# Orbital Mechanics Post #3 Here are some of the basic measurements of an elliptical orbit. It's the shape of an ellipse with the main body at one focus. The satellite will travel around this body in the shape shown in the picture. The velocity of the satellite varies relative to its radial distance to the main body. The highest velocity occurs at periapsis and the lowest at apoapsis.

Apopasis and periapsis are the furthest and closest points in the orbit to the main body, respectively. The names vary when talking about different celestial bodies. Apogee and perigee for Earth, apolune and perilune for the Moon etc.

The dimensions shown are:
a - the semi-major axis
b - the semi-minor axis
ae - the semi-major axis multiplied by the eccentricity

The eccentricity is the parameter that defines how much an orbit varies from a perfect circle. A circular orbit has an eccentricity of 0. Elliptical orbits have an eccentricity between 1 and 0. Parabolic orbits have an eccentricity of exactly 1 and hyperbolic orbits have an eccentricity of greater than 1.