 In this video, we'll discuss an overview of the history of people's understanding of gravity, which has taken centuries to reach where it is today. We'll start in 1543, when Copernicus developed a revolutionary mathematical model of the solar system in which the planets orbit the sun. This idea, known as heliocentrism, is in opposition to the idea of geocentrism, which is a prevailing idea of the time. Geocentrism is the belief that everything in the solar system orbits the earth. Heliocentrism had in fact been proposed by at least 300 BC by Aristarchus, an ancient Greek. However, Aristarchus had only proposed the idea of heliocentrism, and did not have the mathematical model to explain the behavior. Copernicus's theory was not completely accepted by the scientific community until many decades later. During his time, Kepler was one of the few proponents of Copernicus's heliocentrism, and Kepler proposed three laws of planetary motion to further develop this theory. Kepler published his first two laws in 1609 and his final law in 1619. While Copernicus's theory modeled the orbits as circular, Kepler's laws focused around the idea of elliptical orbits. It was almost 70 years later in 1687 that Newton proposed his law of universal gravitation, which described the forces that result in planetary orbits. This one law explained all of Kepler's laws. This was a year that Newton published his great work, known as Principia Mathematica, in which he proposed not only his law of universal gravitation, but also his laws of motion, all of which are the basis of classical mechanics used today. However, his theory of gravity was not directly tested until the Cavendish experiment in 1798. This was the first experiment to provide values from which the universal gravitational constant, Big G, could be found. The experimental method was based on measuring the deflection of a bar due to the gravitational attraction between two larger masses and two smaller masses placed on either end of the bar. Cavendish used his experimental results to find the density of the earth, but it was not until later that his experimental values were used to determine G. Furthermore, it wasn't until 1873 that Newton's law was expressed in the way we use it today. It was at this time that the expression included the gravitational constant. This constant appears to be a fundamental constant of the universe, like the speed of light or the charge of an electron. Then, in 1915, Genius struck when Einstein published his theory of general relativity. This relativistic theory of gravity is the most sophisticated model of gravity we have. It explains the existence of black holes and other gravitational effects that we have since observed. Even though Einstein's theory is the most accurate description we have for gravity, both Kepler's laws and Newton's law of universal gravitation remain useful for most situations, and they have the advantage of being much, much easier to apply. We've already examined Newton's law of universal gravitation in detail, so let's look at Kepler's three laws of planetary motion.