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Letters on Sunspots

The Letters on Sunspots, was a continuation of Sidereus Nunicus, Galileo's first work where he publicly declared that he believed that the Copernican system was correct. This was the first work where Galileo used a ship, which would later become famous in Dialogue Concerning the Two Chief World Systems (Dialogo sopra i due Massimi Sistemi del Mondo).

Galileo often mentions how does not know or understand some things. He mentions that the sun might not be revolving, or that the spots might look different if they were viewed in different parts of the world. However, in one passage, he states that the sun does have a motion, and he wonders what causes this motion. This is where his work established a connection between cosmology and mechanics. Galileo wrote, "I seem to have observed that physical bodies have physical inclination to some motion." Galileo's explanation of the sun's motion was hesitant and ambiguous, and he was aware of these general principles. However, this is the first instance in one of his works where he mentioned the concept of inertia, which would later become Newton's First Law of Motion.

Critical dialogue with Scheiner

When Jesuit Christoph Scheiner first observed sunspot in March 1611, he ignored them until he saw them again in October. When he saw them again, he attributed the sightings to either a defect in the eye, problems with the lens of the telescope, or a possible disturbance in the atmosphere, but he rejected all three hypotheses. He rejected the hypothesis that he had a defect in his eye because others also observed the spots. 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.The first was that no cloud could follow the sun's diurnal motion throughout the day, especially given the sun's diameter. The spots also showed no parallel motion, but the spot's movement across the sun was constant. His final reason was that the spots could be seen through small clouds. Scheiner believed that these dark spots were visible because there were small orbiting bodies around the sun, which would block the sun's light. This explanation would allow him to avoid Aristotle's belief that the sun was a perfect celestial being.

Galileo's First Letter- 4 May 1612

The sunspots had one single motion: they moved across the Sun in a uniform fashion. Galileo argued that the sun was a perfect sphere and that it moves by itself on its own center. The sun carries these spots until they disappear in about one lunar month.

'Flaws' in the Sun

In 807 A.D. there was a sunspot that was large, and it was seen for 8 days. The records in this period stated that Mercury caused this spot. Since Mercury is the closest planet to the sun, its location was unclear, and many argued that Mars could be anywhere. In earlier writings, Ptolemy discussions on sunspots referred to the transit movement of Mercury and Venus. in the twelfth century, Muslim philosopher Averroes's interpretation also discussed the transit of Mercury and Venus. In De constitutione mundi, Giovanni described a similar phenomenon that his father saw, but it may have been possible that he was describing Averroes's observations.

Use of diagrams and illustrations

Since the sunspots were consistently changing positions, Scheiner wanted to present this in his drawings. To do this, he had one page dedicated to observations. This page appeared as a fold out plate with over six weeks of observations. All of his figures are small. except for the observations in the top left corner. Since his drawings were not drawn to scale, he admitted that it a shortcoming, possibly due to inconsistent weather, lack of time, or impediments. However, these small figures could have been confused as satilites orbiting the sun. Scheiner also showed the formation of spots in different orientations. 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.