Here's why:
* Kepler's Laws: Johannes Kepler, a 17th-century astronomer, formulated laws of planetary motion. His first law states that planets orbit the sun in elliptical paths, with the sun at one focus of the ellipse.
* Gravity: The sun's gravitational pull is the dominant force that keeps planets in orbit. An ellipse is the natural shape of an orbit when the gravitational force isn't perfectly uniform.
* Eccentricity: The shape of an ellipse is determined by its eccentricity. A perfectly circular orbit has an eccentricity of 0. Most planets have a slightly eccentric orbit, meaning their paths are slightly elongated ellipses.
Note: While the orbits of planets are elliptical, the eccentricity is usually small, making them appear close to circular.
