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. Had he been able to follow up on his observation the next
day, he would still have seen the spot. Since he knew that Mercury
takes only a few hours to cross the Sun's disk during one of its
infrequent transits, he would have known that what he observed
could not have been Mercury.
A sunspot
The 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.
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."Legend has it that the famous Greek painter
Apelles once hid behind one of his painting to hear what people
said about it. When a shoemaker praised the way Apelles had
rendered shoes in the painting, Apelles revealed himself and
thanked the shoemaker for the compliment, but this man now
proceeded to give his not so complimentary opinions about other
aspects of the painting. Apelles answered "Let the shoemaker stick
to his last." 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 protege 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. 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.
"Helioscopium" used by
Scheiner for his later sunspot observations.
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.
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Sunspot drawings from Scheiner's Rosa Ursina.
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"),The rose refers to
the Sun, Cardinal Orsini was his patron who paid for the
printing. 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.
Sunspot drawing by
Gassendi
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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Sunspots drawings by Hevelius
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