Space & Astronomy

Galileo Galilei

1564–1642 · Astronomer & Physicist

An Italian astronomer and physicist whose telescopic observations and experiments transformed astronomy and established a more mathematical approach to science.

“Philosophy is written in this grand book, the universe, which stands continually open to our gaze.”

BORN

February 15th, 1564

NATIONALITY

Italian

FIELD

Astronomer & Physicist

SUBJECT

Space & Astronomy

Why is this scientist famous?

Making pioneering telescopic observations that strongly supported heliocentrism and transformed observational astronomy.

Galileo greatly improved the telescope and used it to make influential observations of the Moon, Jupiter, Venus, the Sun and the stars. His discovery of moons orbiting Jupiter showed that not everything revolves around Earth, while the full set of phases of Venus contradicted the traditional Ptolemaic model. His observations provided powerful support for a Sun-centred understanding of the Solar System.

Galileo's story

Early Life

Born in Pisa, Italy, Galileo was tutored privately by a monk before entering the University of Pisa to study medicine. He never completed his degree, but his talent in mathematics was so obvious that he was soon appointed as a mathematics professor. He was argumentative, witty, and brilliant.

The Telescope

In 1609, Galileo learned of the recently invented Dutch telescope and quickly built improved versions of his own. He was not the first person to use a telescope for astronomy, but his systematic observations and rapid publication made telescopic astronomy enormously influential. He observed an uneven lunar surface, four moons orbiting Jupiter, vast numbers of previously invisible stars, the phases of Venus and sunspots.

Champion of Copernicus

Galileo argued strongly for the Copernican system in which Earth moves around the Sun. His observations undermined important parts of the traditional Earth-centred Ptolemaic system, although they did not by themselves prove the Copernican model over every competing model. In 1616 Church authorities instructed him not to hold or defend heliocentrism as physical fact. After publication of his Dialogue Concerning the Two Chief World Systems, he was tried by the Roman Inquisition in 1633, found 'vehemently suspect of heresy', required to abjure his position and sentenced to house arrest.

Legacy

Even under house arrest, Galileo continued his work, writing a book summarising his physics discoveries that was smuggled out and published in Holland. His insistence on observation and experiment over authority earned him the title 'father of modern science.' In 1992, Pope John Paul II publicly discussed the Galileo case after a Vatican commission had re-examined it and acknowledged errors made by Church authorities involved in the dispute.

What did they discover?

Jupiter's Moons

Discovered four moons orbiting Jupiter — Io, Europa, Ganymede and Callisto — demonstrating that celestial bodies could orbit something other than Earth.

Phases of Venus

Observed the full range of phases of Venus, which contradicted the traditional Ptolemaic model and supported models in which Venus orbits the Sun.

Lunar Mountains

Used telescopic observations of light and shadow to argue that the Moon has mountains and an uneven surface, challenging the traditional idea of perfectly smooth heavenly bodies.

Laws of Falling Bodies

Used experiments and mathematical reasoning to show that, when air resistance is negligible, falling bodies accelerate in the same way regardless of their mass.

Amazing facts

He was born in the same year as Shakespeare: 1564.

He did not invent the telescope and was not the first person to aim one at the sky, but his improved instruments and published observations transformed astronomy.

He was placed under house arrest by the Church for the last nine years of his life.

Galileo became completely blind during the final years of his life, probably because of eye disease rather than his astronomical observations.

His middle finger is preserved in a glass egg at the Museo Galileo in Florence, pointing skyward.

In 1992, Pope John Paul II marked the conclusion of a long Vatican re-examination of the Galileo case and acknowledged errors by Church authorities involved in it.