These letters record astronomical observations made by the Italian physicist and astronomer Galileo Galilei (1564–1642). 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 467 BCE.
The introduction of the telescope to astronomy in 1609 (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 1609 and soon began building and modifying one from 1610 which allowed him to observe the sunspots, usually invisible to the naked eye. In late 1610, both Galileo and the English mathematician Thomas Harriot observed sunspots, although their observations were unknown to one another. However, it was the observations of Christoph Scheiner in March 1611 that prompted Galileo to write his letters on the sunspots.
Scheiner was a Jesuit mathematician at the German University of Ingoldstadt who published his Three Letters on Solar Spots written to Marc Wesler in January 1612. Scheiner argued that sunspots were satellites of the sun, and preserved Aristotle’s principle of an unchanging universe in his theory. Marc Wesler, a banker and magistrate in the town of Augsburg in Germany and patron of the new sciences, sought Galilieo’s opinion. Galileo resumed his observations of sunspots in April 1612.
In his letters to Wesler, published in 1613, he identified sunspots correctly as markings on the sun, confirming that the sun rotated monthly, as the position of the spots moved. Modern astronomers with new technologies have confirmed that the cyclical appearance of sunspots over long and short periods is connected with increased levels of magnetic activity.
Galileo’s construction and use of a telescope also led him to discover the moons of Jupiter, a finding he published in his book Sidereus Nuncius (Starry Messenger, 1610). Galileo also identified the Milky Way as a collection of stars and saw that the moon had a ragged surface like the earth. These discoveries made Sidereus Nuncius an immediate success.
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