In Kepler's first law of planetary motion, he says that planets orbit around the sun, not in perfect circles, but in ellipses (3.) the Earth also orbits the sun in this shape. The shape of the ellipse is determined by two focii. The closer these two points are to each other, the more the ellipse will resemble a circle.

The moon for example, orbits around the Earth. The moon is not always at the same distance from the Earth in all points of the orbit. So the shape is an ellipse, which is made with Earth as one of its focus(1.).

Eccentricity(6.) describes the flatness the orbit is, or how elliptical the orbit is. Remember Kepler's first law, he says that planets move in elliptical orbits and not perfect circles. So it is simply how different from a perfect circle an orbit is. Pluto is known to have a large eccentricity.

Because of the shape of an ellipse, there are going to be points where a planet is closest to the sun and where a planet is farthest from the sun. This is where Kepler's 2nd law of planetary motion comes in. When the planet is closest to the sun, this is called the perihelion (5.) where a planet would be traveling the fastest at this point of the orbit. When the planet is at its farthest point from the sun, this is called the aphelion (2.) where a planet would be traveling the slowest.

The third law of Kepler describes the relationship between the period and the average distance of the sun. The semimajor axis(4.) is the average distance of the planet object from the sun and is measured in Astonomical units (AU). The period is measured in years.

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2 years ago