Kepler’s Laws of Orbital Motion

Observing Kepler’s Laws of Orbital Motion at Work

Kepler’s laws show the effects of gravity on orbits. They apply to any object that orbits another: planets orbiting the Sun, moons orbiting a planet, spacecraft orbiting Earth.

Kepler’s First Law Describes the Shape of an Orbit

The orbit of a planet around the Sun (or of a satellite around a planet) is not a perfect circle. It is an ellipse—a “flattened” circle. The Sun (or the center of the planet) occupies one focus of the ellipse. A focus is one of the two internal points that help determine the shape of an ellipse. The distance from one focus to any point on the ellipse and then back to the second focus is always the same.

Kepler’s Second Law Describes the Way an Object’s Speed Varies along Its Orbit

A planet’s orbital speed changes, depending on how far it is from the Sun. The closer a planet is to the Sun, the stronger the Sun’s gravitational pull on it, and the faster the planet moves. The farther it is from the Sun, the weaker the Sun’s gravitational pull, and the slower it moves in its orbit.

Kepler’s Third Law Compares the Motion of Objects in Orbits of Different Sizes

A planet farther from the Sun not only has a longer path than a closer planet, but it also travels slower, since the Sun’s gravitational pull on it is weaker. Therefore, the larger a planet’s orbit, the longer the planet takes to complete it.