mars-retrogradeIf you watch the moon, you’d notice that it rises in the east and sets in the west. This direction is called ‘prograde motion’. The stars, sun, and moon all follow the same prograde motion, meaning that they all move across the sky in the same direction.


However, at certain times of the orbit, certain planets move in ‘retrograde motion’, the opposite way. Mars, Venus, and Mercury all have retrograde motion that have been recorded for as long as we’ve had something to write with. While most of the time, they spend their time in the ‘prograde’ direction, you’ll find that sometimes they stop, go backwards, stop, then go forward again, all over the course of several days to weeks.


Here are videos I created that show you what this would look like if you tracked their position in the sky each night for an year or two.


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Mercury and Venus Retrograde Motion

This is a video that shows the retrograde motion of Venus and Mercury over the course of several years. Venus is the dot that stays centered throughout the video (Mercury is the one that swings around rapidly), and the bright dot is the sun. Note how sometimes the trace lines zigzag, and other times they loop. Mercury and Venus never get far from the sun from Earth’s point of view, which is why you’ll only see Mercury in the early dawn or early evening.




Retrograde Motion of Mars

You’ve probably heard of epicycles people used to use to help explain the retrograde motion of Mars. Have you ever wondered what the fuss was all about? Here’s a video that traces out the path Mars takes over the course of several years. Do you see our Moon zipping by? The planets, Sun, and Moon all travel along line called the ‘ecliptic’, as they all are in about the same plane.



 
Download Student Worksheet & Exercises


Exercises


  1. During which of the months does Mars appear to move in retrograde?
  2.  Why does Mars appear to move backward?
  3. Which planets have retrograde motion?

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Comments

15 Responses to “Retrograde Motion”

  1. Nope, the object doesn’t actually have loops in its orbit. As you said, retrograde describes the illusion that the object is looping.

  2. mariamitchellmk says:

    So is retrograde an illusion because of where we are in orbit or do they really have loops in their orbit?

  3. ALL planets exhibit retrograde motion. It doesn’t make sense to include the Earth because it’s your reference point, so by “all” we mean all 7 planets.

  4. gracielabecerra2009 says:

    Hello! Not really understanding this one in general unfortunately. Also, #3 says all planets spin in retrograde motion but I read earth does not. Can you please help me?
    Thank you!

  5. It’s because of where we are in orbit. It’s like we’re catching up to it, so it appears to move backwards, but then it moves forward again when we are past a certain point in our orbit. Do the exercise in the worksheet below the videos to help it to be more clear! 🙂

  6. Julie Germaine says:

    For number two, is it why does Mars appear to move backwards during orbit or during Retrograde?

  8. Caroline Wood says:

    are there any celestial bodies that do the whole orbit backwards?

  10. Yes, if you track the course of Mars night after night, you’d see it start to head backwards at one point.

  11. Caroline Wood says:

    Why do they do this and has the earth ever done this as far as we know(Retrograde Motion)? Do they always do it at the same point in the orbit?

  12. Caroline Wood says:

    Do these planets really go backward like that?

  13. sevy keble says:

    It did? Are scientists tracking it?

  14. I think that’s 3 years worth of tracking. Did you know that Mars started retrograde motion this week?

  15. sevy keble says:

    In the “Mercury and Venus Retrogade Motion” video, how much time is that showing?
    sevy keble 🙁