The orbit of a planet is an ellipse that goes around the sun at one focus point, according to Kepler’s First Law of Planetary Motion
This means that an ellipse will have two focal points (labeled F and F’ in the illustration), and the sun will take up one focal point. It can also apply for small objects (moons or satellites) to orbit a larger body (planet or star). Spacecrafts will orbit around their respective planet or larger body in an elliptical shape (as well as the other conic shapes such as circular, parabolic, and hyperbolic to be covered in upcoming posts).
Now lets interpret these symbols above
What is eccentricity e ?
Eccentricity is the constant that determines the shape (roundness or flatness) of the orbit. It’s the focal length divided by the semi-major axis. (Note: It’s never negative!)
It is important to know the geometry of an ellipse in order to determine the Kepler equations as well and the Energy equations.