Galileo, Castelli, and Stellar Parallax

The story of Galileo Galilei versus the Catholic Church is usually told as a simple drama: enlightened science crushed by religious prejudice. We know this narrative is wrong, as I discussed previously on my blog (Revisiting the Galileo Affair).

But did you know that Galileo tried to measure stellar parallax to prove the Copernican heliocentric system, that his experiment failed, and that he did not tell anyone? – This really struck me.

Here is the story:

In 1617, Benedetto Castelli asked Galileo to observe Mizar, a star that appears single to the naked eye but reveals itself as a double star when seen through a telescope.

Galileo was an important figure, and so was Castelli. He was a Benedictine monk, mathematician, and astronomer — first a student and later a close friend of Galileo. He defended Galileo and the Copernican system throughout Galileo’s troubles with Church authorities. In 1613, when Galileo decided to defend himself publicly against some of his accusers, he chose to do so by means of an open letter addressed to Castelli.

Galileo realized that the double star Mizar could be used to observe stellar parallax and thus confirm heliocentrism. “He observed them [both stars] meticulously for a year, but saw no parallax. He recognized this as observational evidence, given the state of astronomical knowledge at the time, that Earth does not go around the sun,” explains Tim Thompson, former physicist at NASA’s Jet Propulsion Laboratory in California [1].

What Galileo did not know was that the real distances of these stars from us are orders of magnitude greater than his calculations suggested. “Nobody understood optics well enough to realize that the apparent stellar disk they saw in telescopes was just an optical artifact (the Airy disk). They thought it was the physical disk of the star, and therefore all of them, Galileo included, seriously underestimated the distances to the stars, assuming they must be nearby and therefore must also show parallax,” says Thompson.

Galileo could not observe stellar parallax, and this was strong counter-evidence against heliocentrism. What did Galileo do? “He hid his observations and never told anyone what he had discovered.”

Hide observations and tell nobody? Scientists perform experiments and make observations to test their hypotheses, and based on these results, they refine their hypotheses in order to arrive at a strong theory. I told this at various occasions to my high-school students during my recent 18-month assignment teaching Biology and Chemistry. I used this graph for illustration of the scientific method:

And if Galileo hid his results, how do we know?

Several historians of astronomy [2] investigated Galieo’s notes and the correspondence between Galilei and Castelli and found reasonable evidence that Galileo tried for several years to find stellar parallax, not only looking at Mizar but also other double stars between 1617 and 1627, on his own and upon Castelli’s request.

But there is even more to say about Galileo’s lack of scientific transparency. In 1632, Galileo wrote:

“I do not believe that the stars are spread over a spherical surface at equal distances from one center; I suppose their distances from us vary so much that some are two or three times as remote as others. Thus if some tiny star were found by the telescope quite close to some of the larger ones, and if that one were therefore very very remote, it might happen that some sensible alterations would take place among them.”

Chris Graney, astronomer and historian of science from the Vatican Observatory, indicates [3] that Galileo proposes — using the word “if” — that parallax might be observable with double stars at different distances from each other. Yet, he does not mention that he had already found such a pair in 1617 and had not observed any differential parallax. He says, “Should Galileo have published his double star observations? Certainly. From a scientific standpoint, if he is going to promote the ideas that Earth circles the sun, and that the stars are sun-sized bodies at varying distances from Earth — and that for these reasons a double star might reveal differential parallax and thus appear in court to give witness that Earth does in fact move — then, yes, he should also mention that he has already observed exactly the sort of double star system that he describes, and that those observations contradict the ideas he is promoting. When Galileo wrote in the Dialogue about how ‘if some tiny star were found by the telescope’ close to a large star, he was sitting on exactly that sort of observation, right in his notebooks. Scientifically speaking, that’s definitely not cool.”

And he concludes: “Galileo was a great scientist. He should be honored as one of the greats. But, in trying to prove that the Earth moved, he did stuff that scientists are not supposed to do. Galileo was not punished for proving that Earth moved. But some of the things he did while trying to prove Earth’s motion would get him in trouble in the scientific world today.”

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Stellar parallax was first measured by Friedrich Bessel in 1838. In 1851, the Earth’s rotation was demonstrated with Foucault pendulum. The decisive evidence Galileo sought did exist — but it required instruments, theoretical insight, and physical understanding that lay beyond his century.

Galileo was right about the Earth’s motion. He was wrong about the distances of the stars. And when his own careful observations failed to support his expectations, he chose silence instead of transparency.

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[1] Tim Thompson on facebook (16 Feb 2026)

[2] Umberto Fedele (1949). Le prime osservazioni di stelle doppie. Leos Ondra (1999), A New View of Mizar.
Overview articles: Harald Siebert (2005). The Early Search for Stellar Parallax: Galileo, Castelli, and Ramponi. Journal fothe History of Astronomy, 36(3), 251–271. doi:10.1177/002182860503600301; Christopher Graney (2017). Strange Tales of Galileo and Proving: Telescopic Evidence for Earth’s Immobility through Double Stars, on the blog of Vatican Observatory. Christopher Graney (2024). The View of the Double Star Mizar, Twenty Years Later, on the blog of Vatican Observatory

[3] Christopher Graney, (2017), op.cit.

St. John Henry Newman and Science

St. John Henry Newman will soon be declared a Doctor of the Church. Newman’s most influential theological idea is the development of doctrine—the belief that Catholic teachings grow and unfold over time under the guidance of the Holy Spirit. His book An Essay on the Development of Christian Doctrine prepared his own conversion to the Catholic Church and had an impact on Vatican II.

St. John Henry Newman was, by vocation and training, a theologian and pastor. However, given his Oxford education (which included lectures in mineralogy and geology) and his role as Rector of the Catholic University of Ireland, Newman was deeply immersed in the intellectual world of his time, including the rising influence of the natural sciences.

Prior to his canonization in 2019, I prepared an article for the Society of Catholic Scientists, https://www.catholicscientists.org/idea/saint-john-henry-newman-a-co-patron-for-scientists, reflecting on his views on science in general and on the theory of evolution in specific.

And I share here a recent comment by Heinz-Herman Peitz (in a translation from German):

“Thank you for the link to a thoughtful tribute of [St. John Henry] Newman by Berta M. Moritz. She presents Newman as a great interdisciplinary pioneer and as one of the first theologians who did not see Darwin’s theory as a contradiction to the Christian faith. For Newman, apparent conflicts between science and faith do not necessarily arise from scientific error but may just as well spring from a misunderstanding of revelation. In this respect—one might add—he was ahead of the First Vatican Council. That Council, to be sure, also recognized that, since there is only one truth, there can ultimately be no genuine conflicts. But when it came to apparent conflicts, it acknowledged only the opposition between “erring reason” and “infallible revelation.”
Moritz also shows Newman’s continuing relevance today, for example in his understanding of “design.” Here Newman inverts the logic employed by many modern proponents of Intelligent Design: “I believe in design because I believe in God, not in God because I see design.”

Henrietta Swan Leavitt – Measuring Stars

“She had the happy faculty of appreciating all that was worthy and lovable in others, and was possessed of a nature so full of sunshine that, to her, all of life became beautiful and full of meaning.” – Solon I. Bailey in the obituary of Henrietta Leavitt

Henrietta Leavitt (04 July 1868 – 12 December 1921, a dedicated researcher at Harvard College Observatory, made groundbreaking contributions to astronomy. Henrietta Leavitt was tasked with cataloging, observing, and tabulating stars that varied in brightness over time, known as Cepheid variables. Photographic plates taken over an extended period at the Arequipa Observatory in Peru captured night sky images, which were then sent to Boston. Leavitt identified many new Cepheid variables, increasing the number of known Cepheid variables in the Large and Small Magellanic Clouds to nearly 1,800. She discovered that these stars oscillated over periods ranging from days to weeks and that the duration of these oscillations was related to their brightness (or luminosity): the longer the period from minimum to maximum brightness, the brighter the star. She found this pattern, since all stars she observed were at approximately the same distance from Earth. She published her findings in 1908 and in 1912.

The comparison of the apparent brightness of a Cepheid star to its absolute brightness – based on its pulsation period – could now be used to measure the distance of these stars. Up to this time, the only possibility to measure distances was using the star’s parallax, a method that only allowed to measure distances up to a few thousand light-years. With using Leavitt’s relationship between luminosity and pulsation period in Cepheid stars, distances up to 13 million light-years could be measured: she thus revolutionized astronomy.

In 1924, Edwin Hubble discovered a Cepheid variable in the Andromeda galaxy and, with the help of Leavitt’s finding, showed that this object lies far beyond our Milky Way.

Her father was a Congregationalist minister. While we know she was a deeply committed Christian throughout her life, details about her faith are scarce. She was passionately devoted to her work, carrying it out with meticulous precision. Despite significant health challenges, including hearing loss and a prolonged battle with cancer, she persevered. Her legacy is a testament to her kindness, intellectual curiosity, and the power of scientific inquiry.

Had she lived longer, she might have been awarded the Nobel Prize. The mathematician Gösta Mittag-Leffler, a member of the Swedish Academy of Sciences, planned to nominate her for the Nobel Prize in 1924, but her untimely death three years earlier prevented this. The Nobel Prize is not awarded posthumously.

Henreitta Leavitt has a moon crater and an asteroid named after her.

St. Augustine on the Literal Meaning of Genesis

St. Augustine was the first to describe that Revelation is given to us in the Book of Nature and in the Book of Scripture. The metaphor of the two books is routed in God’s Truth as the source of both [1].

He expresses true concern that a literal misinterpretation of the book of Genesis – like the erroneous claim by Young-Earth-Creationists that the universe was created in six (24 hour) days and that earth is 6.000 years old – may lead unbelievers to ridicule Holy Scripture. This warning should be taken seriously:

Usually, even a non-Christian knows something about the earth, the heavens, and the other elements of the world, about the motion and orbit of the stars and even their size and relative positions, about the predictable eclipses of the sun and moon, the cycles of the years and the seasons, about the kinds of animals, shrubs, stones, and so forth, and this knowledge he holds to as being certain from reason and experience.

Now, it is a disgraceful and dangerous thing for an infidel to hear a Christian, presumably giving the meaning of Holy Scripture, talking nonsense on these topics; and we should take all means to prevent such an embarrassing situation, in which people show up vast ignorance in a Christian and laugh it to scorn. The shame is not so much that an ignorant individual is derided, but that people outside the household of faith think our sacred writers held such opinions, and, to the great loss of those for whose salvation we toil, the writers of our Scripture are criticized and rejected as unlearned men. If they find a Christian mistaken in a field which they themselves know well and hear him maintaining his foolish opinions about our books, how are they going to believe those books in matters concerning the resurrection of the dead, the hope of eternal life, and the kingdom of heaven, when they think their pages are full of falsehoods on facts which they themselves have learned from experience and the light of reason?

Reckless and incompetent expounders of Holy Scripture bring untold trouble and sorrow on their wiser brethren when they are caught in one of their mischievous false opinions and are taken to task by those who are not bound by the authority of our sacred books. For then, to defend their utterly foolish and obviously untrue statements, they will try to call upon Holy Scripture for proof and even recite from memory many passages which they think support their position, although “they understand neither what they say nor the things about which they make assertions” (Tim 1,7).”

St. Augustine, The Literal Meaning of Genesis, Book 1, Chapter 19, Section 3

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[1] For an in-depth scholarly article, see: Oskari Juurikkala, The Two Books of God: The Metaphor of the Book of Nature in Augustine, Augustinianum 61/2 (2021), p. 479 – 498

Thomas Aquinas: Fierce animals in Paradise?

Have you heard that all animals were tame in Paradise and only ate plants? I did, but it sounded strange to me. Here is a reply from Thomas Aquinas, 13th century:

“In the opinion of some, those animals which now are fierce and kill others, would, in that state, have been tame, not only in regard to man, but also in regard to other animals. But this is quite unreasonable. For the nature of animals was not changed by man’s sin, as if those whose nature now it is to devour the flesh of others, would then have lived on herbs, as the lion and falcon.”
– Thomas Aquinas, Summa Theologiae, part I q 96, Reply to Objection 2.

Picture: Jan Brueghel the Elder: Earth, or The Earthly Paradise, detail of animals, 1607-08

The 2024 list: Catholic Women Pioneering in Science

I am preparing a talk for the 2024 Conference of the Society of Catholic Scientists, my contribution is titled: “With the Future in Mind: Catholic Women Pioneers in Science”.

The women presented here come from a variety of scientific disciplines and lead lives as diverse as life itself can be, but they all had two characteristics in common: (1) they have had an impact on the science and/or scientific education at their time and (2) their lives were informed, transformed, and inspired by their Catholic faith.

Twenty-four of these women were presented in the 2023 post, and 12 new short biographies have been added.

Lise Meitner, Eva von Bahr, and Elisabeth Schiemann: the power of friendship

Memorial for Lise Meitner, Humboldt Universität Berlin, created by Anna Franziska Schwarzbach

These three female scientists were unique in their personalities and bound together in friendship. Did you know that Lise Meitner – who discovered the principle of nuclear fission – was supported by her previous colleague Eva von Bahr to build up a new life in Sweden when she needed to flee Germany in 1938? And that Elisabeth Schiemann was a leading botanist and an active member of the “Confessing Church”, active in the resistance to the Nazi regime?

I explored this and more in my recent article “Three pioneering women in science: a story of science, faith, and the power of friendship” on the website of the Society of Catholic Scientists.

Darwin and Mendel on Inherited Variability

Darwin’s Origin of Species took many years of compilations and it was even accelerated by Alfred R. Wallace paper on the same subject in 1858. Origin of Species was really groundbreaking, because Darwin not only proposed common ancestry – that was in the air since Humboldt and Lamarck, and “Vestiges of the Natural History of Creation” -, but he proposed also a process for the underlying mechanism of the diversity of species.

His theory of evolution had 2 principles: common ancestry and natural selection. Natural selection can be described with 5 words: Variation, Inheritance, Selection, Time, Adaptation. Inherited variability and a selection pressure to adapt to the environment.

In Darwin’s time, the source of inherited variability was unknown. Darwin considered heredity as a “blending” process and the offspring were seen as essentially a “dilution” of the different parental characteristics.

Revisiting the Galileo Affair

Recently, in a talk to a small group of Catholic women scientists, I suggested we need to see events in the 17th century from the perspective of 17^th century history and scientific knowledge.

Indeed, from today’s perspective, it may seem completely antiquated not to know that the Earth is orbiting the Sun, and not vice versa. In the early 17th century, though, there were (at least) 4 models of our planetary system:

the Ptolemaic system (geocentric),
the Copernican system (heliocentric),
the Tychonic system (geo-heliocentric), and
the Keplerian system (heliocentric, elliptic orbits)

The Galileo affair has indeed often been used as an argument that the Catholic Church was hostile to science and that Galileo was a martyr for science, as it were. This timeline article is intended to set the historical record straight. Based on the scientific knowledge of the time, a heliocentric model was not obvious. While heliocentrism ultimately turned out to be right, Galileo could not present the scientific proofs for it, which came much later. Moreover, Galileo ventured into advising theologians how to interpret Scripture, going beyond his position as a scientist.

A timeline representation of the story is available here (click on the image below:)

St. Giuseppe Moscati: Scientist, Physician, and Pioneer in biochemistry

I learned from my colleague Sinto Varghese that Giuseppe Moscati was not only a physician and a saint, but also a scientist and researcher. Sinto Varghese writes on ‘Church and Science’: you find him on X, formally Twitter: @ChurchNScience, and on FB https://www.facebook.com/CatholicsandScience. I am using his entry with permission.

Giuseppe Moscati (25 July 1880 – 12 April 1927) was an Italian physician who made pioneering contributions to medicine in his time. He seamlessly integrated traditional clinical diagnostic methods, primarily reliant on physical examination, with the insights of physiological chemistry. His published works provide an insight into his scientific contributions, with a particular emphasis on his revolutionary approaches to treating juvenile diabetes. Moscati is rightfully recognized as the first practitioner to introduce insulin therapy in Italy, earning him the distinction of being a pioneer in modern diabetology and endocrinology. He also employed light microscopy to determine the quantity of blood in experimental nephritis.

Science meets Faith

Can you be both, Scientist and Christian at the same time? Yes, you can – yes, we can!