Isaac Newton – England 1643-1727 AD

Newton built on the ideas of those before him explaining many different phenomena in his life time. He explained centrifugal force where anything moving in a circle tries to move away from the centre. A ball on a string will try to fly away but the string stops it.Image of Isaac Newton

He also observed free floating objects would be drawn together. It occurred to him that it was these opposing forces that, when balanced, kept the Moon in orbit around the Earth and similarly kept planets in orbit around the Sun. He called this force gravity.

Newton had found a means by which Copernicus’s system could work.

The evidence for the sun centred system was now so strong that it was impossible to return to Ptolemy’s view but it took another 300 years for the church to admit mistakes had been made in Galileo’s trial.

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Johannes Kepler – Germany 1571-1630 AD

Image of Johannes Kepler Kepler was working at the same time as Galileo and they often wrote to each other. Kepler was able to use information on the position of stars to work out how the planets moved. He found out that the planets’ orbits are not perfect circles but elliptical (egg shaped).As planets move closer to the Sun they speed up, and as they move away they slow down. The more distant a planet is from the Sun the slower it moves. These three things meant that Ptolemy’s system now had several flaws but supported the theories of Copernicus and Galileo.

What was it that kept the planets in their orbits? Kepler thought it had something to do with the Sun.

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Nicolaus Copernicus – Poland 1473- 1543AD

Image of Nicolaus Copernicus

In attempting to clarify Ptolemy’s explanation of the universe Copernicus realised that the Sun was at the centre and that the Earth and the other planets revolved around it. He also realised that the Earth rotated once a day to give us day and night.

As he worked on the fine details his new system became as complicated as Ptolemy’s. Worried about appearing a fool, Copernicus spent most of his life working on his theory before publishing “The Book of Revolutions of the Heavenly Spheres” in the year he died.

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Claudius Ptolemy – Egypt 85-165 AD

Ptolemy revised Aristotle’s model of the universe keeping the stars in their crystal spheres and the Earth at the centre. To account for the apparent backward motion of some planets he proposed that they moved in smaller orbits called epicycles.

Although very complicated, Ptolemy’s system could predict the motion of the planets. This view of the universe was accepted for about 1300 years.

Image of Claudius Ptolemy

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Aristotle – Greece 384-322 BC

Possibly the most influential thinker ever, Aristotle wrote on logic, nature, and physics, the origins of the Earth and universe, on species and much more. He was influential for centuries after the Greek civilization disappeared. At the time of Galileo most people still thought that Aristotle’s view of the universe was right.

He believed that the Earth was at the centre of the universe and the planets and stars were fixed in crystal spheres that moved around it. The word planet is Greek for wandering star.

Aristotle didn’t believe in experiment. He wrote; “what must logically be the case must be true.” With regard to falling objects, he said, that the speed an object falls at is proportional to its weight. So if you have a ball weighing 100g, and one weighing 1kg, the heavier one will fall ten times as fast. If he had experimented he would have realised his mistake.

Image of Aristotle

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Galileo Galilei – Italy 1564-1642

Galileo studied to be a doctor but he was more interested in mathematics and natural philosophy. He taught mathematics in several different colleges and wrote two books, one about the different mass of objects and one about motion.

He taught for many years in colleges, and his reputation as a great thinker began to grow. He heard of a spyglass that had been made by a Dutchman called Lippershey and set out to make his own. He learnt how to make his own lenses and sold the idea of the telescope to the Venetian navy.

Galileo used his improved version of Lipershey’s spyglass to look at the moon and stars. The things he saw proved that Copernicus’ ideas about the universe were true.

He saw that Jupiter had four moons that changed position and sometimes disappeared from view – they were in orbit around the planet. He also observed that the planet Venus changed shape, as the moon does. He knew this could not happen if it was orbiting the Earth; it could only occur if Venus was travelling around the sun. These two facts help prove that the sun was at the centre of the universe and that the Moon and stars were not as perfect as Aristotle imagined.Image of Galileo Galilei

When he turned his telescope to the Moon he could see that there were valleys and mountains, craters and plains just as there were on Earth.

Galileo wrote about what he had seen through the telescope in his book Starry Messenger. This brought him even more fame, dinners were held in his honour at which he would talk about his observations.

He continued to observe the motions of the planets including Saturn and to contribute to the debate on the nature of sunspots. His participation in a controversy about the nature of comets (he was wrong about this) began to attract attention and enemies. He was drawn into the debate about whether Copernicus was right by an ex student of his and the letters he sent in support of Copernicus’ theories were examined by the Inquisition. They found nothing to concern them at this time, chiefly because the Church chose to view Copernicus’ ideas as a mathematical theory for calculating the position of the planets and not a serious physical reality.

However the Pope ordered an enquiry to resolve the matter of Copernicus’ ideas and they condemned his teachings as false. This meant that Galileo was banned from supporting him publicly.

Galileo continued to write and teach, publishing his book The Assayer that outlined his methods of scientific investigation. He was invited to meet and speak with the Pope about his ideas several times and this made him think that that he could write about Copernicus’ theory of the universe without getting into trouble. He was still very careful. He  called the book Dialogue of the Two Chief Systems of the World in which he clearly set out many of his arguments. He thought the book would get around the ban because he had argued for both views in it. It was, however, clear which view the reader was supposed to think was right.

The book was banned by the Catholic Church and Galileo was accused of misinterpreting the Bible and warned to stop teaching and discussing these ideas. A six year illness meant he was unable to travel to Rome to answer the charges against him.

Galileo's drawings of the moon

In 1633 he was eventually put on trial before the Inquisition  who found him guilty of heresy. He escaped the punishment of death by making a public statement saying that everything he had previously said was wrong, but spent the rest of his life a prisoner in his own house.

He continued to experiment and wrote another book, “Discourses and Mathematical Demonstrations Concerning the Two New Sciences” He had to send it out of Italy secretly to allow people to read it.

His experiments with pendulums led him to design the first pendulum clock but he died before he could make it.

In 1992, 350 years after Galileo’s death Pope John Paul II made a speech which admitted mistakes had been made in the trial of Galileo but he didn’t pardon him.

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