Galileo and Heliocentrism

The phases of Venus, sunspots and comets were three phenomena that played an important role in debates about heliocentrism in the seventeenth century. Each group investigated one of these phenomena and how Galileo or one of his contemporaries interpreted it.

Group 1 (James Johnson, Jacob Curtis, Carlos Ybarra, Jessica Robertson, Ryan Fransen, Laine Abernathy, Collin Wade, Jacob Moore, and Daniel Chamberlain). PHASES OF VENUS. When did Galileo first observe the phases of Venus? How did he publish his findings? How did he interpret his findings?

Phases of Venus and evolution of its apparent diameter. Wikimedia Commons.

Galileo first viewed Venus through a telescope in late 1610.  He was able to witness all the phases, as predicted by the Copernican heliocentric model of the cosmos.  Ptolemaic models would not allow for this, as Venus would have had to always appear crescent shaped.  Galileo revealed his findings in 1613.  He used a anagram (a coded message) in a letter to the Tuscan ambassador of Prague.

Watch video of Ptolemy’s model and the phases of Venus.

Watch video of Copernicus’ model and the phases of Venus.

Galileo’s observations of the phases of Venus support Copernicus’ heliocentric system, BUT they are also consistent with Tycho’s geo-heliocentric system.

Group 2 (Clark Bonham, Austin Dickerson, Eric Lopez, Michael McQuaid, Sage Ranallo, Zachary Smith, Justin Thrash). SUNSPOTS. What are sunspots? When did Galileo first observe them? How did he publish his findings? How did he interpret his findings?

Sunspots are small, dark markings in the layer of the sun that can be seen in visible light. They are magnetic storms resulting from magnetic flux within the sun.  The magnetic flux emergence on the Sun’s surface is a group of negative and positive polarities.  The locations of the negative and positive polarities on the opposite sides of the Sun’s surface are responsible for the formation of sunspots.  Galileo first observed sunspots in 1609 – 1610. He described them as appearing like clouds. He saw spots on the sun’s surface that appeared, changed shape, and disappeared over time. Galileo also observed that their rate of motion was not uniform, and therefore they could not be planets since he believed that planets exhibited uniform motion.  Castelli, one of Galileo’s pupils, had previously developed a method to observe the sun through a telescope.  Galileo wrote about his findings in letters to Marcus Welser from 1612-1613. The three letters were published by the Lyncean Academy in the summer of 1613.  Galileo thought the spots were connected to the surface of the sun and were carried around by the sun’s rotation as clouds are carried by the rotation of the Earth and the Earth’s atmosphere. He supported this with mathematical arguments based on his observations of the spots.

Galileo Galilei, Istoria e Dimonstrazioni (Rome, 1613)
Galileo Galilei, Istoria e Dimonstrazioni (Rome, 1613). Courtesy of OU History of Science Collections.


“Sunspots” from Galileo Project

“All About Sunspots” from One-Minute Astronomer

Magnetnetic flux emergence from Scholarpedia

William R. Shea, “Galileo, Scheiner, and the Interpretation of Sunspots” Isis Vol. 61, No. 4 (Winter, 1970), pp. 498-519

Group 3 (Taylor Greene, Kiley Poppins, Conrad Young, Nathan McGuffey, John Stacy, Matt Banks, Michael Rath, and Brandon Curd). SUNSPOTS. Who was Christoph Scheiner? When did Scheiner first observe sunspots? How did he publish his findings? How did he interpret his findings?

Christoph Scheiner. Wikimedia Commons.

Christoph Scheiner (1573-1650) was a German Jesuit.  He was an expert is astronomy, mathematics and optics.  He first observed sunspots in either March or April of 1611, and reported on them in a series of letters to the Augsburg banker Marcus Welser, a patron of the scholars in the Jesuit order.  In the letters, Scheiner used the pseudonym “Apelles.” Apelles was a famous myth as a 4th century Greek artist, who was famous for hiding behind his paintings to listen to what his critics said about them. This is obviously an appropriate pseudonym.  Scheiner believed that the sunspots were satellites of the sun. He wanted to preserve the perfection of the sun. He thought that the sunspots are were bodies moving across the solar disc.  Welser shared the letters with Galileo, who commented on them. Galileo believed that sunspots were on, or near, the surface of the sun. He believed they originated and disappeared there, changed shape, and varied in frequency. To him, this disproved the belief that the sun was perfect.  The Jesuits were upset that Galileo criticized Scheiner. This led them to claim that Galileo plagiarized Scheiner’s work from the Apelles letters in The Assayer.  Scheiner refuted a number of Galileo’s ideas in his own Rosa Ursina Sive Sol, a book in which he defended geocentrism. However, in this book he did end up accepting Galileo’s stance on sunspots despite his earlier feud with him.

Christoph Scheiner, sun spots from Rosa Ursina (1630). Courtesy of OU History of Science Collections.


“Sunspots” from The Galileo Project

Feingold, M., The New Science and Jesuit Science; 17th Century Perspectives. Great Britain, Kluwer Academic Publishers (2003).

Group 4 (Zachary Connor, Courtlan Roland, Nicole Palmeter, Bryce Corlee, Lindsey Hamilton, Sofia Buscarini, Cameron Owens, and Benjamin Kraft). COMETS. Did Galileo believe comets were terrestrial or celestial phenomena? What was his evidence? Where did he publish his views on comets?

Galileo, Il Saggiatore (Rome, 1623). Courtesy of OU History of Science Collections.


Galileo believed that comets were terrestrial phenomena.  Galileo was confined to bed due to illness during the period when the comet was visible, so the evidence that he had to gather was mostly from observations of his contemporaries, especially those of Tycho Brahe. The measurements made by Tycho stated that comets were heavenly bodies.  Galileo argued that comets were optical phenomena rather than real objects which made their parallaxes immeasurable.  Galileo published his views on comets in Il Saggiatore (The Assayer) (Rome, 1623).

Sources: The Galileo Project and The Assayer (trans. Stillman Drake).

Group 5 (Jessica Caballero, Vanessa Maynard, Shannon Dias Viegas, Morgan Blodgett, Jessica Raper, Tanner Linn, Wamika Kumar, and Kim Brooks). COMETS. Who was Orazio Grassi? Did he believe comets were terrestrial or celestial phenomena? What was his evidence? Where did he publish his views on comets?

Orazio Grassi (1583-1654) was a Jesuit mathematician and astronomer.  Born in Savona, in the Republic of Genoa, he studied at the Roman College (physiology, theology, math).  He became a professor of mathematics, first in Genoa and then in Rome.  His specialties included optics and architecture as well as astronomy.  In 1618, three comets were visible and aroused considerable attention from astronomers all over Europe.  The Jesuits selected Grassi to give a lecture at the Roman College on the comets in 1619.  This lecture was subsequently published as De tribus cometis anni MDCXVIII disputatio astronomica (Astronomical disputation on the three comets of 1618) (Rome, 1619).  Grassi declared that comets were celestial phenomena, supporting the views of the Danish astronomer Tycho Brahe.  In this he disagreed with Galileo, who argued that comets were terrestrial phenomena.  The two debated this in print for several years.


Sources: “Orazio Grassi” from Museo Galileo and from Wikipedia








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