Re: What is "retrograde orbital motion?"

Jonas Cross <nope@xxxxxxxxxxx> wrote:

I once read that some of Jupiter's moons have retrograde orbital
motion. I understand what that means. It means that, when the Jovian
"system" is viewed from Jupiter's North Pole, the moons appear to be
orbiting Jupiter in a clockwise direction. But, why do we describe such
motion as "retrograde?" Is it because:

(a) those moons are orbiting Jupiter in the direction opposite from the
way that Jupiter rotates on its axis;

(b) those moons are orbiting Jupiter in the direction opposite from the
way that Jupiter orbits the Sun;

(c) those moons are orbiting Jupiter in the direction opposite from the
way that Earth orbits the Sun (the idea being that Earth sets the standard
for the entire Solar System); or

(d) other (please explain).

The most comon meaning of retrograde would be opposite the direction of
the parent body's rotation, because this has dynamical consequences
(affects tidally driven decay of orbits, for example). In galaxy
pairs, this is the meaning. In Jupiter's case, senses a and b coincide.
(b) also matters, because for moons which are just barely bound to
a planet in view of perturbations by the Sun, retrograde orbits
are more stable and longer-lasting than direct (the astronomical
opposite of retrograde, no matter what the flight-dynamics people
think by inventing "posigrade"; prograde is only a bit better) orbits.
Just as well Venus has no dynamical room to fit satellites, because
we'd have a hard time telling whether they were direct or retrograde
since it would depend on he context. And Pluto and Uranus skip that
problem by being polar with respect to the orbital planes.

Bill Keel
.

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