Why Do the Planets Orbit the Sun in an Elliptical Fashion?
Planets orbit in the shape of an ellipse because of the gravitational interactions between planets and the Sun, along with other celestial bodies. Johannes Kepler in the 1600s was the first to start discussing the shape of planetary orbits, formulating a series of laws to explain their shape and characteristics. Isaac Newton and Albert Einstein contributed additional research and theories to the body of knowledge about how planets move through space.
Kepler determined, according to his first law of planetary motion, that the orbits of the planets are elliptical in shape, with the Sun at one focus of the ellipse and the shape of the ellipse being measured in terms of eccentricity; the more eccentric an orbit, the more elongated it is. Many planets, like Earth, have orbits that are very close to being circular, while others, like Mercury, have more eccentric orbits. Kepler did not determine why the planets orbit in an ellipse, but his groundwork was used by other physicists who came up with concrete explanations.
Newton contributed the factor of gravity, showing how 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 like many hands pulling taffy. Einstein's theory of relativity also plays a role in explaining why planets orbit the Sun in an elliptical fashion, as 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.
Using mathematical rules established by a number of physicists and astronomers, people can accurately calculate the orbit of individual planets as well as entities like comets and they can also 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.
It is important to be aware that many descriptions of planetary orbits are simplistic, placing 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 over time. 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 motion.
Written by Laura Martinelli
