Letter to the Grand Duchess Christina

The scientist Galileo Galilei (1564-1642) wrote his “Letter to the Grand Duchess Christina” in 1615. It explains the relationship between two understandings of the universe, the scientific and religious, and argues that they are compatible. Galileo contends that science deals with the world as we observe it, while religion deals with sacred texts. The letter endures today as an example of how to unite worldviews that seem incompatible. It is also an important document in the history of science. Galileo’s observations of the Sun, Moon, and planets were foundational for the development of modern astronomy and led, after his death, to widespread acceptance of the view that the Earth orbits the Sun.

Galileo’s letter focuses on two competing models of the planetary system. The Ptolemaic model, named after the Alexandrian mathematician Ptolemy (born around 100 CE), puts the Earth at the center with other heavenly bodies orbiting around it. The Copernican model, named after the Polish astronomer Nicolaus Copernicus (1473-1543), places the Sun at the center of the planetary system. (It is called the heliocentric model, from the ancient Greek word for the Sun, helios.) At the time of Galileo’s writing, the Catholic Church accepted the Ptolemaic model.