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Movement of Stars and Planets
Ancient Greek astronomers realized that some objects in the sky move in regular patterns in relation to other objects that remain fixed in position. They named the moving objects planets (from the Greek word for "wanderer"). The five planets they could see with the naked eye were Mercury, Venus, Mars, Jupiter, and Saturn. Although most of the ancient Greeks believed the Earth was at the center of the Universe, Aristarchus of Samos (310-230 C.E.) concluded, based on his calculations of relative sizes and distances of the Earth, the Sun, and the Moon, that the universe was actually heliocentric.
Heliocentrism
Aristarchus's theory was revived in the 16th century by Polish mathematician and astronomer Nicolaus Copernicus (1473-1543). He found out that the only way to preserve the Ancient Greek model of planets moving in perfect circles and at uniform speeds was to put the Sun at the center. Some hundred years later, Austrian mathematician Johannes Kepler discovered that planets move in circular, not elliptical, orbits and at varying speeds. Through his intensive use of a newly-invented telescope, the Italian scientist Galileo Galilei confirmed Copernicus's and Kepler's theories and discovered the moons of Jupiter.
Expansion of the Universe
Based on Newton's theory of gravity and Einstein's theory of relativity, American scientist Edwin Hubble (1889-1953) discovered early in the 20th century that the Universe is constantly expanding. While working at the Mount Wilson Observatory in California, he discovered other galaxies beyond our own Milky Way. By observing and measuring light coming from those galaxies, Hubble proved that the Universe is expanding.
Big Bang
Hubble's discovery confirmed the idea formulated by a Belgian priest and physicist Georges Lemaitre, who said that billions of years ago the universe was a single, tiny particle that started expanding after an enormous explosion. In 1965, Arno Penzias and Robert Wilson accidentally found a proof of this explosion--the Big Bang--while building a large radio receiver in New Jersey. They realized that the constant excess noise they were hearing was cosmic microwave background radiation--the remnant heat left over from the Big Bang.
Exoplanets
In their search for planets in other solar systems, astronomers had found it almost impossible to see objects that do not emit light like stars. This situation changed when they started looking at the stars themselves, trying to spot wobbles in their motion. Thanks to the laws of gravity, an orbiting planet exerts a pull on its star, forcing a shift in the light coming from that star. That shift tells astronomers the size and distance of the planet from its star. The quest now is to find a planet similar to Earth, with the right size and distance from its star to make it habitable. Astronomers think they have found one such planet around a star called Gliese 581, located 20 light years away from Earth in the constellation Libra.