Arguments of Galileo Galilei in support of his astronomical claims

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I do not feel obliged to believe that the same
God who has endowed us with sense, reason and
intellect has intended us to forgo their use.

Being one of the most prominent thinkers and astronomers, Galileo Galilei is interesting to us as the one, who went down into history as one of supporters of Copernicanism, the pioneer of experimental scientific method, the reducer of problems to a simple set of terms on the basis of everyday experience and common-sense logic and finally as the one, who used a refracting telescope in order to make important astronomical discoveries.  In the essay I am focusing my attention on the arguments in support of Copernicanism that is widely presented in the Galileo Galilee’s “Letter to the Grand Duchess Christina of Tuscany” (1615).  When considering those arguments we are to take into account the specification of that time: Church was the power of knowledge monopoly, had the right to prohibit research in the fields arguing with Bible and possession by scientists primitive research tools.

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In order to understand arguments we are to understand under what circumstances Galileo Galilei has written this letter.  Galilei was known as the supporter of Copernicus heliocentricity, which is the view that sun is the center and Earth is revolving around the Sun.  Outcomes of Galileo research were new to the public.  By the year of 1615, he did his observations of stars with the help of self-made telescope.  After careful investigation of the planets, he drew the Moon’s phases as seen through the telescope, showed that the Moon’s surface is not smooth, but is uneven; discovered four moons revolving around Jupiter and many more stars which are invisible with naked eye.  Based on this discoveries he developed his philosophical views and therefore questioned some passages in Bible.  Because his conclusions made him one of the followers of Copernicanism, his thoughts were distinguished among the others and were revolutionary for those times.

Main arguments, which Galilei used for the support of copernicanism, were based first of all on the novelty of investigation.  He is certain that “increase of known truths stimulates the investigation, establishment, and growth of the arts; not their diminution or destruction”.   As to me, this argument is very important as it widening the research limits and stimulates the world progress.  This argument could also be found in his work as a theme, penetrating the text in different variations.

All his arguments are presented in a structured way and are logically connected.  Because existed close relation between science and Church, heresy and its denial are the next leading argument in Galileo’s work.  He is focusing on interpretation of some passages from Bible which are differently applied by theologians.  In the Bible it is said: “And the sun stood still, and the moon stayed . . .” (Josh. 10:13) Galileo is convinced that main mistake of heresy and misunderstanding of Copernicus ideas is in applicability of some Bible passages to the needs and purposes of the Church.  Understandably, Copernicus teachings (as well as Galileo’s) were questioning the all-powerfulness, all-knowing, creator of all-things – God.  And it is not surprising, that the point he is discussing the most was about incorrect Bible interpretation.

Galileo states that there are astronomical arguments and physical effects which might be “derived from many things in [his] new celestial discoveries that plainly confute the Ptolemaic system while admirably agreeing with and confirming the contrary hypothesis”.  As Paul Halsall states, those discoveries could not answer not only Ptolemaic system, but Aristotle’s argument as well.  Copernicalism of Galileo did not prove that heliocentricity is true, because in that case there “would be observable parallax shifts in the stars’ positions as the earth moved in its orbit around the sun”.  But when taking into account technology level of that time, no such a shift could be observed.  Hence, Galileo Galilei is omitting this point and bases his argumentation mainly on his observations of Jupiter, moons around Jupiter and phases of Venus.  After those discoveries he comes to the conclusion that stars were fixed in their relative positions to the earth, and only sun, moon and planets were moving in the space.

Galileo is expressing the thought that oppressive doctrine towards the scientists is causing troubles not only to that doctrine and its followers but to all (for instance) mathematics and mathematicians in general.  It could be read through the lines that Galileo is seeing attempt of Church to have new findings under control and have possibility to regulate people’s minds in the way most suited for the Church.  In my understanding, that was Galileo’s stumbling stone when he started proclaiming Copernicus heliocentric theory as the truth, not a scientific theory.  Therefore moving the debate onto theological grounds for Galileo caused him further implications with Church.

According to the letter to Grand Duches, Galileo is trying to support Copernicanism because he has “never tried to discuss the matters of religion or faith nor does he use argument that depend in any way upon the authority of sacred writings which he might have interpreted erroneously”.  In fact, in his works he is using only astronomical and geometrical demonstrations, is basing his arguments on physical conclusions and exact observations.    But this does not hinder to Galileo be acknowledged in theology as well.  For instance, Copernicus himself was not only priest and canon, but he also knew, that doctrine could be proved only in case of coordination with Bible passages.

Galileo is not trying to give bare arguments, but he also is providing with specific passages in Bible that are contradictory themselves.  Because God has delivered the world for the disputes, he is supporting free “philosophizing about mundane and physical things”.  He is also taking the passage from Bible in his support, which is teaching us that the glory and greatness of God is marvelously distinguished in all his works and divinely read in the open book of heaven.

It could be assumed that Galileo Galilei is trying to reveal the tension between religious dogmas and individual reasoning.  As to me, one of his considerable accomplishments is in self-emancipation, which has begun still during the Renaissance period and found its continuation in the Enlightenment Period.  His contribution is tremendous and is valuated only with the flow of time.


1.      Galilei, Galileo. “Letter to the Grand Duchess, Christina (1615)”.  Brooklin College.  5 June 2004.

2.      “Galileo Galilei”.  Reference Library: Encyclopedia.  Campus program. 7 June 2004.

3.      Halsall, Paul “Galileo controversy”. Catholic Answers. 5 June 2004.

4.      Sharratt, Mariane. “Galileo: Decisive Innovator”. Cambridge (1994)


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