Any object moving freely under gravity traces out a path called an orbit. Both the path of a falling apple and the path of the Moon around the Earth are orbits. The Earth and the other planets move around the Sun in elliptical orbits. This was first described by Johannes Kepler in the early 17th century. Newton later described these orbits as inevitable consequences of his law of universal gravitation. Newton's orbits differ from Kepler's in that, Newton showed they were not simply elliptical orbits, but were more complex. Newton's theory lets us extend our understanding to the motions of stars and galaxies.
Johannes Kepler
Kepler made a breakthrough to understanding in the motion of the planets. Kepler was able to formulate 3 laws of planetary motion based on Brahe's observation of Mars.
The first 2 laws were published in 1609 and the 3rd in 1619.
The orbit of a planet around the Sun is in the form of an ellipse, with the Sun at one focus.
The line connecting a planet to the Sun sweeps out equal areas in equal times.
The square of the orbital period of a planet is proportional to the cube of the semi-major axis.
Isaac Newton
The laws of Kepler describe the motions of the planets, yet, offers no explanation of why the planets move the way they do. Isaac Newton, through his laws of motion and universal gravitation, showed that the planets move under the influence of gravitational attraction that act between the planets and the much larger Sun.
Types of orbits
An orbit's shape is characterized by its eccentricity. When the eccentricity of an orbit is 0, it is a perfect circle. Eccentricities between 0 and 1 are elliptical orbits. Longer orbits have the greater eccentricities. The eccentricity of the Earth's orbit is 0.017. The most eccentric planetary orbit in the Solar System is that of Pluto (0.248). Halley's comet has an eccentricity of 0.97, moving from within the orbit of Venu to beyond the orbit of Neptune.