Kepler Laws of Planetary Motion Lesson for Kids

Instructor: Mary Beth Burns

Mary Beth has taught 1st, 4th and 5th grade and has a specialist degree in Educational Leadership. She is currently an assistant principal.

Johannes Kepler discovered many things about our universe, including the three laws of planetary motion. Come and learn about these three laws, as well as some background about Kepler's life.

Who Was Kepler?

Johannes Kepler is an example of what a person can do despite hardship. Born in 1571 in Germany, he had crippled hands and was visually impaired as a result of smallpox. But he didn't let his disabilities define him. Instead, Kepler went on to be one of the most successful astronomers of all time.

Johannes Kepler
Johannes Kepler painting

In addition to his discovery of the three laws of planetary motion, he also explained how light works in a telescope, how our vision works, and many other important discoveries. Kepler's accomplishments can best be described by the inscription on his gravestone: 'I used to measure the heavens, now I shall measure the shadows of the earth. Although my soul was from heaven, the shadow of my body lies here.'

First Law: The Law of Ellipses

Planets travel around the Sun in an elliptical orbit.

Kepler's first law of planetary motion is known as the Law of Ellipses. An ellipse is a shape that is best described as a squished circle, similar to an oval. Kepler discovered that a planet's orbit has an ellipse shape that travels around one star. In the case of our solar system, that star is the Sun and it is the focus of the planet's orbit.

Second Law: The Law of Equal Areas

Kepler's second law, known as the Law of Equal Areas, is about how fast a planet travels around its elliptical orbit.

Pretend that you are riding a bike around a block with four houses at the same speed. Let's call them House A, House B, House C and House D. While House A and B are close to each other, House C and D are far away from each other. It would take you longer to travel between the houses that are further apart than the houses that are closer together. However, if you were traveling like a planet, this would not be the case, as it would take you the same amount of time to travel to the different houses, regardless of how far apart they are.

Kepler discovered that planets, on their elliptical orbits around the sun, actually move faster when they are nearer to the Sun and slower when they are further away. Therefore, the speed that planets travel is always changing.

So why is it called the 'Law of Equal Areas'? Well, Kepler described this as an imaginary line between the sun and a planet that sweeps equal areas during equal periods of time.

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