Why does Mars orbit in retrograde motion?

I am reading Big Bang by Simon Singh and it is attempting to explain to me how Mars rotates in retrograde motion. Apparently, Jupiter and Saturn exhibit the same behavior.

It states:
Mars moves in the sky toward the left. As the Earth begins to pass Mars it pauses on center, then as earth passes, mars begins to move toward the right in the sky. Then, the further the Earth passes, Mars changes directions once more and moves in the sky back toward the left.

I’m having difficulty understanding this concept. I just can’t visualize it. Could anyone help?

Mars, of course, orbits the Sun in a nice, orderly ellipse with no reversals of direction. Because the Earth does the same (and, being closer to the Sun, takes less time to complete an orbit), an observer on the Earth will see Mars do what you describe. It’s the combined motion of Mars and the Earth that leads to this apparently complex motion.

This page has a nifty Java applet that demonstrates Mars’s retrograde motion. I imagine that many of the other Google hits for “Mars retrograde motion” have other demos.

Think of when you’re driving along the freeway and you slowly pass another car. They seem to be moving backward relative to you, they’re really just moving slightly slower in the same direction and therefore “receding.” The same thing is happening with retrograde motion, only in a circle instead of a straight line.

I believe, from earth, ALL the planets can look retrograde.

Keep in mind that “retrograde motion” describes only the appearance of Mars against the background of the fixed stars, as seen from here on Earth, and not its actual motion in orbit.

The analogy of a car you’re passing appearing to move “backwards” is a good one.

Good Egg, it’s been 20 years since I studied this, but I’m pretty sure that the inner planets, Venus and Mercury, do not exhibit retrograde motion.

commasense, you obviously have never read anything astrological. They claim venus and mercury retorgrade all the time. Well, not all the time, and not for long periods but they do.

There’s a nice animation of the retrograde motion of Venus at the bottom of this page.
The panel on the right shows the lines of sight between the two planets at various points in their orbits, while the left panel shows the projection of those sight lines onto the background of stars.

You’re absolutely right. Once I realized, about 30 years ago, that astrology was total bullshit, I stopped reading about it.

I Googled “Venus” and “retrograde,” and pretty much all I got was astrological Web sites. On real astronomy sites, all they spoke about was the fact that Venus’ rotation is retrograde. Which I took as confirmation of my understanding that in the real world (unlike the fantasy world of astrologers) Venus does not exhibit retrograde motion in its apparent path across the sky.

But digging deeper I found this astronomy lesson from Eastern Michigan University that speaks about the conditions in which Venus and Mercury retrograde. So I grant that it happens, and I was mistaken before.

But I wasn’t willing to take an astrologer’s word for it. (Venus in retrograde is apparently much more important to astrologers than it is to astronomers.)

I just remembered why I thought Venus and Mercury didn’t retrograde: because unlike the outer planets, you can’t see it happen without very special equipment.

It took a few minutes for something I read on that EMU site to sink in: “Inferior planets retrograde near inferior conjunction.” In other words, only when they are very near the Sun in the sky, or even transiting its face. Take a look at Figure 8.1.

So the ancients probably inferred that it happened, but could never see it.

Oh, I stopped reading astrology bunk long ago too. But I remember a lot of it, especially the “never write letters or go on vacation while Mercury is retrograde, as there will be miscommunications all around.” So I figured they knew that bit of real astronomical info, that they can appear retrograde. And it makes one wonder whether a certain politician was born during a mercury retrograde. :wink:

The apparent retrograde motion of the planets caused a lot of headaches with the early geocentric models of the solar system, which assumed a) all orbits were perfect circles instead of ellipses, and b) everything orbited the Earth.

Early astronomers invented little curlicues for the other planets, so that instead of moving steadily in one direction on a circle, the planet would do a quick loop-the-loop and resume its orbit. There’s a picture here.

If you imagine what a loop-the-loop looks like from the side — a deceleration, reverse of direction, and reversal again — that’s retrograde motion. The planet really isn’t doing a curlicue, of course, but it can appear to do so relative to the starfield behind it.

Here’s an analogy that might or might not help, but that I found amusing when I thought of it.

Consider that you’re a track star in a really huge stadium, with gigantic billboards showing beneath the audience stands. You’re running in a fairly small circular track near the center of the stadium.

Mars is another runner, going slightly faster than you, but about a track that’s about twice as far out as yours is, so it takes him quite a lot longer to run around a full circle. You’re both going around counterclockwise without stopping.

Most of the time, if you’re not right on the same part of your tracks, it’ll be easy to see that Mars is going counterclockwise by looking at his progress relative to the billboards. However, when you’re in the process of ‘lapping’ him and passing him by, your perspective will be shifting faster than Mars is moving, and so judging by the billboards, it’ll look like he’s going ‘backwards’, ie counterclockwise.

The real billboards, ie the stars in the constellations of our sky, are unimaginably distant of course, but the analogy still holds up I think. :slight_smile:

And the discussion would not be complete without the mention of where the word “planet” comes from. The Greek word πλανήτης, meaning wanderer (due to the observed loopty-loop movement).

Heh heh - the google ads are all for real estate in Jupiter, FL.

Nitpick: it wasn’t specifically because of the retrograde motions that they were called wanderers, but just because, unlike all the fixed stars, which don’t change their positions with respect to one another, the planets move against that background.

Messy Paint: you don’t say where you are located, but if you have a planetarium nearby, you might stop in some day and ask them to show you retrograde motion. Most planetarium staffers are happy to talk with the public about this kind of thing, and if they do a live star show, they can probably work it into the next one for you. He/she might even offer you a private show. When I gave planetarium shows, I would occasionally talk about and show retrograde motion.

BTW, we haven’t really discussed the actual appearances of planetary motion. So in case there’s no planetarium near you, let me talk about that for a minute.

If you go out tonight and look at the sky, you should be able to see Mars, bright and reddish in the southern sky a few hours after sunset. Over the course of a single night, it would be hard to observe it moving with respect to the stars nearby, but if you could take a snapshot of its position tonight, and again tomorrow, and the next day, you would see that it was moving slowly from west to east. If you had a star chart, you could plot that motion.

If you did, you’d find that the motion was not smooth and regular. It speeds up and slows down, and every so often it stops and goes in reverse (westward), perhaps tracing out a loop before moving forward (eastward) again.

As for the reason, we’ve discussed that: it’s because the motion of the earth, “passing on the inside,” is superimposed on the proper motion of the planet itself. If we could watch from the point of view of the sun, we’d see no retrogradation.

I hope this and the answers from the other posters have helped a little.

thank you all for your examples and in-depth explanations…between reading this stuff and looking at some of the demos provided…its clearer to me now…

commasense, i am in cambridge, mass so theres gotta be some sort of planetarium around boston…i think id like to go talk to one anyway, especially if they typically welcome a conversation…

The inner planets (Mercury and Venus, for those who need a playlist) do not display the apparent retrograde motion of the outer planets. This is because, moving “faster” through their orbits than the Earth, they are never “overtaken”. They will appear to slow down (as they approach their closest pass to Earth) and speed up (as they recede away), but they never show the stop-reverse-return motion that so confounded Aristolians and led the Ptolomaic scholars, speficially Apollonius of Perga to propose the epicycle-based model. This is all due, as previously mentioned, to persistent beliefs stemming from the long disproven theory of geocentricity.

As the inner planets can’t be seen (by eye) at closest approach, being directly into the sun from Earth’s view, the ancients may have believed that they, too, engaged in retrograde motion, but in fact they do not. Instead, they behave, as all celestial bodies do, as described by the Keplerian model, modified only by the addition of general relativity at the extremes of velocity and gravitational distortion, though it was of course Nicolaus Copernicus who suggested the heliocentric model, albeit with circular rather than elliptical orbits. (This led Copernicus to suggest more, but smaller epicycles to explain aberations away from perfect circles.)

This information continues to linger in the guise of astrology, which is as large a bunch of crackpot pseudoscience as has ever been passed off as valid theory. One would think, with irrefutable evidence of astronomy and interplanetary space exploration that it would be as defunct as the four bodily humours and phlogiston theory, but alas, such bunk still guides the decision-making processes of everyone from your flaky hairdresser to certain sage and respected world leaders.

Hey, it’s cheaper than heroin , and perhaps not quite as brain-rotting as daytime television. But I’d rather read tea leaves; at least that way, I’d get to drink a delicious and healthful beverage with my steaming plate of bullshit pie.

Stranger

heavens above is what i use for star charts. Do you happen to have anything better or more useful than this site?

Sadly, I cant see to many stars, but i should be able to find mars anyway right?

The Museum of Science has one. I’ve been there twice in the last six months, and many times before, too, but I don’t think I’ve actually been in their planetarium. But it’s one of the best in the country.

Their Sky Tonight show is the one you want to check out. I’m guessing it’s a live show in which operator is free to talk about whatever he wants. Fridays at 7 pm and Saturdays and Sundays at 12:30. If you go on a clear Friday evening, the roof-top observatory is open after the show.

I wouldn’t be surprised if Harvard, MIT, and other institutions around town have planetariums too, but I don’t know off-hand.

Harvard has an observatory that is open once a month to the public. (At least, that was the schedule last time I went.) I’m sure it’s somewhere on their website.

Uh, obviously my last post referred to Messy Paint’s post #15. The museum has a planetarium, not a Mars.

Mars is one of the brightest objects in the sky and should be easily visible even if the seeing isn’t great because of light pollution in the city. But if you can’t see many other stars, it will be hard to note Mars’ position if you want to try to track its motion over time.

The star chart over at Heavens Above shows tonight’s sky at about 10 pm local time. Note that Mars will be a little to the right (west) of Antares, the brightest star in Taurus. This is a potential source of confusion because Antares is quite bright and also reddish, like Mars. In fact the name, Antares, means rival of Mars, i.e. Anti-Ares, Ares being another name for Mars.

Keep in mind when looking at that chart that the southernmost third or so of that circle will not be visible to us here in the Northern Hemisphere.

Stranger: thanks for straightening us all out on the inner planets. I was pretty sure that they didn’t retrograde, and seeing no references to it at all on most astronomy sites, I wanted to be categorical. But I’ve learned the lesson of hubris, and also to distrust my memory at the advanced age of 50. Score one for my poor old brain.