# 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.

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## 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.

Q:

## What is orbit eccentricity?

A:

Eccentricity is the measure of the "roundness" of an orbit. A perfectly circular orbit has an eccentricity of zero; higher numbers indicate more elliptical orbits. Neptune, Venus, and Earth are the planets in our solar system with the least eccentric orbits. Mercury and the dwarf planet Pluto have the most eccentric orbits.

## Did You Know?

Johannes Kepler was the first to use the term “satellite” to describe orbiting bodies in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus (Narration About Four Satellites of Jupiter Observed) in 1610. He derived the term from the Latin word satelles, which means attendant, because the satellites accompanied their primary planet in their journey through the heavens.

## Pop Quiz

Approximately how many human-made objects are currently orbiting Earth?

A) 2000
B) 4000
C) 8000
D) 16000

8,000: The United States Space Surveillance Network (SSN) has been tracking objects in Earth's orbit since 1957 when the Soviets launched Sputnik I.  The SSN currently tracks more than 8,000 human-made objects orbiting Earth.  The SSN tracks objects that are 10 centimeters in diameter or larger; those now orbiting Earth range from satellites weighing several tons to pieces of spent rocket bodies weighing only 10 pounds.  Less than 600 are operational satellites, the rest is junk.