**A New View of Planetary Orbits**

When Kepler became Tycho Brahe's assistant, Brahe gave him one main job: to interpret data he was collecting on the orbit of Mars. It took Kepler several years to complete this task -- long after Brahe's death -- but the puzzling data led to a revolutionary discovery. Unlike most people of his day, Kepler believed in the Copernican system that the planets revolve around the Sun in circular orbits. But even with this theory, Brahe's measurements did not work out. Eventually, Kepler realized that the orbit of Mars -- as well as the other planets -- had to be elliptical. He also calculated that the Sun was not in the center of this orbit, but at one focus. (Every ellipse has two points, each called a "focus." The sum of the distances to the foci from any point on the ellipse is a constant.) These two revolutionary ideas make up Kepler's first law of planetary motion.

Kepler helped prove the heliocentric theory, which states that the:

Sun and planets orbit a nucleus of helium

Sun and planets orbit the Earth

Earth and planets orbit the Sun

### Correct!

The heliocentric theory states that the Earth and planets orbit the Sun. During Kepler's time, most people believed that the Sun and planets revolved around the Earth. This was called geocentric astronomy.

A planet closer to the Sun will move ??? than a planet farther from the Sun.

faster

slower

at the same pace

### Correct!

According to Kepler's second law, planets move faster when they come closer to the Sun and slower as they move farther away. Kepler's third law uses the time it takes a planet to orbit the Sun to find its average distance from the Sun.

Kepler was the first to coin the phrase "satellite."

### Fact

Kepler used the term "satellite" to describe the moons orbiting Jupiter.

Kepler's laws of orbital motion apply only to planets.

### Fiction

Kepler's laws of orbital motion apply not just to planets, but to all bodies that orbit another body due to gravity.