Letter to the Grand Duchess Christina

Copernicanism, also known as heliocentrism, is the theory that the Sun is at the center of the planetary system. It is opposed to the Ptolemaic theory, which says that the Earth is at the center of the planetary system. Copernicus, and later Galileo, argued for heliocentrism based on observation and evidence.

Doctrine is the officially accepted interpretation of a scriptural teaching. More specifically, it is “the process of conceptualizing the primal—often experiential or intuitive—insights of the faith of a religious community in support of rationally understood belief” (Encyclopedia Britannica). When Galileo refers to doctrine, he means this definition.

Exegesis is the interpretation of a text to understand its meaning. Augustine argues that the Bible expresses its ideas in layers of metaphor. When we read it, we can imagine multiple interpretations of any passage, and it’s impossible to know which one is correct. Galileo leans on this idea to explain why his observations and the Copernican system in general need not contradict biblical passages. The Bible speaks in metaphors, not literal truths; if it says something that is clearly contrary to fact, it must be because the Bible is hiding the truth beneath metaphor.