Kepler's Laws of Planetary Motion
Johannes Kepler was a German mathematician, astronomer, and astrologer who lived from 1571 to 1630. Among Kepler's many accomplishments was his discovery of three natural laws that describe how objects like planets orbit around more massive objects like stars. Today, we call them Kepler's Laws of Planetary Motion.
Remember, in science, a law is a mathematical description of a natural event that is always true. You undoubtedly know by now that the sun is the center of our solar system. Our sun is made of up of 98% of all the stuff in our solar system. To put that in perspective, imagine that the entire solar system is represented by a small pile of 100 grains of sand. 98 of those grains of sand represent the sun and only 2 grains of sand represent all the planets, asteroids, moons, etc. The more massive an object is, the more gravity it produces. The sun is so massive that all the major celestial objects in our solar system are caught in the sun's gravity and are forced to orbit. 
Kepler's First Law of Planetary Motion
Kepler's first law says that each planet orbits the sun in a shape called an ellipse. An ellipse is a circular shape that is proportionally symmetrical. A circle is always an ellipse, but only some ellipses are circles. Other ellipses are circles that have been stretched. If you look at a diagram of the orbital paths of big and small objects in our solar system, you might be able to see that Kepler was right, all objects orbiting the sun have an elliptical orbit. One consequence of Kepler's first law is that objects orbiting the sun are sometimes closer to the sun than at other times during their orbit.
Kepler's first law says that each planet orbits the sun in a shape called an ellipse. An ellipse is a circular shape that is proportionally symmetrical. A circle is always an ellipse, but only some ellipses are circles. Other ellipses are circles that have been stretched. If you look at a diagram of the orbital paths of big and small objects in our solar system, you might be able to see that Kepler was right, all objects orbiting the sun have an elliptical orbit. One consequence of Kepler's first law is that objects orbiting the sun are sometimes closer to the sun than at other times during their orbit.
Kepler's Second Law of Planetary Motion
Since objects orbiting the sun are sometimes closer to the sun than at other times, one question that arises pertains to the speed at which an object orbits the sun. We know that a planet, for instance, never stops moving, but does it ever speed up or slow down? The answer is yes. Kepler's second law says that when a planet is closer to the sun, it speeds up, and when a planet moves away from the sun, it slows down. 
Earth's elliptical orbit is almost perfectly circular, but there is still a slight variation in Earth's proximity to Sol, our sun. It turns out that Earth moves a little bit faster around the sun during the northern hemisphere's winter than at any other point during Earth's year, which is also called an orbital period.
Kepler's Third Law of Planetary Motion
Now that we know that planets move faster when they are closer to the sun and slower when they are far away from the sun, it is easier to understand Kepler's third law. Kepler's third law says that there is a mathematical relationship between the size of a planet's elliptical path around the sun and how long a planet's orbital period (year) is.
Now that we know that planets move faster when they are closer to the sun and slower when they are far away from the sun, it is easier to understand Kepler's third law. Kepler's third law says that there is a mathematical relationship between the size of a planet's elliptical path around the sun and how long a planet's orbital period (year) is.


