Galileo didn't discover heliocentrism.
1517 Martin Luther publishes the Ninety-Five Theses. The Protestant Reformation begins decades before Galileo’s conflict and reshapes the religious and political environment of Europe.
1543 Nicolaus Copernicus, a canon lawyer and church administrator, publishes De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres).
This work introduces the mathematical heliocentric model. The term “Copernican system” comes from this publication.
Copernicus dies the same year.
1570s Church scholars recognize that the Julian calendar has drifted relative to the equinox. Because Easter depends on the lunar cycle relative to the equinox, the error creates theological and practical problems.
1578–1580 Pope Gregory XIII commissions astronomical work to correct the calendar and builds the Vatican observatory tower, often called the Gregorian Tower or Tower of the Winds, to support observations. The tower still exists today.
1582 The Gregorian calendar is promulgated after roughly a decade of astronomical and mathematical work led largely by Jesuit scholars such as Christopher Clavius.
This reform demonstrates that the Catholic Church was actively funding and conducting astronomical research decades before Galileo.
1609–1610 Galileo uses the telescope for astronomical observation and publishes Sidereus Nuncius.
He observes the moons of Jupiter and the phases of Venus, discoveries that undermine the traditional Ptolemaic geocentric system but do not uniquely prove heliocentrism over competing models such as Tycho Brahe’s system.
1616 The Roman Inquisition rules that heliocentrism may not be taught as physical truth.
Copernicus’s book is not banned outright but is suspended until minor corrections are made.
Galileo is instructed not to defend heliocentrism as fact.
1623 Cardinal Maffeo Barberini, who had previously been sympathetic to Galileo, becomes Pope Urban VIII. Galileo initially believes he has papal support.
1632 Galileo publishes Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems).
The book is written as a Socratic dialogue comparing geocentrism and heliocentrism and clearly favors the Copernican position.
The work cites Copernicus directly and does not claim to invent heliocentrism.
Arguments associated with the Pope appear in the mouth of the character Simplicio, which is perceived in Rome as insulting. (calling the pope "simple" or stupid)
Galileo claims proof based on his theory of tides, which is incorrect.
He rejects Kepler’s already published elliptical orbits and insists on circular orbits.
At this time, heliocentrism was still debated scientifically. Many astronomers preferred the Tychonic system because stellar parallax had not yet been observed.
1633 Galileo is tried by the Roman Inquisition and found “vehemently suspected of heresy.”
He is required to recant and is sentenced to house arrest for the remainder of his life.