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The Discoveries of Galileo – Part 4: Venus

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Galileo Galilei


The Discoveries of Galileo – Part 4: Venus

Galileo’s discoveries of Jupiter, sunspots, and the Moon were covered in parts 1 through 3 of this series. This article will cover an important discovery that Galileo made in late 1610 of his observations of Venus. Figure 1 shows the timeline of Galileo’s discoveries during 1609 and 1610.

Over the summer of 1609 Galileo refined his spyglass from about 3x magnification in June to around 8x by October. During this time, he also moved from Padua to Florence, Italy after being appointed by Cosimo II, the Grand Duke of Tuscany, as Philosopher and Chief Mathematician of the University of Pisa.


The Ptolemaic geocentric model was the prevailing belief in the early 1600’s among the Roman Catholic Church and European natural philosophers regarding the movement of the celestial objects: the planets, Sun, Moon, and stars all revolved around the Earth. Ptolemy (100 – 170 AD) was a Greek mathematician, astronomer, and geographer living in Egypt who provided a comprehensive treatise on astronomy titled The Almagest. This series of books was based on Ptolemy’s quarter-century observations and included the known ancient celestial observations recorded from Egypt and Arabia. According to the Encyclopedia Britannica, Ptolemy argued that the Earth “is a stationary sphere at the centre of a vastly larger celestial sphere that revolves at a perfectly uniform rate around Earth, carrying with it the stars, planets, Sun, and Moon—thereby causing their daily risings and settings.”

The Ptolemaic model included a series of circles along which celestial objects coursed through during their annual movement around the Earth. The circles of some objects such as the Moon and planets did not match up to observations, so Ptolemy came up with a mathematical way to align the movements: deferent and epicycle circular paths. To explain the frequently observed retrograde motion of the planets, Ptolemy had them move along a smaller epicycle circle fixed to their larger deferent circle that orbited around the Earth (figure 2).



Recall from the Part 2 article on sunspots that Galileo was corresponding with a wealthy Augsburg merchant named Mark Welser in response to sunspot observations by the Jesuit priest, Christopher Scheiner (aka Apelles). Galileo brings up the topic of Venus in his First Letter on Sunspots dated May 1612 and he states that he had discovered that Venus changed shape two years earlier in 1610. Apelles had suggested that sunspots afforded a method for determining whether “Venus and Mercury revolve about the sun or between the earth and the sun.” Galileo wrote, “This is the fact that Venus changes shape precisely as does the moon.” (Discoveries, p. 93). Later in the letter Galileo forcefully asserted,

As a matter of fact nothing else could be said before the telescope came along to show us that Venus is naturally and actually dark like the moon, and like the moon has phases.” (Discoveries, p. 95).

Galileo explained his observations of the phases of Venus from his drawings of Venus from October 1610 to February 1611 as seen in the top panel of figure 3. Venus varies from a small perfectly shaped sphere on the right to a larger semicircular sphere and then passes into a horned shape on the left which becomes wide and thin as it approaches the Sun. Galileo wrote in the First Letter on Sunspots,

These things leave no room for doubt about the orbit of Venus. With absolute necessity we shall conclude, in agreement with the theories of the Pythagoreans and of Copernicus, that Venus revolves about the sun just as do all the other planets.” (Discoveries, p. 95).

I also inserted the images from Stellarium astronomy software for the same time frame in 1610 – 1611 to confirm how well the software can reach back into time and align to what Galileo saw (figure 3, bottom panel).



Galileo used his lengthy observations of the phases of Venus and inductive reasoning to prove that Venus revolved around the Sun and not below it as many believe at the time. He also noted that the apparent diameter of Venus changes over time. In his Third Letter on Sunspots to Welser in December 1612 he noted,

“For when the planet is nearest the earth, its diameter is more than six times as great as when it is most distant, and although this difference can be precisely observed only with the telescope, it is nevertheless quite perceptible to the naked eye.” (Discoveries, p. 131).

The left drawing in figure 4 shows the imagined phases of the Ptolemaic geocentric model of Venus were it to orbit above or below the Sun or between the Earth and the Sun. Galileo posited that the Ptolemaic model would only produce horned or crescent phases either pointing rightward or leftward. The right drawing in figure 4 shows Galileo’s illustration made during his observation of a full cycle of Venus moving around the Sun. Starting with Venus at position #1, he saw a rightward facing crescent move through phases #3 & 4 where Venus grows smaller and rounder and finally to #6 the leftward facing crescent. The Copernican model below the phase sketches shows the relative position of Venus to the Earth and orbiting around the Sun. These matched well with what Galileo observed. He concludes in his Third Letter on Sunspots,

“The opaqueness of Venus is indubitably proved by the single experience I reported in my first letter; namely, our seeing Venus vary in shape as does the moon. This is the only strong argument for proving Venus to revolve about the sun; let it suffice us, as it leaves no room for doubt.” (Discoveries, p. 133).

Hence, using drawings of the phases of Venus and inductive reasoning, Galileo “indubitably proved” the Copernican model best fit the observational data.

Finally, a few years later in 1615, Galileo solidified his belief in the Copernican model in a lengthy letter to the Grand Duchess Christina of Tuscany. This letter was subtitled “Concerning the Use of Biblical Quotations in Matters of Science.” Galileo wrote,

“Furthermore, it would be necessary to forbid men to look at the heavens, in order that they might not see Mars and Venus sometimes quite near the earth and sometimes very distant, the variation being so great that Venus is forty times and Mars sixty times as large at one time as another. And it would be necessary to prevent Venus being seen round at one time and forked at another, with very thin horns; as well as many other sensory observations which can never be reconciled with the Ptolemaic system in any way, but are strong arguments for the Copernican.” (Discoveries, p. 195-196).

Galileo goes on to argue that Copernicus’ book should not be banned but should be viewed in light of the new astronomical evidence being observed nightly.

Thus, through observational astronomy in the early 17th century, Galileo was able to open new cracks in the foundation of the Ptolemaic model. Next month I will submit the final article of this series, part 5, about Galileo’s observations of the Milky Way and several large constellations which created still more cracks in that foundation.


Ed LaBelle has been a member of AAS since 2010. He has worked as an engineer in the semiconductor industry for 30 years and is the founder of the Psalm 19 Astronomy Society based in Austin.

End Notes:

The Discoveries of Galileo – Part 1: Jupiter,


The Discoveries of Galileo – Part 2: Sunspots and Their Movements,


The Discoveries of Galileo – Part 3: The Moon,


  • Special Ed, Piero DP, Mattimac and 6 others like this


Nice read, thank you kindly
    • E-Ray and ziggeman like this



Thanks for Part 4.  smile.gif   Interesting to read and follow his reasoning as to why the Copernican system had it right.  And all because he could see the previously unknown phases with a telescope.


In my "following in the footsteps of Galileo" sketching project with my Galileoscope, the only observation of his I have left to do is to to record the phases of Venus.

    • E-Ray likes this
Apr 04 2021 11:41 AM

Galileo's observation of the gibbous phase for Venus is arguably the most crucial of his many important discoveries. Yet historians often overlook its significance. This article clearly explains why it disproved the geocentric hypothesis. Well done, E-Ray!

    • Special Ed and E-Ray like this

Thank you so much for researching and sharing this with the community. It is eye opening.  Did Galileo coin the term "naked eye", since you have it in your quote from him above?

    • E-Ray likes this

Hi Quaternion, Good question. I'm not sure where the term "naked eye" came from and could be word selection from the translation that Stillman Drake made from Italian to English.



Thank you so much for researching and sharing this with the community. It is eye opening.  Did Galileo coin the term "naked eye", since you have it in your quote from him above?

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