In 200 AD, astronomers believed that planets moved round the earth in small circles called the epicycle. The Heliocentric model later explained that all planets including the sun revolved around the sun. This model also explained that the retrograde motion of planets did so as the earth which moves faster than the other planets overtook the other planets. Other planets are categorized as superior or inferior to the earth. Time that is able to elapse between the configurations that are identical and consecutive is known as the synodic period.
It was Copernicus who determined the sidereal period of other planets. He was also able to know the distance between the planets and the sun. Kepler was able to come up with a law that stated that every planet has an orbit that about the sun know as an ellipse. He also came up with the semi major axis. This happens to be the distance between the planets and the sun. The measure of the deviation from the perfect circle is known as the eccentricity. Kepler had three laws to explain the motion of planets and this was a major achievement in the field of astronomy. Galileo was able to discover a lot more with the aid of a telescope. He was able to know that the earth was not at the centre of the universe. With his telescope, he was able to know that the earth was not at the centre of the universe and that the earth was just like any other planet moving around the sun. Isaac Newton later came and introduced three laws. Inertia, force and action and reaction. He was also able to come up with the laws of the universal gravitation. The law of gravitation states that two bodies attract each other with a force directly proportional to each and every mass of the two bodies. This force is also inversely proportional to the square of the distance between them. Using the law of gravitation, Kepler’s three laws can be summed up. Newton’s was able to note that every