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100
Aristotle (B.C.)
Aristotle's model of the universe had Earth at its center, also known as a geocentric model, and outside lay a sphere in which the stars were fixed. Celestial bodies, such as the Sun and the Moon, revolved in circles around the Earth. Aristotle also believed that the planets perfectly circled the Earth, and that all objects fell onto the Earth. Aristotle's theory satisfied people at the time, since they thought nature to be perfectly symmetrical. -
150
Ptolemy
Claudius Ptolemy modified Aristotle's theory, which failed to explain all types of movement seen in the sky. Ptolemy created a system of circles and epicycles that was meant to explain the motion of any celestial object. His model was also useful in making astronomical calculations. His model, however, like Aristotle's, maintained the idea of geocentrism, which held that the Earth was the center of the universe. -
Newton
Newton made the frist great unification of laws that could explain both earthly and celestial phenomena. He used the idea of heliocentrism, which stated that the Earth and planets revolve arond the Sun at the center of the solar system, and through his inverse square law proved that planets moved in ellipses, not perfect circles. He also used the concept of gravity to explain the motion of objects on Earth, and created the F=ma law. His work was extremely powerful and unaltered for 200 years. -
Cavendish
Henry Cavendish added on to Newton's work on gravity through his famous "Cavendish Experiment," in which he used two lead balls to approximate the density of the Earth. In order to do so he measured the gravitational attraction between the two balls. The results of Cavendish's experiment also led to the calculation of the universal gravitational constant and the mass of the Earth, both values that are highy used and important today. -
Einstein
Einstein was a physicist who developed the general theory of relativity. He linked gravity and mass, and gravity and energy, thus concluding that mass and energy affect the gravitational field. He also concluded that the gravitational field is not a force as Newton had thought, but a curvature in space. Therefore, bodies are affected by gravity not because of a force being exerted on them but because they follow space's curvature, which is affected by mass and energy, such as the Sun.
