Galileo Galilei (1564-1642) needs no formal introduction, being one of the most famous astronomers and scientists in history. After four years, Galileo had announced to his father that he wanted to be a monk. This was not exactly what father had in mind, so Galileo was hastily withdrawn from the monastery. In 1581, at the age of 17, he entered the University of Pisa to study medicine, as his father wished. At age 20, Galileo noticed a lamp swinging overhead while he was in a cathedral. Curious to find out how long it took the lamp to swing back and forth, he used his pulse to time large and small swings. Galileo discovered something that no one else had ever realized: the period of each swing was exactly the same. The law of the pendulum, which would eventually be used to regulate clocks, made Galileo instantly famous.
One of the instrumental figures in the Scientific Revolution, Galileo’s achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism. Galileo has been called the "father of modern observational astronomy", the "father of modern physics", the "father of science", and "the Father of Modern Science". According to Stephen Hawking, "Galileo, perhaps more than any other single person, was responsible for the birth of modern science".
The Letter to The Grand Duchess Christina, written in 1615 by Galileo Galilei, was an essay on the relation between the revelations of the Bible and the new discoveries then being made in science. Christina, daughter of Charles III of Lorraine and granddaughter of Catherine de' Medici, was the widow of Ferdinando I de Medici, Grand Duke of Tuscany 1587-1609, who had appointed Galileo to the professorship of mathematics at the University of Pisa in 1588. By 1615, with the controversy over the Earth's motion widespread and increasingly dangerous, Galileo revised this letter and greatly expanded it; this became the Letter to the Grand Duchess Christina.
