shapes of the spots.
"Helioscopium" used by Scheiner for his later sunspot observations.
High-resolution image available.
Sunspot drawings from Scheiner's Rosa Ursina. High-resolution images available for
left,
middle,
right, drawings.
Scheiner's definitive sunspot studies were followed up by others. In France Pierre Gassendi made numerous observations (not published until 1658); in Gdansk Johannes Hevelius (1647) and in Bologna Giovanni Battista Riccioli (1651) did the same. There is, therefore, a reasonably good sunspot record for the years 1610-1645.
Sunspot drawing by Gassendi.
Sunspots drawings by Hevelius. High-resolution images available for
left and
right drawings.
After this time, however, sunspot activity was drastically reduced. When, in 1671, a prominent sunspot was observed, it was treated as a rare event. Sunspot activity increased again after about 1710. The period of low activity is now referred to as the Maunder Minimum, after Edward Walter Maunder (1851-1928), one of the first modern astronomers to study the long-term cycles of sunspots. Modern studies of sunspots originated with the rise of astrophysics, around the turn of the century. The chief early investigator of these phenomena in the United States was George Ellery Hale (1868-1938), who built the first spectro-heliograph and built the Yerkes and Mount Wilson observatories, including the 200-inch telescope on Palomar Mountain.
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