New Mexico Museum of Space History
International Space Hall of Fame

Galileo Galilei
Inducted in 1991

Photograph of Galileo Galilei

The Father of modern science; expanded man's understanding of the Universe.

Galileo Galilei pioneered the experimental scientific method, and was the first to use a refracting telescope to make important astronomical discoveries. He is often referred to as the "father of modern astronomy" and the "father of modern physics". Albert Einstein called Galileo the "father of modern science."

Galileo Galilei was born on February 15, 1564 in Pisa, Italy but lived in Florence, Italy for most of his childhood. His father was Vincenzo Galilei, an accomplished Florentine mathematician and musician. Galileo studied medicine at the University of Pisa from 1581 to 1585 but left without a degree, returning to Florence to tutor mathematics. He went back to the University of Pisa in 1589 then moved to the University of Padua in 1592 to teach geometry, mechanics, and astronomy.

At the beginning of his career, Galileo taught the accepted astronomical theory of the time; that the Sun, stars, and all the planets revolved around the Earth. This was the geocentric model of the Universe, revered as dogma since Ptolemy and Aristotle more than a thousand years before. While at the University of Padua Galileo learned of Nicolaus Copernicus' theory (published in 1543) that the Earth and all the other planets revolved around the Sun. Soon Galileo's observations with the newly invented telescope convinced him of the truth of Copernicus' sun-centered, or heliocentric theory.

On January 7, 1610 Galileo used a refracting telescope of his own design to discover three of Jupiter's four largest moons and the fourth large moon four nights later. The telescope showed the moons appearing and disappearing periodically, due to their movement behind Jupiter, which he correctly deduced as proof they were orbiting the planet. The existance of a another planet with smaller bodies orbiting it flatly contradicted the geocentric model of the universe, in which the Earth was the center of creation and all other planets orbited it.

In 1610, Cosimo de' Medici, Grand Duke of Tuscany, named Galileo as his personal mathematician. This position brought Galileo back to Florence, and enabled him to devote more time to astronomy. With his telescope Galileo became the first European to document sunspots, which refuted Aristotle's belief that the sun was a perfect sphere without mark or blemish. Galileo had already shown this was not true for the Moon in 1609 when he used his telescope to discover lunar mountains and craters. Galileo concluded that the Moon was "rough and uneven, and just like the surface of the Earth itself," and not the smooth sphere Aristotle envisioned.

Galileo also turned his telescopes towards the planet Venus, and saw it had a set of phases similar to that of the Moon. This was in line with the heliocentric model of the solar system since all phases of Venus should be visible if it orbited the Sun from a closer distance than the Earth. Galileo was also the first to show the Milky Way was not a nebulous mass but rather millions of stars packed so densely that they appeared to be clouds. He also carried out revolutionary experiments in motion and mechanics.

By 1616, Galileo's support for the heliocentric theory had drawn the ire of the Roman Catholic Church. The Inquisition had burned astronomer Giordano Bruno at the stake in 1600 for similar heresies. Protected by the the powerful Medici family, Galileo defended heliocentrism, and claimed it was not contrary to Scriptural passages. In 1632, he published his first scientific masterpiece, the Dialogue Concerning the Two Chief World Systems. In this work, he compared the Ptolemaic-Aristotelian theory to the Copernican model, showing that the Copernican system was logically superior.

Galileo was soon summoned to Rome, accused of willfully disobeying the Papal order against propagation of Copernicus' theory. In 1633, the Inquisition found Galileo guilty of the charge, and forced him to publicly withdraw his belief in the heliocentric theory, and sentenced him to life imprisonment. Due to his advanced age he was permitted to serve his term under house arrest at his villa outside Florence. Galileo died, still under house arrest, on January 8, 1642.

A devout Catholic, Galileo did not intend to challenge the authority of the Church with his discoveries as he felt he was only revealing God's work by showing the true nature of the Universe. Galileo believed "the language of God is mathematics," and the Universe was best understood through mathematics. Galileo was "rehabilitated" by the Catholic Church in 1741, and on October 31, 1992 Pope John Paul II formally apologized for the injustice done to Galileo.

A lunar crater and another on Mars are named for Galileo Galilei, as is asteroid 697 Galilea. NASA's Galileo probe to Jupiter (launched in 1989) also commemorates his memory, as does the appelation "Galilean moons" to the four Jovian satellites he discovered.

Galileo Galilei quotes:

"All truths are easy to understand once they are discovered; the point is to discover them."

"I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use."

"I have never met a man so ignorant that I couldn't learn something from him."

"The sun, with all those planets revolving around it and dependent on it, can still ripen a bunch of grapes as if it had nothing else in the universe to do."

"You cannot teach a man anything; you can only help him discover it in himself."

"In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual."