Orbital eccentricity facts for kids

Examples of four orbital trajectories with different eccentricities

In astrodynamics, orbital eccentricity shows how much the shape of an object's orbit is different from a circle.

Eccentricity ( $e\,\!$ ) is defined for all circular, elliptic, parabolic and hyperbolic orbits. It can take the following values:

for circular orbits: $e\,\!$ is equal to zero,
for elliptical orbits: $e\,\!$ is more than zero but less than 1,
for parabolic trajectories: $e\,\!$ is equal to 1,
for hyperbolic trajectories: $e\,\!$ is more than 1.

Finding eccentricity

Here is a formula to find eccentricity:

$e_{obj}=\frac {r_a-r_p} {r_a+r_p}$

Where e_obj is the eccentricity, r_a is the apoapsis (far point) of the object's orbit, and r_p is the periapsis (near point) of the object's orbit. The near and far points are the apsides.

Images for kids

Elliptic orbit by eccentricity 0.0 · 0.2 · 0.4 · 0.6 · 0.8
Plot of the changing orbital eccentricity of Mercury, Venus, Earth, and Mars over the next 50000 years. The arrows indicate the different scales used, as the eccentricities of Mercury and Mars are much greater than those of Venus and Earth. The 0 point on this plot is the year 2007.

Orbital eccentricity facts for kids

Finding eccentricity

Images for kids

See also