An orbit is the regular, repeating path an object in space takes around another, larger object, such as a planet moving around the Sun. This movement is a continuous, predictable trajectory shaped by the interaction of two fundamental concepts of physics. Planets remain in their paths because they are constantly subject to two opposing, powerful influences that maintain a constant, dynamic balance.
Gravity: The Sun’s Anchor
The first influence is gravity, the attractive force that pulls all objects with mass toward one another. The Sun accounts for over 99.8% of the total mass of the solar system, exerting an immense gravitational field that dominates the space around it. This powerful field acts as a tether, constantly pulling the planets inward toward the Sun’s center. Without this attractive force, the planets would simply fly off into space along a straight path.
The magnitude of this gravitational pull is governed by a principle relating the masses of the two objects and the distance separating them. The force of attraction is directly proportional to the product of the two masses, meaning the Sun’s immense mass creates an immense pull. Conversely, this force is inversely proportional to the square of the distance between the objects. This means that the farther a planet is from the Sun, the weaker the gravitational influence becomes.
Inertia: The Forward Momentum
The second influence is inertia, the tendency of any object in motion to remain moving at a constant speed and in a straight line. When the solar system formed from a spinning cloud of gas and dust, the planets inherited a significant sideways velocity. This initial movement created the forward momentum that keeps each planet moving tangentially to its orbit.
If the Sun’s gravitational force were suddenly to vanish, the planet’s inertia would cause it to continue traveling straight ahead, shooting out of the solar system. The vacuum of space offers practically no resistance to motion, meaning the planet’s forward motion represents its natural tendency to escape the Sun’s pull.
The Delicate Equilibrium of Orbit
Planetary orbits are the result of a continuous tug-of-war between the Sun’s inward pull of gravity and the planet’s forward inertia. The two forces are balanced in a precise, dynamic way, allowing orbit to be accurately described as a state of perpetual, controlled free-fall.
The planet is constantly falling toward the Sun, but its tremendous sideways speed ensures that it continuously misses the star. Gravity acts to change the direction of the planet’s velocity, bending its straight-line path into a curve. The forward momentum carries the planet far enough that the surface of the Sun curves away beneath it at the exact rate the planet falls.
This required speed, known as orbital velocity, must be precisely matched to the strength of the gravitational field at that distance. If a planet were moving too slowly, the Sun’s gravity would overwhelm its inertia, causing it to spiral inward. Conversely, if the planet were moving too fast, its inertia would overcome gravity, and it would escape the solar system entirely.
Why Planetary Paths Are Elliptical
While orbits are often visualized as perfect circles, the path a planet takes is actually an ellipse—a slightly stretched or flattened circle. This elliptical shape is a natural outcome of the varying interplay between the planet’s inertia and the Sun’s gravity. The Sun is not located at the exact center of the ellipse but at one of the two focal points.
The planet’s distance from the Sun is constantly changing throughout its orbit. The point where the planet is closest to the Sun is called perihelion, and the point where it is farthest away is called aphelion. As the planet moves toward perihelion, the Sun’s gravitational pull increases, causing the planet to speed up. When the planet swings away toward aphelion, the gravitational force weakens, and the planet naturally slows down.