History & Culture
Our Island Universe:
Two Small Pieces of Glass Ushered in a Revolution in Science
A January 1610 announcement fundamentally changed how we view the cosmos.
By Shanil Virani
Image courtesy of the History of Science Collections, University of Oklahoma Libraries
In January 1610, Galileo Galilei noted the positions of four dots near the planet Jupiter.
[Image courtesy of the History of Science Collections, University of Oklahoma Libraries]
Between January 7 and January 13, 1610, a series of observations was made that would forever change how we would view the cosmos. The observer detailed in this log book a discovery made using a relatively new invention at that time. The observer had discovered four small, point sources of light very close to the (giant) planet Jupiter. On January 10, one of them disappeared for a short period. The observer attributed the disappearance of the object as being hidden behind Jupiter.
Given his extensive observations, he was now forced to conclude that these four points of light were orbiting Jupiter and not Earth. The observer, Galileo Galilei, and his two small pieces of glass would usher in a scientific revolution that reverberates to this day.
Until this discovery, and for some 1,500 years prior to Galileo, our ancestors accepted the model from the early Greeks that said Earth was at the center while everything else in the sky — our Moon, the Sun, the planets, and the stars — all orbited around us. We’re at the center of it all according to that long-standing notion that originated with Aristotle and was fortified by Claudius Ptolemy.
The idea of a Sun-centered solar system did not originate with Copernicus even though we often give him credit. In fact, there is now evidence that Aristarchus of Samos some 2,200 years ago proposed a Sun-centered model. He even put the planets in their correct order from the Sun. However, presenting ideas contrary to that of Aristotle’s was unpopular given his giant reputation in intellectual thought.
This is the telescope Galileo Galilei used for his revolutionary observations.
[Image courtesy Museo Galileo, Firenze]
This is the telescope Galileo Galilei used for his revolutionary observations.
[Image courtesy Museo Galileo, Firenze]
Galileo Galilei sketched the cratered surface of our Moon and revealed it was not a perfect spherical body.
[Image courtesy of the History of Science Collections, University of Oklahoma Libraries]
While Aristarchus’s original text has been lost, there are fragments remaining that tell us he advocated for a Sun-centered model. This idea would not resurface until the 16th century when a Polish astronomer, Nicholaus Copernicus, revived the idea of a Sun-centered cosmos to offer a different model to counter issues that had been building against the Earth-centric model, commonly referred to as "Almagest" — Arabic for the "Greatest," which is how Ptolemy’s model had come to be known.
In the same time frame that Galileo was making his observations, Johannes Kepler was forced to also reconsider Aristotelian cosmology. In 1600, Kepler began to work for Tycho Brahe, arguably the world’s foremost observational astronomer. At the time, Brahe had compiled an unprecedentedly accurate data set of planetary positions that Kepler wished to access. Upon Brahe’s death in 1601, he was finally granted complete access. While Copernicus published his Sun-centered model in 1543, it was largely ignored as his model made no better predictions than Ptolemy’s. Why? He still had “perfect circles” for the orbits of planets. Kepler, using Brahe’s exquisite observational data, found “perfect circles” didn’t fit the data, but instead — elliptical orbits did — and Kepler’s first law of planetary motion was born.
Even though Copernicus’s heliocentric model predates Galileo’s discovery by nearly seven decades, it was Galileo and his telescope that provided the evidence in support of this idea. Using the new technological invention of the time — a telescope — Galileo observed the moons of Jupiter, that Venus goes through the same set of phases our Moon goes through, that there are craters on our Moon, and that the Sun has spots on it, all with his 2-inch-diameter telescope. These observations are incompatible with the philosophical Earth-centered model of Ptolemy and Aristotle. And so away went the geocentric universe and the idea of the cosmos as “perfection personified.”
SHANIL VIRANI is an astronomer, educator, and a science communicator with the Astronomical Society of the Pacific. He is the co-author of Daughter of the Stars, a coffee-table astrophotography book about light pollution and what we lose when we lose the night, and also the host of the “Our Island Universe” podcast available via SoundCloud.
However, Galileo's ideas were so contrary to the principles of Aristotelian Cosmology that the Catholic church found him "vehemently suspect of heresy," forced him to recant his discoveries, and to spend the rest of his life under house arrest. It wasn't until 1992 (!!) that the Catholic Church recognized they made a mistake, that Galileo was right, and so they exonerated him.
Nevertheless, the correct idea that the Sun is at the center of our solar system and the beginning of a scientific revolution started 414 years ago, in January of 1610. ✰
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