Galileo and his contributions to astronomy

On February 15, 457 years ago, the world witnessed the birth of one of the most influential contributors to astronomy and pioneers in science and physics: a true polymath. Born in 1564, Galileo Galilei, known as the father of astronomy, revolutionized how we understand the system our Earth revolves in and the way we place ourselves in it, thanks to his many controversial claims and observations of the Milky Way and our Solar System.

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Galileo Galilei

His career was launched while studying at the University of Pisa, where he got involved in projects touching on various fields, notably the fine arts, engineering, and sciences. In 1589, he was appointed chair of mathematics at the university and subsequently moved on to teach astronomy and geometry, amongst others, at the University of Padua, where he gave birth to one of his most influential inventions, which would shatter the field of astronomy in the following years: the refracting telescope.

Based on a model developed by Hans Lippershey, Galileo pioneered a telescope with around three times magnification. He then improved to 30, one with which he could observe upright images of the Earth and use them as a means of funding by selling them on markets.

Observing the Moon

His first astonishing astronomical discovery came about when he pointed his telescope for one of the first times to the sky. Although he was not the first to observe the Moon, he was the first to deduce that the “strange spottedness” that Thomas Harriot had previously observed was due to lunar mountains and craters, moving away from the established belief that the Moon was a smooth sphere.

Jupiter’s satellites

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The Galilean Moons (Image Courtesy: ESA)

Galileo’s ambitions escalated as he then pointed his telescope towards Jupiter. This was indeed not in vain, for as he proceeded to observe the planet, he made an astonishing discovery. He observed that it had three “stars,” “invisible by their smallness.” However, after continued observation, he had not been recording “star-like” behavior, for they moved relatively to Jupiter in an orbiting movement. Thus, what he had believed to be stars actually turned out to be three of Jupiter’s four moons.

This was Galileo’s first of many controversial discoveries as it blatantly went against a fundamental principle of Aristotelian cosmology. Indeed, the fact that bodies were orbiting another planet than Earth was contrary to the well-settled belief that “all heavenly bodies should circle Earth.”

A few years later, Astronomer Ole Romer calculated the speed of light using Io, one of the four Galilean moons. Please read this article to know how he did that.

Phases of Venus

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The phases of Venus (Image Courtesy: UCA)

When Galileo turned his telescope to Venus, he pushed his opposition to the Geocentric theory even further and confirmed that planets were orbiting the Sun and not the Earth. In effect, observing Venus had allowed him to observe different sets of lighting phases that were only possible if Venus moved around the Sun, allowing Earth to see its surface under different lightings. Indeed, had Venus moved around Earth, an observer would have always seen it in the same constant light. This allowed Copernican Heliocentrism to become a primordial adversary in the face of the established Ptolemy’s geocentrism.

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Saturn’s Rings

After discovering the four Jovian moons, Galileo pointed his telescope at Saturn in July 1610. He was bewildered at what he saw. To Galileo’s surprise, Saturn wasn’t just one planet, but three! A big middle planet with a small planet or moon on each side, and the three were almost touching. So Galileo drew it like this:

Galileo Rings of Saturn

He described the rings as the planet’s handles. Theories of handles and ovals and triple planets lasted another 40 years until Dutch astronomer Christian Huygens finally discovered the rings of Saturn.


After exploring many of our solar system bodies, Galileo had to turn to the biggest body and the only star in our system. Using the naked eye and his telescope, he discovered sunspots, darker areas on the Sun’s surface, which posed yet another challenge to the rather clerical beliefs in astronomy. Indeed, the universe and Solar System were believed to be heavenly perfect, yet these small areas of imperfection went against this claim. Besides, the changing position of these spots was yet another piece of evidence in favor of heliocentrism.

Broader legacy

As such, other than his reputed heresy with which he was accused at the time, his observation and deductions in astronomy laid the foundations for the tools we use and the celestial bodies we observe today. His rigor and consistent methodology brought about the origins of what we know as “the scientific method,” which today lies at the foundation of all experimental sciences.

Learn Astrophysics at Home:

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