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Retrograde Motion
Ancient astronomers observed planets that seemed to change their motion eastward at times and begin a westward movement for a couple of weeks. Observers called this "retrograde motion" and tried to develop theories as to why planets seemed to do this. This false notion was due to the belief that Earth was the center of the universe.
("Planetary motion." World of Scientific Discovery. Gale, 2007. Science in Context. Web. 11 Dec. 2013.) -
Jan 1, 1473
Heliocentric Universe
Aristarchus originally suggested the theory that the planets revolved around the sun in the 260 BC. But it was Nicholas Copernicus in the 16th century who carried out investigations which supported this concept , and he ultimately sparked the change from the perception of a geocentric universe. This corrected the concept of retrograde motion, which indeed is just Earth passing by another planet.
(Drake, Stillman. Discoveries and Opinions of Galileo. Garden City, NY: Doubleday, 1957.) -
Elliptical Orbits
Tycho Brahe was a Danish astronomer who collected data concerning the motion of Mars. His assistant Johannes Kepler tried to construct Mars' orbit with the data, but found it impossible to create a circular route. He finally began to work with an oval path, and Brahe's findings corresponded exactly. This lead to the discovery that all planets orbit in a ellipse.
("Planetary Motion." Astronomy & Space: From the Big Bang to the Big Crunch. Gale, 2007. Science in Context. Web. 11 Dec. 2013.) -
Kepler's Three Laws
Kepler's discovery of elliptical orbits became his first law of planetary motion. The second law that he discovered is that an imaginary line connecting the Sun's center and the planet's center will create equal areas and periods of time. His third and final law states that the ratio between the squares of any two planets' year's and the ratio of their cubed average distances away from the sun are equal.
("Kepler's Three Laws." Kepler's Three Laws. The Physics Classroom, 1996. Web. Dec 2013.) -
Changing Velocities
Through the studies of Kepler, Copernicus, and Galileo, the concept the world had that planets traveled at a constant speed came to a halt. They discovered through many observations and calculations that planets moved at constantly changing velocity and that the distance from the Sun is inversely proportional to its speed.
(Drake, Stillman. Discoveries and Opinions of Galileo. Garden City, NY: Doubleday, 1957.) -
Newton's Law of Gravity
Based on Kepler's laws and his own laws of motion, Newton derived the universal law of gravitation, which can be simply stated as the equation F=G(m1m2/r^2), where F is the gravitational force between two objects, G is the gravitational constant, the ms are their masses, and r is the distance between them. This is the reason that planets stay in an orbit.
("Planetary Motion." Astronomy & Space: From the Big Bang to the Big Crunch. Gale, 2007. Science in Context. Web. 12 Dec. 2013.) -
Orbital Predictions
English mathematician John Couch Adams and the French Urbain Le Verrier both worked at the same using calculus and some of Sir Isaac Newton's laws to accurately predict an unknown planet's current position in space and its mass only on the gravitational effects on it. This advance in mathematics greatly assisted our current understanding.
(Drake, Stillman. Discoveries and Opinions of Galileo. Garden City, NY: Doubleday, 1957.) -
Curvature of Space and Time
Albert Einstein presented the general theory of relativity in the early 20th century. He was the first to suggest the idea that space was curved. He theorized that gravity caused a object to curve on a path surrounding a body with a larger mass. This concept that objects could stretch space explains why the moon does not fall in a straight line toward the Earth.
("Planetary Motion." Astronomy & Space: From the Big Bang to the Big Crunch. Gale, 2007. Science in Context. Web. 12 Dec. 2013.)
