What is "retrograde orbital motion?"

Jonas Cross 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).

I think the canonical answer is (a), although both (b) and (c) also
happen to be true, and of course, that isn't coincidental, since both
the Earth and Jupiter were formed in the same primordial cloud of gas
and dust. (I think there's a law that says you have to use the word
"primordial" there.)

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Jonas Cross:
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...


Brian Tung:
I think the canonical answer is (a), although both (b) and (c) also
happen to be true, and of course, that isn't coincidental, since both
the Earth and Jupiter were formed in the same primordial cloud of gas
and dust. (I think there's a law that says you have to use the word
"primordial" there.)

1. Your eloquence never ceases to amaze me. I was going to answer Mr.
Cross's question in this way:

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

*Bingo*

But you made that impossible.

2. I think that "primordial" (literally, "existing at or from the
beginning of time") is superfluous here, even in the figurative sense
in which you and I understand it -- very old. Why does it have to be
said that the cloud of dust and gas is primordial? Whether it dates
from perhaps 6 billion years BP or three weeks BP, the essential
information in this context is that, for the Earth and Jupiter, it is
the /same/ cloud.

Just killing time here, waiting for my lunch to cook. Move on, nothing
to see here, etc.

Davoud

--
usenet *at* davidillig dawt com

Davoud wrote:
But you made that impossible.

Sorry about that.

2. I think that "primordial" (literally, "existing at or from the
beginning of time") is superfluous here, even in the figurative sense
in which you and I understand it -- very old. Why does it have to be
said that the cloud of dust and gas is primordial? Whether it dates
from perhaps 6 billion years BP or three weeks BP, the essential
information in this context is that, for the Earth and Jupiter, it is
the /same/ cloud.

That was my point--it's one of those things that seems almost de rigeur,
for no particular reason. It's rare that a science program refers to
that cloud and doesn't *somewhere* use the word "primordial" to describe
it.

Just killing time here, waiting for my lunch to cook. Move on, nothing
to see here, etc.

What's your lunch cooking? Itself? Isn't that a little macabre?

(Bump, set, ...)

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Davoud:
Just killing time here, waiting for my lunch to cook.

Brian Tung:
What's your lunch cooking? Itself? Isn't that a little macabre?

Uh-uh! I was ready for that one. "cook: verb, intrans. (of food) be
heated so that the condition required for eating is reached."

Macabre? That would be the sentient food animal at Milliways. He
described to the patrons the virtues of eating his various parts, then
said "I'll just nip off and shoot myself..." or words to that effect.

I'm having boiled fresh* white corn and boiled potatoes for lunch. They
are cooking themselves now.

Davoud

*fresh: adj. picked on my wife's farm within the past hour.

--
usenet *at* davidillig dawt com

Brian Tung wrote:
Jonas Cross 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).


I think the canonical answer is (a), although both (b) and (c) also
happen to be true, and of course, that isn't coincidental, since both
the Earth and Jupiter were formed in the same primordial cloud of gas
and dust. (I think there's a law that says you have to use the word
"primordial" there.)


The canonical answer is indeed (a). The inner satellites of Uranus and
the three known satellites of Pluto are considered to be in direct
orbits, not retrograde, even though the planets themselves are in
retrograde rotation.

-- Bill Owen

is there anything you dont know - dont answer that!

Davoud wrote:

Davoud:
Just killing time here, waiting for my lunch to cook.

Brian Tung:
What's your lunch cooking? Itself? Isn't that a little macabre?

Uh-uh! I was ready for that one. "cook: verb, intrans. (of food) be
heated so that the condition required for eating is reached."

Macabre? That would be the sentient food animal at Milliways. He
described to the patrons the virtues of eating his various parts, then
said "I'll just nip off and shoot myself..." or words to that effect.

I'm having boiled fresh* white corn and boiled potatoes for lunch. They
are cooking themselves now.

Davoud

*fresh: adj. picked on my wife's farm within the past hour.

--
usenet *at* davidillig dawt com