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Nicolaus Copernicus proposed a heleocentric model of the Solar System, in which the planets, including Earth, orbit the stationary Sun. He also created the concept of orbit sizes based on the size and distance of the object from its star.
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One of the last astronomers to work without telescopes, Brahe observed planetary motion, which allowed later astronomers to create new models of the solar system. He also made observations of comets, and believed that they were further away from the Earth than the moon.
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Galileo made major improvements to the telescope. With the telescope, he confirmed the phases of Venus, and strongly backed the idea of heleocentrism.
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Kepler created the laws of planetary motion, which built the foundation for Isaac Newton's theory of universal gravetation. He also believed that planets with larger orbits have a slower speed.
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Gioanni Cassini was an astronomer who, among other things, discovered markings on the surface of Mars and determined the rotational periods of Mars and Jupiter.
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Isaac Newton formed the laws of motion and universal gravitation, which helped to explain the movement of celestial objects. His discoveries were major factors in validating the previously theorized heliocentric model of the solar system.
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Halley focused a lot on lunar observation and discovered a comet, named Halley's Comet, which he accurately predicted would return 53 years after he observed it.
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Charles Messier published an astronomical catalogue consisting of nebulae and star clusters that came to be known as the 110 "Messier objects". It help astronomical observers distinguish between permanent and transient visually diffuse objects in the sky.
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Herschel discovered the planet Uranus as well as infrared radiation. He also discovered two moons of Uranus as well as two moons of Saturn, and composed music.
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Annie Jump Cannon created the Harvard Classification Scheme and the development of contemporary stellar classification.
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Albert Einstein developed the general theory of relativity and is best known for his mass–energy equivalence formula E = mc2. He also investigated the thermal properties of light which laid the foundation of the photon theory of light and developed the special theory of relativity.
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Established the field of extragalactic astronomy and showed that the recessional velocity of a galaxy increases with its distance from the earth, implying the universe is expanding.
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Frank Drake is most notable as one of the pioneers in the search for extraterrestrial intelligence, including the founding of SETI. Developed the Drake equation, and is the creator of the Arecibo Message.
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Carl Sagan is best known for his contributions to the scientific research of extraterrestrial life, including experimental demonstration of the production of amino acids from basic chemicals by radiation. He also discovered the high surface temperatures of Venus.
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Stephen Hawking was the first to set forth a cosmology explained by a union of the general theory of relativity and quantum mechanics. He also came up with the framework of general relativity, and the theoretical prediction that black holes emit radiation, often called Hawking radiation.
