On 2/10/23 9:11 PM, Wayne Green wrote:
The ecliptic is the "mean invariant plane" of the solar system.
I will not go beyond your first line.
First, the ecliptic is not a plane.
Neither it is Earth's orbit, as some believe.
The ecliptic is a great circle on the celestial sphere.
From a geocentric point of view, it is the projection on the celestial sphere of Sun's orbit around the Earth.
We also speak of the ecliptic plane, which is trivially the plane containing the ecliptic circle.
It's better to use "invariable" than "invariant". The former means "something which does not change in time"; the latter is an ancient term and in modern physics and astronomy has a different meaning, when applicable.
The invariable plane of the solar system is perpendicular to its total angular momentum, which is dominated by Jupiter's motion, mainly because ot its mass.
Actually the angle between the ecliptic plane and the invariable plane is almost 1.6¡ã.
To be sure, the ecliptic plane is not exactly stationary: Earth's motion is perturbed by the other planets. So it is necessary, when speaking of the ecliptic, to state the reference epoch (date).
But the ecliptic motion is very slow: about 47 arcseconds per century if I remember right.
--
Elio Fabri
