Here's why:
* Kepler's Laws of Planetary Motion: Johannes Kepler, through his observations, discovered that planets don't orbit in perfect circles. His first law states that the orbit of a planet around the Sun is an ellipse, with the Sun at one focus of the ellipse.
* Gravity's Influence: The gravitational force between the Sun and a planet is not constant throughout the orbit. It's stronger when the planet is closer to the Sun and weaker when it's farther away. This varying force causes the orbit to deviate from a perfect circle.
Important Note: While elliptical, most planetary orbits are nearly circular. However, the eccentricity (a measure of how elongated an ellipse is) varies for different planets. For example, Earth's orbit is almost circular, while Mercury's orbit is much more elliptical.
