Galileo Galilei's Solar Planet Model

Galileo Galilei (1564 – 1642) made such significant contributions to human understanding of the cosmos and Earth's place in it that he often receives credit for heliocentrism, the view that the Earth revolves around the sun and not the other way around.

What Galileo actually did was provide observational support for a theory that had been put forward by Polish astronomer Nicolaus Copernicus (1473 – 1543), who died twenty years before Galileo was born.

Copernicus completed his treatise just before he died, and it was banned by the Catholic Church, but nevertheless, it spawned a movement that eventually resulted in the adoption of the heliocentric model. The movement became known as the Copernican revolution, and it lasted for about 100 years.

Galileo's main contributions to the revolution were observational data, which he obtained with a telescope he built himself. He was the first astronomer to scan the heavens with a light-magnifying instrument and is sometimes referred to as the father of observational astronomy. He published his observations, and they were so significant that the Catholic Church tried him as a heretic and confined him to house arrest for the rest of his life.

To put Galileo's achievements in perspective, it helps to understand the political and social climate that prevailed during his life. The Church was a powerful conservative institution, and its influence was felt throughout Europe. It had subscribed to the view that the Earth was the center of the universe since its foundation, and it didn't want to change. Anyone who challenged the view was subject to torture and execution.

Evidence exists that a Greek astronomer, Aristarchus of Samos (c. 310 BCE – c. 230 BCE), believed the Earth rotates around the sun. None of his writings have survived, but he is mentioned by Greek philosophers Archimedes, Plutarch and Sextus Empiricus. His view, like that of Democritus, who believed in atoms, was at odds with Aristotle and Plato, whose philosophies dominated Western thought throughout the first 1,500 years of the Christian era.

The Aristotelian view was that the Earth is at the center of the universe, and it was surrounded by a series of concentric spheres, each corresponding to one of the planets. Christian thinkers liked this view, perhaps because it supported creation stories in the Bible, but it didn't do a very good job of explaining the motions of the planets, especially retrograde motions, when the planets appear to reverse their direction of movement.

Along came Persian astronomer Ptolemy (c. 100 CE – c. 170 CE) to propose that each planet revolved in a large circle around the Earth as well around a smaller one with its center on the large circle. He called the large circle the deferent and the smaller one the epicycle. In addition, the center of the deferent could be offset from the Earth by an amount known as the equant.

Combining these into a complicated scheme that became the Ptolemaic system, the positions of the planets could be reasonably well predicted, and astronomers used this model until Copernicus came along.

Like all scientists and philosophers, Copernicus sought the simplest answers to why the universe is the way it is, and the Ptolemaic system was anything but simple. He realized one small change in perspective was all that was needed to fix that – at least most of it.

With an acknowledgment to Aristarchus of Samos (which he later deleted), Copernicus published his treatise De Revolutionibus Orbium Coelestium (On the Revolutions of the Celestial Spheres) in 1543, the year of his death.

In the Copernican model, the sun is at the center of the universe, not the Earth. That largely eliminated the need for epicycles and equants, but not completely, because Copernicus believed that planetary orbits are circular. The truth is that they are elliptical, but that wouldn't be known until Johannes Kepler figured it out in 1605.

Because he died soon after his treatise was published, Copernicus didn't have to face any backlash from the Church. It's probable he planned it that way. His book was indeed banned by the Church in 1616, and it remained on the banned list until 1835. Giordano Bruno, an Italian astronomer and mathematician who adhered to the Copernican view, was not so lucky: He was burned at the stake in 1600 for refusing to recant his Copernican philosophies.

Galileo was outspoken, flamboyant and creative, and he is credited with many achievements, including the confirmation of the Copernican theory.

Upon hearing of the invention of the telescope by the Dutch in 1608, Galileo built his own, which was capable of 30× magnification. He used it to study Jupiter, which no one had ever seen close-up before, and noticed four stars surrounding it. He realized they were moons, and in 1610 published a short treatise entitled Siderius Nuncius (The Starry Messenger), which contradicted the Aristotelian worldview and made him a celebrity.

In the document, he called the moons the "Medician Stars" to gain favor with the grand duke of Tuscany, Cosimo II de Medici. Cosimo II was not above flattery, and he granted Galileo the powerful post of mathematician and philosopher to the Medicis, which gave him a platform from which to espouse his theories.

Galileo made three other observations that were important corroborations of Copernican theory, and he used his post to publicize them. The first was that the moon had mountains, and the second was that the sun had dark areas called sunspots, both of these contradicted Aristotle, who taught that the planets are perfect and flawless.

The third observation provided perhaps the most important of all for Galileo's support of the heliocentric theory: he was able to observe that Venus had phases, like the moon. This could only be explained if the planets orbit the sun, not the Earth.

When the Church banned Copernicus' book in 1616, it summoned Galileo to Rome and forbade him from teaching heliocentric theory. He agreed, but in 1632, he published another book in which he compared the geocentric and heliocentric theories. He claimed to be neutral, but no one was fooled.

The Church summoned him back to Rome and demanded that he recant under penalty of torture. Galileo was 70 at the time, and he knew what had happened to Bruno, so he agreed a second time. The Church sentenced him to house arrest for the rest of his life.

After having constructed his "spyglass," which was how telescopes were known at the time, Galileo made his important observational discoveries. All of these observations, taken together, were proof for him that the sun was at the center of the universe. We now know it's actually at the center of the solar system, but that phrase hadn't been coined yet.

While observing sunspots, which he didn't realize was a dangerous thing to do, he noticed that they moved across the face of the sun, and that inspired a revolutionary idea. The sun rotates on its axis. The fact that the Earth has an axial rotation was part of Copernican theory, but the discovery that the sun also rotates was new.

His observations of the phases of Venus was proof that Venus orbits the sun, but this wasn't exactly news to scientists of the time. Although they had never observed the phases, they already suspected as much, and simply assumed that both Venus and Mercury orbited the sun while the sun orbits the Earth. Taken with his other observations, though, observation of the Venus' phases was fairly conclusive support for the idea that all planets orbit the sun, not just Venus.

Galileo is known for a host of other scientific breakthroughs. He devised an experiment to measure the speed of light. Most people at the time believed that the speed of light was infinite, but not Galileo, who believed that although light travels very fast, its speed is finite and measurable. He devised an experiment, but never tried it (and it probably wouldn't have worked).

Although he didn't invent the telescope, Galileo did invent a number of measuring devices that are used to this day, including the compass and a type of thermometer that measures temperature by the heights of suspended containers of ethanol in a large vertical tube filled with water.

Galileo was the first to recognize that falling bodies are all subject to the same force of acceleration and, in the absence of air drag, they fall at the same rate. He was the first to realize that the trajectory of a cannonball had vertical and horizontal components that could be depicted on a graph and analyzed separately.

Galileo's flamboyance my be one reason why he receives so much credit for the heliocentric theory. Despite that, he was an ardent Catholic for his entire life. Here are some other facts about Galileo:

Was Galileo a priest? The answer is yes and no. When he was young, he went to study medicine at a Jesuit monastery, where he took his priestly vows. Not long after that, however, he decided his true calling was to be a monk, not a priest. He was defrocked, and his father withdrew him from the monastery.

Was Galileo married? Galileo had a common-law wife, and together they had three children, but because he never married his wife (perhaps because he still took his priestly vows seriously), his children were illegitimate. He could not provide his daughters with a dowry, so they had to live in convents for their entire lives.

Galileo had a "Me Too" moment. Perhaps a little too flamboyant and creative, Galileo was accused of being inappropriate with his students, and his professorship at the University of Pisa was terminated. Nevertheless, he still has fans, including Albert Einstein, who called Galileo the father of modern physics and of modern science in general.

The "Leaning Tower" experiment is a myth. One of the most famous stories of Galileo has him dropping two balls from the Tower of Pisa to confirm his theory of gravity. Even though Galileo was born in Pisa and taught there, evidence that this actually happened is scanty. It was more likely a thought experiment.

Was Galileo vindicated? Although he died under house arrest, Galileo has definitely been vindicated by history,. When NASA sent a probe to explore Jupiter in 1989, it was named Galileo. Interestingly, three years after that, the Vatican absolved Galileo.