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Eratosthenes used the sun to measure the size of the round Earth. This discovery is considered important because most people during his time believed that the world was flat. His measurement of the sun (24,660 miles) was also 211 miles off of the true measurement.
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Claudius Ptolemy set up a solar system model in which the sun, stars, and other planets revolved around Earth. He was able to combine what he saw of the stars' movements with mathematics, especially geometry, to predict the movements of the planets. He worked out that the planets must move in epicycles, smaller circles, and the Earth itself moved along an equant to make his predictions true. Even though Earth being in the center isn't true, it was accepted for hundreds of years.
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Nicolaus Copernicus proposed a model of the solar system that involved the Earth revolving around the sun. He used his own observations of the movements of the planets to back up this idea. This idea was too radical for the scholars of his time to accept, but they eventually changed the way many scientists viewed the solar system. Earlier star watchers also believed in the same idea, but it was Copernicus who brought that idea to the world.
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Fascinated by astronomy but disappointed with the accuracy of tables of planetary motion, Tycho decided to record planetary positions ten times more accurately than the best earlier work. He built vast instruments to set exact sights on the stars and used multiple clocks and timekeepers. He achieved his goal of measuring to one minute of arc. This was a tremendous feat before the invention of the telescope.
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Johannes Kepler, using detailed measurements of the path of planets kept by Tycho Brahe, determined that planets traveled around the sun, not in circles but ellipses. In so doing, he found his three laws of planetary motion:
1) The planets move in elliptical orbits with the sun at a focus.
2) In their orbits around the sun, the planets sweep out equal areas in equal times.
3) The squares of the times to complete one orbit are proportional to the cubes of the average distances from the sun. -
Using the three laws of planetary motion by Johannes Kepler, Sir Isaac Newton was able to set up universal gravitation or gravity. Known today as Newton's laws, he calculated three laws describing the motion of forces between objects.
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In the early 20th century, Albert Einstein suggested that the laws of physics are the same throughout the universe, that the speed of light in a vacuum is constant, and that space and time are linked in an entity known as space-time, which is distorted by gravity. This new way of looking at the universe went beyond current understanding at the time.