Information Description These letters record astronomical observations made by the Italian physicist and astronomer Galileo Galilei — Here, Galileo records sunspots which are dark areas of irregular shape seen periodically on the surface of the sun. Observations of sunspots in ancient China were recorded as early as 28 BCE, and it is possible that the Greek philosopher Anaxagoras may have observed a sunspot in BCE. The introduction of the telescope to astronomy in combined with the ability to project an image to avoid direct observation with the eye meant that observers could view the sun consistently and safely in some detail for the first time. Galileo had heard about telescopes in and soon began building and modifying one from which allowed him to observe the sunspots, usually invisible to the naked eye. In late , both Galileo and the English mathematician Thomas Harriot observed sunspots, although their observations were unknown to one another.
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He used eight other telescopes and all of them had visible spots on the Sun. He also rejected that there was a disturbance in the atmosphere for four reasons. His final reason was that the spots could be seen through small clouds. Then, under the pseudonym Apelles latens post tabulam Apelles hiding behind the painting ,  he presented his description and conclusions about them in three letters to the Augsburg banker and scholar Mark Welser.
Scheiner wanted to remain anonymous, because he believed in order to avoid involving the Jesuit order and the church generally in an area of controversy. These later letters were different in tone to the first three, as they hinted that Galileo was claiming credit for having discovered the phases of Venus , when in fact proper credit was due to others. Having read these second three letters, Galileo replied with a third of his own, much sharper and more polemical in tone than his earlier ones.
In the case of Letters on Sunspots his critical support appears to have been helpful in ensuring that publication was not prevented by influential Dominicans of the Sacred Palace.
Galileo wanted to claim divine inspiration for his findings and show how they accorded with Holy Writ; the censors wanted to keep unusual new ideas at a safe distance from core tenets of the faith. His key conclusions are that sunspots were real and not merely optical illusions; and that they were not static, but moved.
The Sun carries these spots until they disappear from view at its rim in about one lunar month. But he is not yet able to detach himself completely from the fantasies he absorbed in the past, to which his intellect sometimes returns and lends assent by force of long-established habit. This is not in fact a disagreement about the direction of the spots, but a reminder of the conventions used by astronomers to describe them. He concludes that they could not therefore be features carried around the Sun on its surface by a regular rotation.
Galileo responds that this would be the case if Apelles had shown that the spots were solid bodies, whereas it is obvious to observers that they are changing shape as they move around the Sun. He therefore says that Apelles has not proved that they could not be on the surface of the Sun. Apelles had included a diagram in his third letter to demonstrate how he believed this could be explained in terms of the spots being small moons, which went through phases.
Galileo maintained that this was doubtful. As the dark area of sunspots approach the limb of the Sun, it appears from observation that the area of darkness reduces from the side facing away from the Sun — i.
If they were moons, the area of darkness would diminish from the side facing the centre of the Sun. He shows however that of all the things we observe on Earth, it is clouds that share the most characteristics with sunspots.
Galileo responds that there is no comparison in either case; the moons of Jupiter Medicean Stars move with an absolute regularity he has already described, while Saturn simply bears no comparison to the description Apelles provides of it.
Here Galileo provides two simple in-line sketches to show what he means. Galileo assures his reader that he can confirm, after long observation, that Saturn never changes its shape, as Apelles claims, and never will. Likewise planetary orbits appear constant in their speed, whereas Apelles has shown that sunspots move rapidly in the centre of the Sun but more slowly at its edges. To accompany these proofs Galileo provides 38 detailed illustrations, which allow the reader to see how his observations relate to his calculations.
Further observations confirm what Galileo originally believed — that the spots were on or very close to the Sun, which carries them round as it rotates on its own axis. The growth in apparent size of the gaps between spots as they approach the centre, and their apparent diminution towards the edges of the Sun, likewise confirm this.
The apparent distance to the observer from C to F is seven times smaller than the actual distance on the surface of the Sun from C to H; however, if the spots are just a small way above the surface of the Sun, the apparent distance from C to F corresponds to the actual distance from R to N, which is less than a third the length of C to H. Thus by measuring the differences in the apparent distances between spots as they move across the Sun, it is possible to know with certainty whether the foreshortening corresponds to the proportion CF:CH or to some other proportion.
The changes in apparent distance observed leave no doubt on this question. This, he says, means they must be low against the sun and thin, rather than high above its surface and thick. These were not arguments Apelles had advanced; rather, he had also argued against them. However he believes that this does in fact happen. Therefore, some spots can doubtless, or rather necessarily, be seen twice by us. This is by way of explaining to the reader that the thirty-eight illustrations which follow are highly accurate i.
This is included in his series of illustrations. However, if the sunspots are not on the surface but above it, at C and E rather than B and L, the differential time of their transit is reduced. The closer the sunspots are to the surface, the more their relative transit times can vary; the further they are from the surface, the less difference in speed will be observed.
At first he insisted they were all spherical, like little moons; now he says they are irregular in shape, forming and dissolving. He previously said that the spots were at various distances from the Sun, wandering between it and Mercury, but he no longer maintains this view.
If sunspots moved at differential speeds, this tended to suggest they were moons moving independently of the Sun itself. Galileo says that in his own observations he has never seen this differential rate of movement, but that spots always move at a constant speed relative to each other. First Galileo demonstrates that points on two different sunspot trajectories at two different latitudes produce lines which maintain a constant proportion with each other at any point in the rotation. From this he concludes that Apelles is simply wrong, and it is not possible for one spot to traverse the Sun in sixteen days, while another takes only fourteen.
He and recalls how Apelles depicts them coming into view, foreshortened, before appearing at their full width.
He then demonstrates that for the spots Apelles had observed to change in apparent size as they did, they would need to be on the face of the Sun, because if they were even a short distance above its surface the foreshortening effect would be remarkably different.
In his First Letter, he had argued that Saturn never changes its shape , and never will. Now, he agrees, it has changed shape. He does not try to prove his earlier views right in spite of new facts, but makes cautious predictions about how its appearance may change in future.
Saturn] and perhaps no less than the emergence of the horned Venus, agrees in a wondrous manner with the harmony of the great Copernican system, to whose universal relations we see such favourable breezes and bright escorts directing us.
Although the behaviour of sunspots was the main topic of their debate, they also touched on other disputes, such as the phases of Venus and the moons of Jupiter. I wait to hear the spoutings of great things from the Peripatetics to maintain the immutability of the skies. Many who had never seen them found the idea of them morally and philosophically repugnant.
Moons of Jupiter[ edit ] Galileo had discovered the moons of Jupiter in Scheiner claimed that the sunspots, with their irregular movements, were like the moons of Jupiter whose positions were similarly hard to predict.
To counter this argument, Galileo published tables of predictions for the future position of the moons of Jupiter, so that astronomers could easily distinguish between the regular, predictable movements they followed with the ephemeral and irregular sunspots. Firstly, it showed that the traditional Aristotelian model of the universe must be wrong, because that model assumed that the Sun had only a diurnal daily motion around the earth, and not a rotation on its own axis.
Secondly, it showed that there was nothing necessarily unusual about rotation of a body in space. In the Aristotelian system, night and day were explained by the Sun moving round a static Earth.
This explanation was that the Earth rotated on its own axis once every day. While the rotation of the Sun did not prove Copernicus right, it proved his opponents wrong and made his ideas more likely to be true. Phases of Venus[ edit ] In the Letters on Sunspots Galileo responded to claims by Scheiner about the phases of Venus , which were an important question in the astronomy of the time.
There were different schools of thought about whether Venus had phases at all — to the naked eye, none were visible. The fact that there was a full phase of Venus, similar to a full moon when Venus was in the same direction in the sky as the Sun meant that at a certain point in its orbit, Venus was on the other side of the Sun to the Earth.
This indicated that Venus went around the Sun, and not around the Earth. This provided important evidence in support of the Copernican model of the universe. Copernicus is not mentioned by name. I see young men Latin], take it into their heads that in these crabbed folios there must be some grand hocus-pocus of logic and philosophy much too high up for them to think of jumping at.
I want them to know, that as nature has given eyes to them, just as well as to philosophers, for the purpose of seeing her works, she has also given them brains for examining and understanding them. Most readers of the time did not have a telescope, so could not see sunspots for themselves — they relied on descriptions and illustrations to make clear what they looked like.
Although the sunspots were constantly changing position, Scheiner presented his observations over a period of six weeks in a single fold out plate. He admitted to his readers that his drawings were not made to scale, and that other factors such as variations in the weather, lack of time, or other impediments may have reduced their accuracy. Sometimes the configurations of the spots were linear following consecutive days, but the orientations became more complex over time that there was a lack of an obvious pattern.
This extensive visual representation, with its large scale and high-quality reproduction, allowed readers to see for themselves how sunspots waxed and waned as the sun rotated. This sense undermined the claims made by Scheiner before any argument was mounted to refute them.
Originally from Strasbourg and a convert from Protestantism, Greuter moved to Rome and set up as a printer specialising in work for the Jesuit order. His work ranged from devotional images of saints through to mathematical diagrams. This relationship may have recommended him as one whose involvement in a publication would perhaps ease its path through censorship; in addition his craftsmanship was outstanding, and he devised a novel etching technique specially in order to make the sunspot illustrations as realistic as possible.
Galileo drew sunspots by projecting an image of the Sun through his helioscope onto a large piece of white paper, on which he had already used a compass to draw a circle. He then sketched the sunspots in as they appeared projected onto his sheet. In Letters on Sunspots Galileo did as Copernicus had done — he elaborated his ideas on the form and substance of sunspots, and accompanied this with tables of predictions for the position of the moons of Jupiter.
In part this was to demonstrate that Scheiner was wrong in comparing sunspots with the moons. More generally, Galileo was using his predictions to establish the validity of his ideas — if he could be demonstrably right about the complex movements of many small moons, his readers could take that as a token of his wider credibility. This approach was the opposite of the method of Aristotelian astronomers, who did not build theoretical models based on data, but looked for ways of explaining how the available data could be accommodated within existing theory.
From the middle of the seventeenth century the debate about whether Scheiner or Galileo was right died down, partly because the number of sunspots was drastically reduced for several decades in the Maunder Minimum , making observation harder. Furious, he decided to stay in Rome and devote himself to proving his own expertise in sunspots.
His major work on the topic was Rosa Ursina —
GALILEO LETTERS ON SUNSPOTS PDF
Dainos However, for him, he was protecting it from the commission of a fatal error. To accompany these proofs Galileo provides 38 detailed illustrations, which allow the reader to see how his observations relate to letterrs calculations. Sunspots are dark areas of irregular shape on the surface of the Sun. When, ina prominent sunspot was observed, it was treated as a rare event.
Kigasar 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. Since his drawings were not drawn to scale, he admitted that it a shortcoming, possibly due to inconsistent weather, lack of time, or impediments. This approach was the opposite of the method of Aristotelian astronomers, who did not build theoretical models based on data, but looked for ways of explaining how the available data could be accommodated within existing theory. Many who had never seen them found the idea of them morally and philosophically repugnant. Skip to main content.