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Aristotle (384 - 322 B.C.) concluded that the earth is round because it always casts a curved shadow on the moon when passing between the sun. This belief was largely abandoned in the middle ages.
Aristarchus (312-230 B.C.) was the first to believe in a sun centered also known as heliocentric universe. In this model, every planet orbits around the sun. Aristarchus used geometry to calculate the relative distances from earth to he sun and from Earth to the moon. He was right that the sun is much larger than the earth and farther away than the moon, but his measurements were too small. However, he did learn that the sun was many times more distant than the moon and many times larger than Earth. Though there was evidence to support the heliocentric model, the Earth-centered view, dominated Western thought for nearly 2000 years. 
Geocentric Model, was thought that the moon, sun and the known planets - Mercury,Venus, Mars, and Jupiter orbit earth. Beyond the planets was a transparent, hollow sphere on which the stars traveled daily around the Earth. 
The Heliocentric model was first thought up by Aristarchus. This was a sun centered universe. He used geometry to calculate the size of the earth and later the sun. 
In a 13 volume work published by Ptolemy in 141 AD. Ptolemy presented a model of the universe that was called the Ptolemaic system. It accounted for the movements of the planets. The precision with which his model was able to predict the motion of the planets allowed it to go unchallenged for nearly 13 centuries. 
Nicholaus Copernicus (1473-1543) was the first great astronomer to emerge after the Middle Ages. He was convinced that the Earth is a planet, just like the other five that were known that made daily motions of the heaves, he reasoned, could be better explained by the rotation of the earth. 
Tycho Brahe (1546-1601), He persuaded King Frederick 2 to build an observatory. Be built the angle-measuring device, His observations of mars were far more precise than any made previously.

Johannes Kepler (1571-1630)The path of each planet around the sun is an ellipse with the sun at one focus. Each planet revolves so that an imaginary line connecting it to the sun sweeps over equal areas in equal time intervals. The square of the the length of time it takes a planet to orbit the sun is proportional to the cube of its mean distance from the sun. 
Galileo Galilei (1564-1642) His most important contributions were his descriptions of the behavior of moving objects. He used a telescope. His five discoveries were: The discovery of 4 satellites or moons orbiting jupiter, the discovery that planets were circular discs as opposed to points of light as was previously thought, the discovery that venus had phases like the moon, the discovery that the moons surface was not smooth, the discovery that the sun had sun spots.
Sir issac newton, He formulated the universal law of gravitation, (1642-1727), according to newton every body in the universe attracts every other body with a force that is directly proportional to their masses and inversely proportional to the square of the distance between the centers of mass.