At one point, it was believed that the planets orbit the sun in a perfect circle. At first glance this seems logical,and in fact some objects do orbit circularly. With the sun's gravitational pull on the planets being constant, why wouldn't the resulting orbit be circular? With the development of physics, and discoveries by scientists like Kepler and Newton, we learned that planets orbit in an elliptical manner — in the shape of an elongated, or squished, circle.

The size and shape of elliptic orbits, also known as eccentric orbits, vary among orbital paths. The eccentricity of the way the planets orbit is determined by the shape of the path's ellipse. For example, the eccentricity of the Earth’s orbital path is a near perfect circle, whereas the eccentricity of Pluto is a longer and thinner ellipse.

The best explanation for why planets orbit elliptically is by way of Kepler’s Laws of Planetary motion. These three laws, discovered by German astronomer Johannes Kepler in 1605, provide a better understanding of how the planets moved. Kepler based his theories on Copernicus’s theories; specifically, the fact that the sun, not the Earth, was at the center of the universe and not just the center of the solar system.

Kepler's first law explains, that planets orbit in the shape of an ellipse with the sun being one of the focal points of the orbit. Unlike circular orbits, whose two focus points reside in the same location — the center of the circle — elliptical orbits have two focus points in different locations. In the case of the sun and the planets, the sun is not at the center of the orbit. Why? Because planets in the solar system gravitationally affect the orbits of other planets squishing what might otherwise have been a perfectly circular orbit. Instead, the sun is one focus for the planet’s gravitational pull. Usually, nothing is at the other focus point. Because the two focus points are not in the same location, the resulting orbital path is elliptical.

The second law of Planetary Motion has to do with the speed at which the planet orbits. That is, the imaginary focal line joining the planet and the sun will create an equal distance and travel speed to the orbit. Because of this, the planet’s orbit will speed up when it is closest to its foci.

The final Kepler law has to do with the time that it takes a planet to orbit the sun. This third law is associated with the following equation: (planet 1)² / (planet 2)² = (radius 1)³ / (radius 2)³ where "planet 1" is the period of revolution of one planet, "planet 2" is the period of revolution of the second planet, "radius 1" is the radius of the first planet's orbit, and "radius 2" is the radius of the second planet's orbit. Essentially, this equation describes how a shorter distance and axis can lead to a faster orbit, as exhibited by the planet Mercury, while planets that are a further distance from the sun, such as Pluto, take years to complete a single orbit.