In , a Polish priest named Nicolaus Copernicus proposed that the Earth was a planet like Venus or Saturn, and that all planets circled the Sun. The theory gathered few followers, and for a time, some of those who did give credence to the idea faced charges of heresy. But the evidence for a heliocentric solar system gradually mounted. When Galileo pointed his telescope into the night sky in , he saw for the first time in human history that moons orbited Jupiter.
If Aristotle were right about all things orbiting Earth, then these moons could not exist. Galileo also observed the phases of Venus, which proved that the planet orbits the Sun. At about the same time, German mathematician Johannes Kepler was publishing a series of laws that describe the orbits of the planets around the Sun. In , Isaac Newton put the final nail in the coffin for the Aristotelian, geocentric view of the Universe.
While Copernicus rightly observed that the planets revolve around the Sun, it was Kepler who correctly defined their orbits. At the age of 27, Kepler became the assistant of a wealthy astronomer, Tycho Brahe, who asked him to define the orbit of Mars. Brahe, who had his own Earth-centered model of the Universe, withheld the bulk of his observations from Kepler at least in part because he did not want Kepler to use them to prove Copernican theory correct.
Using these observations, Kepler found that the orbits of the planets followed three laws. Eventually, however, Kepler noticed that an imaginary line drawn from a planet to the Sun swept out an equal area of space in equal times, regardless of where the planet was in its orbit.
For all these triangles to have the same area, the planet must move more quickly when it is near the Sun, but more slowly when it is farthest from the Sun. It was this law that inspired Newton, who came up with three laws of his own to explain why the planets move as they do.
By unifying all motion, Newton shifted the scientific perspective to a search for large, unifying patterns in nature. Law I. Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed theron. The law is regularly summed up in one word: inertia. Law II.
The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. The strength of the force F is defined by how much it changes the motion acceleration, a of an object with some mass m.
Law III. To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. Within the pages of Principia, Newton also presented his law of universal gravitation as a case study of his laws of motion. All matter exerts a force, which he called gravity, that pulls all other matter towards its center.
The strength of the force depends on the mass of the object: the Sun has more gravity than Earth, which in turn has more gravity than an apple. Galileo was the first to observe the craters in the mountains of the Moon. He saw the compound nature of Saturn, what scientists now realize are rings. He saw the moons of Jupiter. He saw the phases of Venus.
Not only did Galileo describe the appearance of mountains on the Moon, but he also measured them. It is characteristic of Galileo as a scientist of the modern school that as soon as he found any kind of phenomenon, he wanted to measure it.
It is all very well to be told that the telescope discloses that there are mountains on the Moon, just as there are mountains on Earth. But how much more extraordinary it is, and how much more convincing to be told that there are mountains on the Moon and that they are exactly four miles high.
Galileo had earlier promised church officials that he would not advocate the Copernican system, at least not publicly. But in a book, which was published in Italian, he supposedly presents an even-handed account of both the Earth-centered and the Sun-centered view.
The book adopts the literary form of a debate, a conversation among observers of Copernican and Ptolemaic leanings, with an educated layperson asking questions of these people. Galileo puts the arguments of the Ptolemaic viewpoint in the mouth of a narrow-minded Aristotelian and someone who bears resemblance to Pope Urban VIII, who was the Pope at the time. So, Galileo was convicted of heresy. He was forced to recant the Copernican view, and he was placed under house arrest for the rest of his life until his death in The scientific instruments developed by Tycho Brahe, and his accurate observations of the movement of planets refined the Copernican model.
The work of Johannes Kepler further developed this. People remember Galileo for his pioneering use of the telescope. Henceforth, humanity's understanding of the universe and our place in it would be forever changed.
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An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, A comparison of the geocentric and heliocentric models of the universe. Credit: history. Credit: Wikipedia Commons. Source: Universe Today. Citation : What is the heliocentric model of the universe? This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission.
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