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Some planetary orbits occasionally can appear to move backwards to an observer on the Earth?

Does anyone know concise, clear, web-based visualizations, animations or tutorials that clearly show how this might come about?

I have difficulties imagining it "in my head". A link to an animated solar system simulation would be fine.

Here is a good static picture,but does anyone know even better ones?

And yes, I know that Wikipedia.com has some links, too.

Retrograde Motion http://wiki.astro.com/astrowiki/en/Retrograde_Motion

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This great animation was linked from the English Wikipedia page on app. retr. motion: alienworlds.glam.ac.uk/retrogradeOrbits.html And it is even customizable /you can select different viewpoints at different lat/lon values on the Earth. –  knb Nov 16 '11 at 20:45

3 Answers 3

up vote 7 down vote accepted

Here is an animation that I created to illustrate retrograde motion. You'll see Earth and Mars in orbit around the sun and a line going from Earth, through Mars, and to the "fixed stars." A glowing green path will follow the motion of Mars as seen from Earth projected against the stars. (linky)

I always try to use a diagram in my explanations because it's almost impossible to explain it just in words -- it is a very visual phenomenon (well, by definition it's a visual phenomenon).

Text to go along with the movie I've linked to would be: Take Earth and Mars at relatively near points in their orbit and draw a straight line between the two, projecting from Earth, through Mars, and towards the "fixed stars." Earth travels in its orbit faster than Mars, so as the planets move, the line you draw through them will start to slow down relative to the stars. When Earth catches up with Mars in the orbit, Mars will appear to star tot move backwards relative to the stars. As Earth-Sun-Mars form a right angle, Mars will again appear to stop in the sky and reverse direction as they both continue in their orbit.

See, even me re-reading that doesn't make much sense without the movie to explain it. Maybe someone will be able to contribute a better explanation.

However, there are two additional points to make, as well. First, all planets (and asteroids) show prograde and retrograde motion. The duration of each part is dependent upon how far away it is in the solar system and whether it's inner to Earth (Mercury, Venus) or exterior. Also, most planets spend most of their time in prograde motion. Retrograde means it goes against the direction it normally goes. Stars over the course of the night move East to West, as do all objects in normal orbits as seen from Earth. But relative to the stars, the sun, moon, and planets normally appear to go west to east. This is prograde motion even though it's "against" the stars. When the planet flips and goes east to west relative to the stars (again, over the course of one day, everything goes east to west, we're talking now about motion over the course of many days relative to star positions) then it's in retrograde motion.

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Um, I have to dispute almost every point in the last paragraph. You have the preponderance of retrograde and prograde reversed, and in this context both terms refer to motion of the planets against the background of stars, so it is misleading to introduce the terms as with/against the motion of stars. And you have the directions backwards. Planets ALWAYS move E->W, like every distant object, over the course of one night. However, their usual relative motion, against background stars, (prograde,) is W->E. The rare, retrograde relative motion is E->W superimposed on the 24-hour E->W. –  Andrew Nov 11 '11 at 3:20
    
en.wikipedia.org/wiki/Apparent_retrograde_motion. Great animation, though. –  Andrew Nov 11 '11 at 3:21
    
Yeah, got my terms mixed up, I wrote that when I was watching the clock to run for the bus. But, I disagree with your point that I got my overall motions wrong -- it's just how I think of the motion. I always think about it relative to the stars, so I just didn't clarify (which I now have) about the diurnal motion. –  Stuart Robbins Nov 11 '11 at 4:23
    
+1 for the animation. But it's definitely not the prettiest. Is there no prettier version out there in the vastness of teh interwebz? –  Warrick Nov 11 '11 at 10:02
    
To add, the way I normally (when I remember) remember prograde and retrograde is that the sun, over the course of many days relative to the stars, appears to move west to east. Going with the sun (the planets when prograde) is what ancient cultures who worshiped the sun would probably think of as "good" -> "pro." When it goes the opposite direction of the sun - relative to the stars over many days), that's against normal motion -> "retro." –  Stuart Robbins Nov 11 '11 at 21:49

Imagine passing another car on the highway. When you're far away, you only see it moving forward against the background trees, but when you get right up next to it, it can appear to move backwards for a moment. That's the most concise explanation I came up with in all the years I taught astro lab.

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You both ignored the fact that the reason the planets "appear" to move East to West is because of Earth's rotation on it's own axis.

Check the following link - it has a few animated GIFs that make it much clearer.

http://www.lasalle.edu/~smithsc/Astronomy/retrograd.html

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Planets moving East to West isn't retrograde motion, which is the relatively rare event when planets move backwards with respect to the background of the stars. Planets, stars, the sun, and the moon always move East to West because of the Earth's rotation. Look at the link you give. It explains it quite well. –  Peter Shor Mar 17 '13 at 16:28
    
Furthermore, the accepted answer did not ignore the fact that planets "appear" to move East to West over the course of a night, but explains it in the last paragraph. Please do not criticize other answers without reading them first. –  Peter Shor Mar 17 '13 at 16:37

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