"The orbit of every planet is an ellipse with the Sun at one of the two foci."
Semi-Major Axis (a)
Half the longest diameter. Determines orbit size and period.
Eccentricity (e)
0 = perfect circle, near 1 = very elongated. Describes how "squished" the ellipse is.
Foci (F1, F2)
Two special points inside every ellipse. The Sun sits at one focus.
Perihelion & Aphelion
Closest and farthest points from the Sun. The planet speeds up at perihelion and slows at aphelion.