The eight planets orbit the sun in an elliptical fashion primarily because of gravitational interactions. The sun has a gravitational pull, as do most planets; other celestial bodies do, too, and the ways in which these forces interact and either attract or repel each other causes orbiting. Most physicists and astronomers believe that the planetary orbits should be perfectly circular. That they are actually elliptical, many say, has more to do with outside forces and variance errors than anything else. German astronomer Johannes Kepler was the first to publish material proving elliptical orbits, and his theories are still considered definitive. They were added to and expanded on by Isaac Newton and Albert Einstein, among others.
Solar System Basics
The solar system is widely believed to contain eight planets, Earth included, that pass around a central sun at various intervals, each on their own elliptical track. Mercury, Venus, Earth, and Mars together make up what is known as the “inner” solar system. These planets rotate the most quickly. Much further out one finds the “outer” system, made up of Jupiter, Saturn, Uranus, and Neptune. These planets sit much farther from each other than do those in the inner rings, and their orbits tend to be much bigger, too. All of the orbits are elliptical in shape, though with the exception of Mercury they tend to appear almost perfectly circular. It is usually only through intense mathematical calculations that people discover they’re actually elliptical.
Influence of Eccentricity
Kepler was the first to identify the elliptical shape in the late 1600s. He came up with three related “laws of planetary motion” that quantify orbital movement with some precision. Through these laws he was able to explain that the planets move on a plane with the sun at one focus and determined that the shape of the ellipse should be measured in terms of eccentricity; namely, the more eccentric an orbit, the more elongated it is. Kepler did not determine why they orbit in an ellipse, but his groundwork was used by other physicists who came up with concrete explanations.
Importance of Gravity
Newton’s studies concluded that gravity plays a major role. Through a series of calculations he was able to show that planets pull on each other and the sun, as the sun also exerts a gravitational pull on them. This has the effect of squashing orbits that one might expect to be circular in a closed system, as the gravitational pulls act on each other. One way to think about this is to envision many hands pulling taffy.
Curvature of Space
The physical shape of the expanse of space also contributes. Einstein's theory of relativity also helps complete the explanation of why planets orbit the Sun in an elliptical fashion, since part of the shape of the orbits is a result of the curvature of space caused by planets acting on the space-time around them. The “bending” of space that results has a proportional impact on movement, and it forces what might otherwise be circular to flatten out and elongate.
In most cases the only accurate way to measure orbits and calculate planetary velocity and movement is to undertake a number of somewhat complex mathematical computations. People can calculate the orbit of individual planets as well as entities like comets using the mathematical rules established by a Kepler, Newton, Einstein, and those that followed, and they can also use equations to track the degree of change over time. This information is useful for a number of applications, from programming telescopes for observation to determining the degree of threat posed by an approaching comet or asteroid.
Changes Over Time
It is important for people to remember that many descriptions of planetary orbits are simplistic for the sake of easy comprehension, and many place the sun as a fixed body in space that planets move around. In fact, the sun is in motion along with the planets, and as they move through space the precise shape of the orbits also changes. This should be kept in mind when looking at discussions of the way the planets orbit around the sun, as the entire orbital system is, in fact, actually moving.