Orbital Eccentricity

The orbital eccentricity defines the form of the orbits of the celestial objects. The form of the Orbite S is in form of ellipse, of polar equation (origin with the hearth): $r= \ frac {p} {1+e \ cos \ left (\ theta \ right)}$ where E is the Excentricité. It thus gives an precise indication of their form. Thus more the eccentricity is large, plus the ellipse is crushed; a contrario , an eccentricity of zero is that of a Cercle. It will be also noted that an eccentricity equalizes to 1 corresponds to a parabolic trajectory , and an eccentricity higher than 1 corresponds to a hyperbolic trajectory , but this trajectory not being closed, one does not speak any more an orbit.

One can as say as it is the relationship between the distance separating the hearth S and the main roads from the ellipse.

Eccentricity of planets of the solar system

Phenomena modifying the eccentricity

See also: Cycles of Milankovitch

When two bodies are in gravitational rotation one around the other, the eccentricity of the orbits is theoretically fixed at the beginning and cannot change. Actually, two principal phenomena can modify it. On the one hand, the two stars are not insulated in space, and the interaction of another planets and bodies can modify the orbit and consequently the eccentricity. Another modification, more foreseeable, is due to the effect of tide. Let us take the concrete example of the moon revolving around the ground. As orbit of the moon is not circular, it is subjected to forces from tide, which is exerted differently according to the point of the orbit where the moon is, and vary continuement during the revolution of the moon. The materials inside the moon thus undergo forces of friction, which are wasteful of energy, and which tend to make the orbit circular, to minimize this friction. Indeed, the synchronous circular orbit (the moon always showing the same one vis-a-vis the ground) is the orbit minimizing the forces of tide. When two stars are in rotation one around the other, the eccentricity of the orbits thus tends to decrease.

In a standard “planet/satellite” system (body of low mass in rotation around a body of high mass), the time necessary to reach the circular orbit (time of circularization) is much higher than the time necessary so that the satellite always presents the same one vis-a-vis the planet (time of circularization). The moon thus always presents the same one vis-a-vis the ground, without its orbit being circular.

The eccentricity of the terrestrial orbit is it-also variable over very long periods (in hundreds of million years), the current value is approximately 0,0167 — but in the past it already reached a maximum value of 0,07.

See too

Objects of the solar system classified by eccentricity

Random links: