Why are planetary orbits elliptical, not circular?

Planetary orbits are elliptical due to the gravitational pull of the sun and the planet's initial velocity.

The shape of a planet's orbit is determined by two main factors: the gravitational pull of the sun and the planet's initial velocity. The sun's gravity pulls the planet towards it, while the planet's velocity tries to keep it moving in a straight line. The combination of these two forces results in an elliptical orbit.

The German astronomer Johannes Kepler was the first to propose that planets move in elliptical orbits, not circular ones. His first law of planetary motion, known as Kepler's First Law, states that "The orbit of every planet is an ellipse with the sun at one of the two foci." This means that the sun is not at the centre of the orbit, but slightly off to one side.

The reason for this is due to the nature of gravity and motion. When a planet is closer to the sun, it moves faster because the sun's gravitational pull is stronger. Conversely, when the planet is further away, it moves slower because the gravitational pull is weaker. This variation in speed as the planet moves around its orbit causes the orbit to be an ellipse rather than a perfect circle.

In a circular orbit, the gravitational force and the planet's velocity would have to be perfectly balanced. This is highly unlikely to occur naturally. In reality, the planet's velocity and the sun's gravitational pull are not perfectly balanced, leading to the elliptical shape of the orbit.

In conclusion, the elliptical shape of planetary orbits is a result of the interplay between the sun's gravitational pull and the planet's initial velocity. This is a fundamental concept in the study of planetary motion and celestial mechanics.