Demystifying the Optical Illusion of Retrograde Motion

Demystifying the Optical Illusion of Retrograde Motion

Demystifying the Optical Illusion of Retrograde Motion

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By Dr. Gary Deel, Ph.D., JD
Faculty Director, School of Business, American Military University

Imagine you take a midnight stroll on a dark summer evening, and during your walk, you happen to notice an object in the sky behaving oddly. It looks like a star, but it moves across the sky in a strange way… forward, then backward, then forward again. You’re probably thinking you are about to have a close encounter with ET.

Interestingly, this kind of behavior happens regularly in the night sky, but it has nothing to do with extraterrestrials. Retrograde motion is the name we’ve given to the optical illusion that we see here on Earth when a planet appears to move forward, backward, and forward again in the night sky.

Planets’ Retrograde Motion Has Long Puzzled Astronomers throughout History

Granted, this apparent motion happens over a period of weeks or months, not seconds. But it was no less perplexing to astronomers over centuries of recorded history.

In fact, observers wondered about this curious night sky behavior for thousands of years. But it wasn’t until Copernicus and Galileo in the 16th and 17th centuries that the mystery was finally solved through the assertion of the heliocentric model.

We call the retrograde motion that we see “apparent” because the planets which exhibit it appear to be changing directions from our vantage point, but in reality, there is no such change in the planets’ motion. Instead, it is just an illusion caused by the dynamics of the solar system.

Why the Illusion of Retrograde Motion Occurs

We see this illusion because planets orbit the Sun at different speeds. Planets orbiting closer to the Sun go around faster than planets orbiting further out.

As an analogy, visualize a figure skater spinning on one leg in the middle of an ice rink. When the skater tucks her arms and legs in close to her body, she spins more quickly. But when she extends her arms and legs outward, her spin slows down. The same thing happens for planets, causing them to orbit at different speeds.

Because of these differences, observers on a given planet (e.g. Earth) see that the other planets appear to move around in odd ways as the vantage point from which other planets are seen changes.

This perceptual illusion is called the parallax effect. And the closer a planet is to us, the more extreme the illusion is.

Experimenting with the Parallax Effect

The simplest way to demonstrate the parallax effect is to hold one finger out at arm’s distance and eye level and close one eye. Note the location of your finger relative to the background. Then switch eyes by closing the open eye and opening the closed one.

You should observe a subtle difference in the location of your finger against the background. Then, slowly bring your finger in toward your nose and repeatedly switch open eyes back and forth. Notice that the change in location of your finger against the background from one eye to the other gets larger as your finger gets closer to your eyes.

Other Celestial Objects May Also Appear to Move in Different Ways

Retrograde motion is not a phenomenon that is exclusive to planets. This illusion occurs with comets, asteroids and any other bodies close enough to Earth to exhibit the parallax effect relative to background stars.

But we commonly talk about retrograde motion with respect to planets. The effects are far more regular and easier to observe than with rarer, fainter objects.

Mars is one example. Every two years or so, Mars appears to move across the sky at night and does a complete reversal of movement. It then doubles back and continues on its original path.

Why does this movement occur? Mars takes about two Earth years to complete one orbit around the Sun. As a result, the Earth passes Mars in its orbit every two years or so.

And what happens when we on Earth pass Mars? We see it from a different perspective against a different background of stars.

An Experiment in Planetary Motion to Do at Home

Try this experiment at home. Go out to your backyard with a friend. Ask your friend to walk in a large, slow circle around the yard.

Now, inside the large circle your friend is making, walk in a faster, smaller circle in the same direction of rotation. For example, if your friend is walking slowly in a circle that is 25 feet in diameter, walk more quickly in a circle that is 15-20 feet in diameter. As such, you should pass your friend repeatedly as you make faster, smaller circles inside of your friend’s larger, slower circles.

After you both set off walking, take a picture of your friend with a camera or smartphone every few steps from wherever you both happen to be at each moment. Once you’ve made a few laps in your circles and taken at least a dozen or so pictures, stop and review your photos in sequence.

If you did this experiment correctly, you should notice in the photos that your perspective on your friend and that background behind your friend should have changed. During your circular walk, you would have moved from behind, to alongside, to ahead of your friend, over and over again. And this means that at each moment you would have seen your friend from a different vantage point, depending on where you both happened to be in your “orbits.”

Note that your friend did not change his or her circle or speed. It was the combination of your changing positions relative to each other that created this effect. The same motion occurs with planets and their orbits, and this is why we see the illusion of retrograde motion.

Scientists have wondered for centuries about the source of the mysterious motion observed by planets and other objects in our night sky. And it turns out that, as with most complicated problems, the solution is dependent on one’s perspective. Apparent retrograde motion just happens to embody that point, literally.

About the Author

Dr. Gary Deel is a Faculty Director in the School of Business at American Military University. He holds a JD in Law and a Ph.D. in hospitality/business management. Gary also holds a bachelor’s degree in space studies and is an avid student of the astronomical sciences.

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