How did Galileo realize that the Earth orbits the sun? He realized it when he observed the orbit of Venus.
Galileo was born in 1564 in Pisa, Italy. Galileo is famous for saying that the Earth orbits the sun rather than the sun orbits the Earth, but he was not the first person to come up with this idea. The Earth orbiting the sun is known as heliocentrism. The idea that everything orbits Earth is known as geocentrism. Before there was any real knowledge of astronomy or complex physics, the geocentric model appeared to make perfect sense. To someone standing on Earth, it is a stable, unmoving body. The moon and the sun both appear to orbit around it once a day. The stars also appear to rotate around the Earth once a day. There was no reason to question this model.
Aristotle was the main proponent of the geocentric idea. He believed that there was no evidence for a moving Earth, so it must be stationary. He rationed that if the Earth were moving, there would be strong winds blowing over the surface and if you threw a ball up in the air, it would land behind you because the Earth would have moved.
Geocentrism was the common idea until the 16th century, but that doesn’t mean it was the only idea. A philosopher in Greece in about 350 BC, Philolaus, put forward the notion that the Earth was one of several planets that orbited around a central star. Aristarchus of Samos in about 270 BC measured the size of the sun and calculated it to be six times larger than the Earth. He realized that something smaller (the Earth) would probably orbit something larger (the sun). Between 1031 and 1450, several Islamic mathematicians and astronomers began to question the geocentric model.
In Europe, the notion of a heliocentric solar system was stymied by the scientific and religious thinking of the time. The Bible had the Earth at the center of the universe and any deviation from that could bring all of Roman Catholicism and religion into question. However, as mathematics and astronomy began to develop, someone would arise to question the status quo.
In 1515, Nicolaus Copernicus, a mathematician and astronomer in Poland, made some observations of the heaves that made him question the geocentric view. There were several problems with the geocentric model. The first was that for the stars to be orbiting the Earth, they would have to be travelling insanely fast. There was no knowledge of the speed of light at that time, but the stars would have had to travel faster than light. Also, the planets have what is known as “retrograde” motion. That means that the motion of a planet appears to slow down and then go backwards before speeding up again. This makes sense if the Earth is also orbiting the sun because our orbit is bringing us closer and farther away to the other planet, but it is difficult to explain with a geocentric model. To make the geocentric model work, Claudius Ptolemaeus in the 2nd century AD had come up with a system of equant points, which were points that bodies rotated around. These equant points were not in the center of each orbiting body and could be used to explain the retrograde motion. Copernicus did the math and he worked out that a heliocentric system explained the retrograde movement far more satisfactorily than the equant point system.
Copernicus came up with the idea before 1515, but he was loath to publish because of what he thought it would do to the church, and because, other than the math, he had no actual proof. He finally published in 1543, shortly before he died. After his book was published, there was very little controversy. The criticism actually came years later and it was on two fronts. The first was his math and his lack of evidence. The second was from a theological standpoint. The math argument was that you couldn’t use math to prove physics and astronomy because it was too low brow. The theological argument was basically that the Bible says the Earth is the center of the universe.
So, how did Galileo realize that the Earth orbits the sun? He saw it, is the easy answer. Copernicus didn’t have the benefit of a telescope, which had been invented by a Dutch spectacle maker in 1608, called Hans Lipperhey. Galileo heard about the invention and built his own. He started by observing the moon and then turned his attention to Jupiter. He was able to see four of Jupiter’s moons. At first he thought they were stars, but he soon realized they were moving. When one of them vanished, he realized that it had gone behind Jupiter and that the things he was observing were moons orbiting another planet. This caused a tremendous upset because if the Earth was the center of everything, it shouldn’t be possible to have moons orbiting another heavenly body.
The second thing that convinced him was Venus. He used his telescope to observe Venus and he saw that it had phases, just as the moon does. If Venus and the sun were both orbiting Earth, it would be impossible to see all of the phases from Earth. The only way it could be possible to see them was if Venus and all of the planets were orbiting the sun.
Galileo went on to write his observations down in various books and texts, which brought him into conflict with the Roman Catholic Church. He ended up being forced to recant what he knew was true and died under house arrest. The Pope issued an “apology” four centuries later. But, that is not the topic for today. Galileo realized that the Earth went round the sun and not the other way around when he observed the moons of Jupiter and the phases of Venus. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Galileo_Galilei
https://en.wikipedia.org/wiki/Geocentric_model
https://en.wikipedia.org/wiki/Heliocentrism
https://en.wikipedia.org/wiki/Nicolaus_Copernicus
https://astronomy.com/magazine/ask-astro/2017/01/proof-earth-revolves-around-the-sun
https://phys.org/news/2016-01-heliocentric-universe.html
https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question46.html
https://earthobservatory.nasa.gov/features/OrbitsHistory
https://solarsystem.nasa.gov/news/307/galileos-observations-of-the-moon-jupiter-venus-and-the-sun/