That Dialogue on Opposing World Systems

Galileo, Copernicus
Galileo and Copernicus    (Gilgub/Flickr)

The title “Dialogue Concerning the Two Chief World Systems” certainly sounds like a heavy topic. It was heavy in 1632 when Galileo published it. The two systems were the Ptolemaic and the Copernican theories of cosmology. It is less controversial and easier to understand today.

Ptolemy, following the tradition of Aristotle, believed that the Earth was the center of the universe, and everything — Sun, Moon, planets, and stars — revolved around it.

Copernicus, on the other hand, posited that the Sun is the center of the universe, and though we seem to be standing still, we are in fact hurtling through space as we circle the star.

I used to have a quotation in my middle school classroom for my students that said “You are not the center of the universe” – Copernicus. Nicholas didn’t say exactly that quote, and he wasn’t specifically referencing my young teen students, but it was a good point-of-departure quote for discussion.

Galileo had spoken with Pope Urban VIII earlier and discussed his tide theory as proof that the Earth moved through space – not that the Sun was the center of the universe. The Pope granted him permission to write “Dialogue on the Tides” but that the Copernican theory should be treated as hypothetical in the book. Wisely, Galileo wrote the book as a series of discussions between two philosophers. One believed in Copernicus, one believed in Ptolemy, and a neutral but well-educated layman served as a moderator. That got it past the Catholic censors.

But Galileo was Copernican all the way and the popular book did not please Pope Urban VIII who had Galileo tried by the Inquisition. They ruled that he was “vehemently suspect of heresy” and too close to endorsing Copernican theory and the book was placed on the Catholic Church’s Index of Forbidden Books.

Galileo was ordered to recant and recite weekly psalms of penitence. He spent the rest of his life under house arrest, and none of his later books were permitted to be published in his lifetime.

The Dialogue on Opposing World Systems remained on the Index of Forbidden Books until 1835. Change is slow in religion – but not in science.

Further Reading

The Essential Galileo

A Moveable Feast

robin eggs
Spring robin eggs

As with many holy days, “Easter” comes from pagan traditions. Anglo Saxons worshipped Eostre, the goddess of springtime and the return of the sun after the long winter. Eostre, in legend, once saved a bird whose wings had frozen during the winter by turning it into a rabbit. Because the rabbit had once been a bird, it could still lay eggs, and that rabbit evolved into the Easter rabbit/bunny.

Eggs had long been a symbol of fertility. In winter they were scarce, so their return in spring were part of the seasonal celebrations. People exchanged decorated eggs at this time as far back as the 11th century.

Easter Sunday, the Christian celebration of Jesus’ resurrection from the dead three days after his crucifixion, is a moveable feast. The date is based on the cycles of the moon. The New Testament says that Jesus was resurrected on the first Sunday after the first Full Moon of spring which places it as early as March 22nd and as late as April 25th.

The almanac also reminds me that today is the day in 1633 that Galileo Galilei was put on trial by the Catholic Church during the Inquisition. He supported the theory that the Earth revolves around the sun and not that Earth was the center of the universe. Galileo took a plea bargain and plead guilty to avoid imprisonment or execution. He was sentenced to an unlimited period of house arrest in his home in Florence. It only took 359 years (1992) for the Catholic Church to formally admit that Galileo’s views on the solar system are correct.

A Father and Daughter and Science and Religion


I think it is pretty safe to assume that everyone has heard of Galileo Galilei. Not as well known is his eldest daughter.

Galileo was born on February 15, 1564, in Pisa, Italy. A mathematics professor, he made observations with implications for the future study of physics. He constructed a telescope (did not invent it) and made significant improvements to it. He supported the Copernican theory of a sun-centered solar system and so was accused twice of heresy by the Catholic church.

He had three children, but was closest to his eldest, Virginia. He saw her as much like himself in intellect, sensibility, and with an always-seeking spirit.

Virginia was born of his illicit affair with Marina Gamba of Venice. Her birth was in the summer of a new century – August 13, 1600.

That year was also when a Dominican friar, Giordano Bruno, who believed the Earth traveled around the Sun, was burned at the stake.

On her thirteenth birthday, Virginia entered a convent and remained there for the rest of her short life. She was devout but loved her father and remained in constant correspondence with him.

I learned about her when I read back in 1999 Galileo’s Daughter by Dava Sobel which used whatever surviving letters (never published in translation) between them as a major source. It is a good tale of that divide and connections between science and spiritual belief that still exists.

Virginia became Maria Celeste as a nun. We can surmise that Celeste might be a celestial nod to her father. In the convent, she was the apothecary – a kind of science of that time. She sent her father herbal treatments. She asked her father for financial help for the convent. She may have helped him prepare some manuscripts.

It is not really clear how father and daughter reconciled his heresy and her devotion. But they did. Love conquers all?

Galileo was not an atheist. He remained a Catholic and believed in the power of prayer.

Unfortunately, though letters from Maria Celeste were discovered among Galileo’s papers, his responses to her have been lost. Maria Celeste’s letters are published as Letters to fathertranslated and edited by Dava Sobel.

We remember Galileo mostly for the telescope, which he found out about in 1609. It was a Dutch gadget and initially known as a spyglass or eyeglass. It was curiosity that made faraway objects appear closer and they were being sold in Paris. Galileo saw it as a device of use in the military and promoted it as that to the Italian government.

He improved the design, as others were also doing in other countries, grinding and polishing lenses himself. The Venetian senate was so amazed and obsessed with using it to look for distant ships from bell towers of the city, they renewed his contract at the University of Padua for life, and Professor Galilei’s salary jumped to 1000 florins per year (a 500% raise from his starting pay).

That telescope cause a huge shift in the way we perceive the world we live in and the universe beyond or world.

Galileo used his improved telescope to make detailed drawings of the Moon’s phases, and he discovered 4 of Jupiter’s 67 moons (Io, Europa, Ganymede, and Callisto), though he considered them to be planets. In a 1610 letter, Galileo commented on them and said “I render infinite thanks to God for being so kind as to make me alone the first observer of marvels kept hidden in obscurity for all previous centuries.”

He reminds me of Charles Darwin in that both had a hard time with their discoveries knowing that this new knowledge would clash with existing religious beliefs. Galileo wrote a famous letter about science and religion and that conflict obviously concerned him – and his daughter, and probably some of you reading this today.

Galileo at age 42.
Galileo at age 42, when Virginia was 6, in a portrait by Domenico Robusti.