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ASTA01 at UTSC – Lecture 9. Chapter 3 The Origin of Modern Astronomy Ancient astronomy The Copernican revolution De Brahe and Kepler Galileo Newton. Galileo Galilei (1564-1642).

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asta01 at utsc lecture 9
ASTA01 at UTSC – Lecture 9
  • Chapter 3The Origin of Modern Astronomy
  • Ancient astronomy
  • The Copernican revolution
  • De Brahe and Kepler
  • Galileo
  • Newton
galileo galilei 1564 1642
Galileo Galilei (1564-1642)
  • Galileo Galilei was born in the Italian city of Pisa, and studied medicine there but later convinced his father that he should study mathematics and natural science (i.e., physics).
  • He discovered that the pendulum swings with a period independent of how far it is deflected, constructed thermometer etc.)
      • Eventually, he became professor of mathematics at the university at Padua, where he remained for 18 years.
      • During this time, Galileo adopted

the Copernican views.

telescopic observations
Telescopic Observations
  • It was the telescope that drove Galileo to publicly defend the heliocentric model.
    • Galileo did not invent the telescope.
      • It was apparently invented around 1608 by lens makers in Holland.
      • Galileo, hearing descriptions in the Fall of 1609, was able to build working telescopes in his workshop.
telescopic observations1
Telescopic Observations

Also, Galileo was not the first person to look at the sky through a telescope.

However, he was the first to observe the sky carefully and apply his observations to the main theoretical problem of the day: the place and nature of Earth among planets

telescopic observations2
Telescopic Observations

What Galileo saw through his telescopes was so amazing that he rushed a small book into print, Sidereus Nuncius (The Starry Messenger).

In the book, he reported two major discoveries about the solar system.

telescopic observations3
Telescopic Observations

First, the Moon was not perfect. (Aristotle’s philosophy held that the Moon was perfect)

It had mountains and valleys on its surface.

Galileo used the shadows to calculate the height of the mountains.

Galileo showed that it was a world like Earth,

This put a division of the world into heaven and earth, two completely different realms, into question.

telescopic observations of jupiter moons
Telescopic Observations of Jupiter & moons
  • Second, Galileo’s

telescope revealed four

new ‘planets’ circling


    • Today, these ‘planets’ are known as the Galilean moons of Jupiter
    • 3 have underground oceans of water, 1 has active volcanoes

By Galileo, Jan 7-24, 1610

siderius nuncius 1610
Siderius Nuncius (1610)

Medicean ‘stars’ (Medicea Sidera)

Were announced as ‘planets’,

In fact they are neither:

Io, Europa,

Ganimedes and Callisto,


the four largest moons of Jupiter

known as its Galilean satellites

telescopic observations4
Telescopic Observations

The moons of Jupiter supported (but not proved!) the Copernican model over the Ptolemaic model.

Critics of Copernicus had said Earth could not move – because the Moon would be left behind.

However, Jupiter moved and kept its satellites. Galileo's discovery suggested that Earth, too, could move and keep its Moon.

telescopic observations5
Telescopic Observations

Also, Aristotle’s philosophy included the belief that all heavenly motion was centred on Earth.

Galileo showed that Jupiter's moons revolve around Jupiter: there could be centres of motion other than Earth!

telescopic observations6
Telescopic Observations

Galileo noticed the P vs. arelation (period vs. distance) : Jupiter's innermost moon had the shortest orbital period and the moons further from Jupiter had proportionally longer periods.

In this way, Jupiter’s moons made up a harmonious system ruled by Jupiter – just as the planets in the Copernican universe were a harmonious system ruled by the Sun.

telescopic observations7
Telescopic Observations

This similarity didn’t constitute proof

Nevertheless, Galileo saw it as an indication that the solar system could be Sun-centred and not Earth-centred.

In the years of further exploration with his telescope, Galileo made additional fundamental discoveries.

telescopic observations8
Telescopic Observations
  • When he observed Venus, he saw that it was going through phases like those of the Moon.
    • In the Ptolemaic model, Venus moves around an epicycle centred on a line between Earth and the Sun.
      • If that were true, it would always be seen as a crescent.
telescopic observations9
Telescopic Observations
  • However, Galileo saw Venus go through a complete set of phases, including full and gibbous. This proved that it did indeed revolve around the Sun:
  • a proof of either Tychonic or Copernican system
telescopic observations10
Telescopic Observations

Sidereus Nuncius (1610) was popular and made Galileo famous.

In 1611, Galileo visited Rome and was treated with great respect.

He had friendly discussions with the powerful Cardinal Barberini, supporter of arts and sciences, a later pope Urban VIII

Church officials and Jesuit priests supported him

Civil authorities as well

The largest opposition was offered by Academia (university scientists)

telescopic observations11
Telescopic Observations

However, as he was outspoken, forceful, and sometimes tactless, he offended many important people who questioned his telescopic discoveries.

Some critics said he was wrong.

Others said he was lying.

Some refused to look through a telescope lest it mislead them (one particular philosopher)

Others looked and claimed to see nothing – hardly surprising given the awkwardness of those first telescopes.

When Galileo visited Rome again in 1616, Cardinal Bellarmine interviewed him privately and ordered him to cease public debate about models of the universe.

Galileo appears to have mostly followed the order.

Galileo’s “1st trial”


The Inquisition (formally named the Congregation of the Holy Office the Index)

banned books relevant to the Copernican hypothesis.

Censorship and control over printing presses was a common phenomenon in the secular and church domains alike, in the 1500- and 1600’s. (In England, only 2 universities and 21 printing offices were approved and licensed by the crown, with a total of 53 presses).

what happened with de revolutionibus
What happened with De revolutionibus

De Revolutionibus itself was only suspended for a few years pending revision of places where strong but unsupported claim were made and/or it went too much into religious perspective.

It was recognized as useful for its predictions of planet positions and allowed into circulation after the modifications.

de revolutionibus
De revolutionibus

Everyone who owned a copy of the book was required to cross out certain statements and add handwritten corrections that stated the Earth’s motion and the central location of the Sun were only theories and not facts.

This is a situation that you will recognize as recurring today in connection with textbooks discussing biological evolution.

Heliocentric theory was indeed a theory not properly proven by observational evidence

Tycho Brahe’s geo-heliocentric system was explaining observed phenomena just as well.

dialogo and trial
Dialogo and Trial

In 1623 Galileo’s friend Cardinal Maffeo Barberini became Pope, taking the name Urban VIII.

Galileo went to Rome in an attempt to have the 1616 order to cease debate lifted. The attempt was unsuccessful

Nevertheless, Galileo began to write a massive defence of Copernicus’s model, completing it in 1629.

After some delay, Galileo’s book was approved by both the local censor in Florence and the head censor of the Vatican in Rome.

It was printed in 1632.


The book was called Dialogo Dei Due Massimi Sistemi (Dialogue Concerning the Two Chief World Systems).

It confronted the ancient astronomy of Aristotle and Ptolemy with the Copernican model.


Galileo wrote the book as a debate among three friends.

Salviati is a swift-tongued defender of Copernicus.

Sagredo is intelligent but largely uninformed.

Simplicio is a dim-witted defender of Ptolemy.


The book was a clear defence of Copernicus.

Some claims were not supported by evidence, although intentionally or not, presented as such, for instance the sunspots or the Earth tides.

dialogo and trial1
Dialogo and Trial

Also, Galileo exposed the pope’s authority to ridicule and provoked the church authorities.

Urban VIII was fond of arguing that, as God was omnipotent, God could construct the universe in any form – while making it appear to humans to have a different. Thus, observations may be misleading.

The pope privately asked Galileo to keep a balanced discussion of two world systems and also to mention his own arguments.

dialogo and trial2
Dialogo and Trial

Galileo placed the pope’s argument in the mouth of Simplicio.Thepope took offense and ordered Galileo to face the Inquisition.

Galileo was interrogated by the Inquisition and threatened with torture [no evidence of that; 4 private meetings were held. 3 out of 10 judges declined to sign the mild sentence].

The Inquisition condemned Galileo, not primarily for heresy but for disobeying the orders given him in 1616. The authorities felt they must react to Galileo. They placed Dialogoon the Index of prohibited works.

dialogo and trial3
Dialogo and Trial

In 1633, at the age of 70, kneeling before the Inquisition, Galileo read a recantation admitting his errors.

Tradition [or a myth] has it that as he rose he whispered, “E pur si muove” (“Still it moves”) – referring to Earth.

Galileo did acknowledge that he made mistakes. Eg., he claimed unjustly that tides or sunspots prove Copernicus was right. In our current science myths these proceedings are painted as a brutal trial with threats of Guantanamo-like torture.

the myth of galileo as science martyr
The myth of Galileo as science martyr

Although he was formally sentenced to life-long home arrest, he was actually confined to certain luxurious homes, including a suite with a view toward Vatican gardens and the villa of the ambassador of Tuscany and his own villa, where Galileo received numerous visitors, and had an active social and scientific life.

His daughter was a nun at a convent; she overtook the second part of the sentence according to which once oer week recantations of 4 psalms were to be made.

the myth of galileo as science martyr1
The myth of Galileo as science martyr

Not being able to write much about theoretical astronomy (to which, unlike Kepler, Galileo did not contribute anyway; Galileo did not even fully believe in Kepler’s laws), the old scientist concentrated on and achieved much later fame by describing the subjects of kinematics and dynamics (about motion and forces in Physics).


Galileo died in 1642, 99 years after the death of Copernicus.

The next year, Isaac Newton was born, a man to actually provide a large number of proofs supporting the Copernican order of the universe.

350 years later, in 1992, Pope John Paul II made a formal statement acknowledging the unjust condemnation of Galileo by the Catholic Church, saying that both sided made mistakes.