
Period
The period of an orbit, usually denoted $T$, is the total time taken for an object to complete one full revolution on its orbit.

Kepler's First Law
The planets orbit the sun in ellipses, with the sun at one focus.

Kepler's Second Law
The radius of a planet sweeps out a constant area per unit time.

Kepler's Third Law
\begin{equation} T^2 = \frac{4\pi^2a^3}{GM} \end{equation} where $T$ is the period, $G$ is the gravitational constant, and $M$ is the mass of the sun.

Eccentricity
The eccentricity is a measure of the elongation of an ellipse. It is defined as: \begin{equation} \epsilon = \sqrt{1  \frac{b^2}{a^2}} \end{equation} where $a$ and $b$ are the semimajor and semiminor axis lengths respectively.

Aphelion
The point of a planet's furthest distance from the sun on its orbit.

Perihelion
Is the point of a planet's closest approach to the sun.

Apogee
For an earth orbit, the furthest distance from the earth.

Perigee
For an earth orbit, the closest distance to the earth.