The Greeks passed the torch eastward toward the Syrians, the Hindus, and the Arabs. The Arabs made new star catalogs in the 9th and 10th century and developed tables of planetary motion. The Arabs were good observers, however, they made few useful contributions to astronomy. As with many other sciences, the Arabs work (in this case, Ptolemy’s Almagest) filtered into Europe and sparked new interest. Europeans, at first, were content in making tables of planets or short digests on his theory. Later, both the German philosopher Nicholas of Cusa and the great Italian inventor Leonardo da Vinci questioned Ptolemy’s ideas of a motionless earth and its centrality.

Astronomy took a drastic change in the 16th century with the thoughts of Polish astronomer, Nicholaus Copernicus. Educated in Italy and a canon of the Roman Catholic Church, he spent his life studying astronomy. His most famous work, De Revolutionibus orbium caelestium (On the Revolution of Heavenly Bodies) (1543), critically analyzed Ptolemy’s earth-centered theory. He said it was better explained by having the sun at the center, the earth and planets orbiting around it. His motive wasn’t purely scientific; he thought that the sun was the only celestial object worthy of the spot of being in the center.

Little attention was paid to Copernicus’ heliocentric theory, until Galileo decided to turn his recently made telescope toward the heavens. He found the evidence needed in the phases of venus. He also discovered the rings of Saturn and observed moons orbiting Jupiter. Convinced Copernicus was right, Galileo began writing and speaking. He was condemned and tried by the ecclesiastical authorities. The theory would not die, though.

The only difference, though a big one, between the geocentric system and the heliocentric was the center. The constant speed and circular orbits of Ptolemy were still around. Tycho Brahe observed the moon, planets, and the sun from 1580-1597 atop his island observatory in Copenhagen and later in Germany. Based on the data collected, his assistant, Johannes Kepler, formulated his laws of planetary motion. His laws state that planets move not in a circular orbit with constant speed, but elliptical with varying speed and that their relative distances can be determined by observing periods of revolution.

To explain Kepler’s laws, the inventor of Calculus, Sir Isaac Newton, reasoned there must be an attractive force between the planets and the sun. this force depends on the mass of the two objects and the distance between them. Newton’s discover is the universal law of gravitation.