Here's why:
* Gravity: The sun's gravity pulls on the planet, keeping it in orbit. But the planet's velocity also plays a role.
* Velocity: If the planet were moving straight towards the sun, it would be pulled directly in. If it were moving perfectly perpendicular to the sun, it would fly off into space.
* Balance: The orbit is a balance between the sun's pull and the planet's velocity. The planet's path is curved because it's constantly falling towards the sun, but it also has enough velocity to keep missing the sun.
Think of it like a ball on a string:
* If you swing a ball on a string, it moves in a circle. The string acts like gravity, keeping the ball from flying off.
* The faster you swing the ball, the wider the circle.
* A planet's orbit is similar, except the "string" is gravity, and the "ball" is the planet.
While an ellipse is not a perfect circle, it's a very close approximation. This is why we often depict planets orbiting the sun in circular paths, even though it's a simplification.
