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What Were Galileo's Contributions to Astronomy?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.


Galileo had three big moments that defined his professional life. One was his studies on physics, another was the conflicts he had with people in academia and the clergy. This article will examine the astronomical work of his life and perhaps reveal new insights into the man who revolutionized science forever.

Copernican System

One of the first mentions of astronomy by Galileo was in 1590 when he conveyed his belief in Copernicus’ work on heliocentrism. Galileo also references Kepler’s works as well. Upon hearing about this, Kepler urged Galileo to be more open about it politically and if need be move elsewhere if safety was a concern. Galileo never did move away, but slowly his views trickled out into his work. He might not have moved because of employment or because of his family of 3 children (Taylor 57-8).

Galileo started to write about astronomy. In one of his documents, he discussed many topics including geography, cosmography (or what we refer to as the latitude/longitude system), eclipses, and the phases of the moon. The purpose of the work may seem confusing to modern readers, for Galileo wrote it in the old style of science, i.e. with no evidence or procedures but instead with crazy theories. But when we compare this work with Dialogues, which he wrote later in his life, and refute many of the concepts here, we almost feel like his sole intention was exposing people to these ideas just to show how the modern science technique was superior to crazy undefendable ideas (59-60).


Not too long after this, a big shift in the stellar viewpoint occurred on October 10, 1604. A new star seemed to appear in the sky and was even visible at times during the day. But according to Aristotelian cosmology, the universe was constant and unchanging, yet here was evidence that was contrary to that. Fortunately, the Aristotelians had a convenient explanation: it was simply an atmospheric disturbance. However, when scientists found it to have an immeasurable parallax, they realized that it was far away and not possibly something in the atmosphere. Galileo was not satisfied with this though. What was the nature of this new star? It had upset the balance of the heavens, and his curiosity took over. This led him to the use of an instrument that aided in his most famous discoveries and ultimately to his legacy to science (60).

That breakthrough was the telescope, something that is attributed to him but was actually developed by Hans Lippershey, a toy maker. It made use of refraction, or the bending of light rays, versus reflection via mirrors like modern telescopes. By gathering the light with the proper curvature and material for the lenses and by placing them at the appropriate distance from each other, distance objects could be magnified to several times their original size, allowing for study of distant (and seemingly small) points of light. After reading up on the lens work of Lipperkey, Galileo grounded and polished his own lenses and even worked at the design of the telescope to improve performance starting in June or July of 1609. Galileo’s design used a tube of lead and two lenses of differing radii of convergence with one convex and the other concave. Those lenses had their flat sides meeting each other. Not too long after Galileo built this telescope, he began to test it and make further improvements. And finally, in January of 1610, the telescope was pointed to the sky and the floodgates of knowledge were opened (Taylor 61-2, Brodrick 30).

The Starry Messenger

The Starry Messenger

The Starry Messenger

It was through that primitive telescope that he first saw mountains on the moon which went against the conventional thought of the time that the moon was smooth. And yet here Galileo saw otherwise, though he wasn’t the first to notice but was the first to publish results about it. And then he turned the telescope to Jupiter on January 7, 1610 and noticed small points of light around it. He began to record their positions night by night from February 26 to March 2 and came to a startling conclusion: they were in fact objects orbiting the planet. He could even predict where they would be in the future based on their orbital motions! Galileo also looked at the Pleiades star cluster and saw over 40 new stars there. He published these findings as well as his new viewpoints on the Milky Way in Sidereus Nuncius (in English, The Starry Messenger) on March 4, 1610. The book is dedicated to the Grand Duke Cosino de Medici of Tuscany and in honor of said gentleman the new satellites of Jupiter are named the Medicean Stars. Even though Galileo did not suspect they were in fact stars but something more groundbreaking, he wanted more evidence before making such a bold claim (Taylor 62-3, Brodrick 34-5, 38).

Galileo started that book with the moon observations mentioned before. When he saw the dark regions all over its face, they looked like seas and thus that was their name, though in Italian we say mare. Around them, Galileo could see clear indications of elevation and craters especially when the moon was waxing or waning. From there, he goes into detail about some observations on the Milky Way and the stars in it. When he looked at the planets of the solar system, some of them seemed to be a disc in the sky rather than a point of light. Yet when looking generally at the heavens, he found that stars did not enlarge to the point of being a definite circle, but the number of stars seen did increase. He found that nebulas seem to be clusters of stars, and that the band of the Milky Way was also a collection of stars. After this, he ends the book with a description of his Medicean Stars and how to find them based on his data with 3 being found on January 7, 1610 and another one on the 13th. He does call them planets, for at that time it meant something that moved against the fixed stars of the heavens (Taylor 64-5, Pannekock 228).

The Medicean Stars

The Medicean Stars

New Findings

Shortly after releasing that book, Galileo continued his astronomical studies and came across a huge discovery. He was able to show that the Copernican motion of the moon around the Earth was indeed true and that other objects in the sky did not orbit the Earth as demonstrated by the Venusian phases. Amazing stuff here, especially with the tech of the time. But to be careful and make sure no one could claim his discoveries for themselves, Galileo released his findings as a riddle and waited an appropriate time for someone to come forward with the solution. He released the answer in November of 1610 (Taylor 65-6).

Of course, technological shortcomings meant that some findings did not hold up to reality. Take for example Saturn. Galileo trained his telescope to it in July of 1610 and found that it seemed to have 2 other planets next to it. Of course, we know now that those are rings but to a person who never knew such was possible and had such low resolution couldn’t help but draw from his frame of reference. It wouldn’t be until 1655 when Huygens observed the rings for an extended period of time and noted that they shifted and were round in nature (Taylor 66, Pannekock 230).

After his riddle was revealed, Galileo presented another one in December of 1610. Many tried to solve it including Kepler but to no avail. Galileo relented on New Year’s Day of 1611 and released the answer. This time it was the discovery of Venusian phases, just like our moon. Note that this wasn’t definite proof of the Copernican system, for the Ptolemaic system too could have such a planetary alignment (Taylor 66-7, Pannekock 230).

His final great discovery of astronomy was sunspots, though history didn’t give him credit initially. That is because he held off publishing the results and sometime later in January of 1612 Christopher Scheimer sees them. Galileo initially felt they were planets that were close to the Sun but then in September calls them clumps of dense matter around the Sun. Galileo wouldn’t publish his findings until March 22, 1613 when the Lyncean Academy releases his three letters. There he critiques Scheimer’s findings and claims the sunspots are actually clouds of material that rotate with the Sun around it. This totally went against Aristotelian conventions, for the clouds according to Galileo are formed by a rotating Sun. Again, this challenges the viewpoint of an unchanging heavens (Taylor 67-8).

Venusian phases as seen by Galileo.

Venusian phases as seen by Galileo.

Discoverer of Neptune?

Looking through Galileo's records offers astronomers records of celestial positions from hundreds of years ago, essentially priceless information. Sometimes what one spots as a star ends up being something special but is left unrecognized for what it is. Such is the case with Galileo's observations of Neptune, over 200 years before its official discovery by Le Verrier and Adams. Looking through Galileo's records shows that he spotted Neptune twice, on December 28, 1612 and January 28, 1613, but didn't note anything special about it. Why did Galileo bother to draw the star sometimes but not other times? Well, Galileo was studying the Jovian system and it just so happens that January of 1613 was a rare occultation between Jupiter and the then unknown Neptune. Luck put Galileo in a spot to see a new planet but his focus was on Jupiter and its moons, plus the orbit of Neptune is very slow and so noting changes in its position is challenging, but it is worth noting that Galileo noted he thought he saw a change in position with respect to another star now known as SAO 119234, but he never followed up on this. It seems like he did plan to, though, because he included a separate drawing noting the distance between Neptune and that star. But after the 28th, Jupiter had moved far enough so that Galileo couldn't put Neptune, SAO, and Jupiter into his frame of view, and without that comparison to Jupiter he simply had to move on (Drake)

If this wasn't interesting enough, his noted observations of it are off from what our celestial modeling shows it should have been by only 1 arc minute! Can we attribute this to a mistake on Galileo's part? Unlikely for several reasons. One is the same error in the two observations separated by a month, but you could of course attribute this to systematic error. However, over 50 years before the cross-hair micrometer was invented for accurate astronomical measurements, Galileo developed a spiritual predecessor to it using a grid at the end of his telescope. If you know distances between the grid and the telescope you can gather proportions which then allow you to find the angular measurements needed and also allowing you to note the changing distances between us and what we observe (moving away would reduce angular size and vise versa). By looking at the ratios between the angular measurements of Jupiter and the distances to the moons we can see that Galileo was very close to modern observations. Besides these factors favoring Galileo's accuracy, nothing else would have been in the area Galileo spotted during those nights, plus Galileo only noted a star if he predicted it would be occulted by Jupiter. Face it, people: Galileo did spot Neptune but simply didn't uncover its secrets. The error found points instead to some missing mass pulling on Neptune's orbit that we have yet to account for. So Galileo almost found another planet and developed a prototype standard for celestial measurements, not bad at all (Ibid).

Galileo's chart showing the position of Jupiter (center) and the four major moons (the circles) as he observed them.

Galileo's chart showing the position of Jupiter (center) and the four major moons (the circles) as he observed them.


Just because Galileo didn’t end up discovering anything else in astronomy didn’t mean he was done with that field. Written from 1625 to 1629, Dialogues was meant to compare and contrast the Ptolemaic and Copernican systems. It was in the form of 4 main dialogues: Causolls’ work, Earth motion, Ptolemaic and Copernican theories, and finally the tides. You could almost call it the anthology of the best work of his life, for it utterly destroys the Ptolemaic system forever and leaves the Copernican theory as supreme. To get around this, Galileo tried to express the ideas as beliefs and not truths. He finished the book in 1630, by which point he was 66 and in poor health (Pannekick 112).

Upon a modern examination of the book, it is clear that Galileo was conveying more than one message. Take for example the Preface. Galileo state that the Copernican theory is not condemned because of people ignoring the facts when in fact he felt that was certainly the case. To further help disguise his intentions, he arranged the book like a conversation between people over a several day period. Each day would cover different topics, and so on the first day the Aristotelian viewpoints were discussed, showing that heir viewpoints on non-changing heavens, motions, etc., were false. Also, debated that first day was the perfect sphereness of the moon and why that was not actually reality (118, 121, 124).

Works Cited

Brodrick, James. Galileo: The Man, His Work, His Misfortune. Harper & Row Publishers, New York, 1964. Print. 30-4, 38.

Drake, Stillman and Chalres T. Kowal. "Galileo's Sighting of Neptune." Scientific American , Vol. 243, No. 6 (December 1980), pp. 74-81.

Pannekick, A. A History of Astronomy. Barnes & Noble, New York: 1961. Print. 228, 230.

Taylor, F. Sherwood. Galileo and the Freedom of Thought. Great Britain: Walls & Co., 1938. Print. 57-68, 101-3, 112.

For more information on Galileo, see:

© 2017 Leonard Kelley


name on January 09, 2020:

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philani on October 04, 2018:

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