[1] What is a Retrograde Motion?


[2] Did Mars some years back stopped spinning? If yes, when, how and why?


.HavE a Great fanTastic Nice bEautiful WonDerful eXcelLent dAy and NIght

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afbS

1. Retrograde motion is just the appearance that a planet that is further away from the Sun than Earth has from our perspective. It happens when the Earth pushes around in it's orbit and 'overtakes' the outer planet. From us it looks like the planet has done a loop in the sky, because it appears to 'go backwards' for a while and then return on it's original path. Here's a good diagram:
http://alpha.lasalle.edu/~smithsc/As.../marsmovie.gif

http://alpha.lasalle.edu/~smithsc/As...nican-move.gif

2. I don't think that Mars stopped spinning 'some years back'. :-?

with regards

Venus and Mercury also exhibit the east to west motion when they are at conjunction, as they are overtaking Earth then.

Also, Venus's rotation is considered retrograde motion, since it is in the opposite direction of the general direciton of rotation of most of the planets.

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My bad.....I've been out of astronomy for nearly a year now and I'm just starting to get back into it. :P But then along with Venus you could consider Pluto (and maybe Uranus) to have retrograde rotations as well.

with regards

Sure, I think so, although their axes are more on the "side", past a ninety degree tilt

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What about satellites orbiting their respective planets in the opposite direction (ie. opposite direction to the way the planet rotates)? I'm pretty sure they are considered to be in "retrograde motion".

with regards

No, it didn't. Planets don't stop spinning. Just out of curiosity, what makes you think it might have?

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As above, so below

Just before I get criticized too much, I didn't mean that planets never stop spinning. I just meant that they don't stop spinning "some years back." Over millions of years, sure, or after some tremendous collision perhaps.

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As above, so below

You talking about synchronous rotation?

BTW, I think that is the first time I have seen somebody quote themselves in a post straight after. :-?

with regards

I wasn't actually thinking of synchronous rotation, but rather a state in which there would be no centripetal force from any rotation. I can't see how it could happen in nature, but maybe if you took a fairly small planet and attached a bunch of rocketships to it and fired engines for a while, you could make the rotation such that there would be no centripetal force. I think this situation has something to do with Mach's principle.

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As above, so below

I don't know an awful lot about astrophysics and the like, but I doubt such a situation would become possible (like you suggested). A fairly small planet? Even a body only several kilometres in diameter would be difficult to force it into a non-rotational situation (and keep it that way!). If an object is in orbit around another, wouldn't it have centripetal force anyway?

with regards

One of the best visual examples of 'retrograde motion' is the opening to the TV series 'M*A*S*H'. The camera chopper approaches two other helicopters from the rear. It moves out to the right and passes them, making it appear as if the one chopper is flying backwards for a bit.

I actually had someone tell me that was the case once...'course, he was so discombobulated he made Jell-O® look stable...

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Username24:

Why do you keep asking all these strange questions?

Are you reading something somewhere that concerns you?

Or are you just bored?

I read on an astronomy website that venus does not have retrograde rotation, but is upside down instead. I prefer to think of it as upright and spinning oddly, but everyone has their reasons.

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J Pax

I don't mind him asking a whole bunch of questions, but he could at least have the decency to answer back to the people who have taken time out of their life to reply to his querie.

with regards

You can look at it that way, of course

Weirdly, a lot of astro websites, planetscapes.com for an instance, list both its rotational period as negative, and its tilt of axis as 180 degrees. It's just a convention, then, and has to be taken in context.

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Retrograde motion is really just a perspective effect that depends on where the Earth and any given planet or body happens to be in it's orbit at the time. It's been known and recorded for thousands of years, it's predictable and it has no implications for the safety or otherwise of the Solar system. As discussed, there are conventions about how proper and retrograde motion are described.

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By asking questions we sometimes get the wrong answers, from wrong answers we learn to ask the right questions.

This webpage, as well as many others, also include retrograde rotation in the category of retrograde motion. I think that is what the OP is mostly concerned about, but I could be wrong and they haven't been back

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