The semi-minor axis and the semi-major axis are related through the eccentricity, as follows:  Note that in a hyperbola can be larger than . In astrodynamics the orbital period of a small body orbiting a central body in a circular or elliptical orbit is:  where:  Note that for all ellipses with a given semi-major axis, the orbital period is the same, disregarding their eccentricity. The specific angular momentum of a small body orbiting a central body in a circular or elliptical orbit is:  where:  In astronomy, the semi-major axis is one of the most important orbital elements of an orbit, along with its orbital period. For Solar System objects, the semi-major axis is related to the period of the orbit by Kepler's third law (originally empirically derived),  where is the period and is the semi-major axis. This form turns out to be a simplification of the general form for the two-body problem, as determined by Newton:  where is the gravitational constant, is the mass of the central body, and is the mass of the orbiting body. Typically, the central body's mass is so much greater than the orbiting body's, that may be ignored. Making that assumption and using typical astronomy units results in the simpler form Kepler discovered. The orbiting body's path around the barycenter and its path relative to its primary are both ellipses. The semi-major axis is sometimes used in astronomy as the primary-to-secondary distance when the mass ratio of the primary to the secondary is significantly large (formula_34); thus, the orbital parameters of the planets are given in heliocentric terms. The difference between the primocentric and "absolute" orbits may best be illustrated by looking at the Earth–Moon system. The mass ratio in this case is . The Earth–Moon characteristic distance, the semi-major axis of the "geocentric" lunar orbit, is 384,400 km. (Given the lunar orbit's eccentricity e = 0.0549, its semi-minor axis is 383,800 km. Thus the Moon's orbit is almost circular.) The "barycentric" lunar orbit, on the other hand, has a semi-major axis of 379,730 km, the Earth's counter-orbit taking up the difference, 4,670 km. The Moon's average barycentric orbital speed is 1.010 km/s, whilst the Earth's is 0.012 km/s.