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Second Iteration
Nicholas Copernicus (1473-1543)
Copernicus was a strong believer in the Greek concept of heavenly motion. Therefore, he was upset with Ptolemy's introduction of the equant and eccentric and he set out to revise the description of the universe and bring it back into line with Aristotle's concept of natural motion. He did this by updating Aristarchus' heliocentric model. He started work on this model in about 1513 but did not finish it until just before his death in 1543.
He placed the 5 known planets and the earth orbiting the sun in perfect circles at a constant speed. They were ordered based on the time it takes each planet to complete a trip through the ecliptic (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). The moon orbits the earth instead of the sun and the stars are in a thick, stationary shell out past Saturn. The daily motion of all the objects is due to the rotation of the earth on its axis and no longer to the daily rotation of the stars dragging the rest of the shells along for the ride.
Such a revolutionary concept was bound to generate strong disagreement. The idea of a rotating earth was especially troubling. All the same objections that were used to refute Aristarchus' earlier work were still used. Namely, there is no sensation of earthly motion and there is no observed parallax. Copernicus only answered the last objection. He believed, correctly, that parallax did exist but that the stars were so far away that it was too small to be measured. He wasn't proved correct until more than 300 years after his death when Bessel measured parallax for the first time.
Copernicus escaped the Inquisition because a friend of his had slipped in a preface to his book stating that this model did not represent physical reality and it was only intended as a convenient device to calculate the positions of the planets. Legend has it that he died the same day the book was released. As people began to accept the model, and ignore the preface, the Church did finally ban his book in 1616. It was removed from the list in the 1820s.
His model was not more accurate than Ptolemy's as is often taught. The main reason for its adoption among scientists of the day was its elegant simplicity. Here was a model that made predictions just as well as the old one but relied on a lot fewer circles and geometric gimmicks. Mathematics had advanced significantly in the time since Aristarchus so Copernicus was able to use his model to calculate the relative spacings of the planets. This enrichment page will show you how he did this.
The movie shown below plots out the motion of Mars and the earth in the heliocentric model. This particular animation shows the eccentricity of Mars' orbit, something that was not suggested until several years later by Kepler.
Galileo Galilei (1564 - 1642)
Galileo helped out Copernicus in many ways. He introduced the concept of inertia to physics. Inertia is the tendency of an object to resist changes in its state of motion. When an object is thrown upward from the earth, it returns to the initial launch point. The ancients said this shows that the earth is stationary. Galileo says that the object's inertia causes it to travel the same distance around the sun as the earth so therefore it returns to the same place. Galileo had noticed that when an object falls it speeds up. He attributed this to a force he called gravity exerted by the earth on every body. Gravity also keeps us attached to the earth as it rotates on its axis. Galileo also discovered that Venus exhibits a phase cycle similar to the moon's. There is one difference though and that is that Venus cannot be seen at its new and full phases because the sun is in the way. Galileo reasoned that therefore both the earth and Venus must orbit the sun. Galileo also discovered that Jupiter is accompanied by four moons. These were later given the names Io, Europa, Ganymede, and Callisto. They show that the earth is obviously not the center of all rotation in the universe. He also discovered that there are many stars that cannot be resolved with the naked eye, suggesting that the universe was infinite. These last few discoveries he made with the help of a telescope. He also used his telescope to study the surface of the sun and discover sunspots; a mistake he later paid for by losing his sight. He did not invent the telescope as is often taught but he does appear to be the first person to use it for astronomy.
He published his discoveries and support for the Copernican model in two books published in 1616 and 1632. He was unusual for the time because he wrote in Italian rather than Latin like most scholars. He also took great pains to make his books interesting often writing them in the form of dialogues rather than dry, boring dissertations. After his first book, "Starry Messenger", was published he was warned by the Church not to publicly support Copernicism again. He abided by this edict until 1632 when he published "A Dialogue on the Two Chief World Systems". This book's outright support for the Copernican model and its ridiculing of the Ptolemaic model earned Galileo a trial before the Inquisition. He was accused of heresy and sentenced to house arrest for life. This "conviction" was overturned nearly 360 years later by John Paul II. However, he got off easily compared to fellow Italian Giordano Bruno who was burned at the stake in 1600 for teaching Copernican ideas.
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