Retrograde Motion and Its Misconceptions

Ever since we humans first looked up into the night sky, we have been fascinated by what we have seen there. Millions of stars, planets, and galaxies, forming patterns and shapes, moving and changing constantly. Throughout our history, scientists have peered into our night sky and tried to explain its many mysteries. One such mystery, which befuddled humanity for thousands of years, is an effect we now call “Retrograde Motion”. Many great minds have taken on the challenge of explaining this anomaly, with explanations differing wildly as science progressed and our understanding of the solar system advanced.

The Geocentric Model of the Universe

Early astronomers used to believe that the Earth lies at the center of everything, with the heavens revolving around it. Yet how could early astronomers find out that the night sky is not merely filled with light-emitting stars, but also with planets? After all, the naked eye can only see so much, and, without a telescope to aid it, it cannot differentiate between a light-emitting star trillions of kilometers away, and a light-reflecting planet hundreds of millions of kilometers away. However, they picked up on the fact that, in our night sky, there existed a few outliers, whose motion differed from that of the others. Early astronomers noticed that five heavenly bodies had some peculiar behavior in our night sky, and built the geocentric model of the universe based around that. They realized that these peculiar heavenly bodies were not like the other stars, but planets, and that they orbited the Earth in different spherical planes. They theorized that the planets orbited the Earth in the order: Mercury, Venus, followed by the Sun, then Mars, Jupiter, and Saturn (the naked eye is unable to see Uranus and Neptune, as they are far too dim). But what about the other stars? Well, they theorized that they were fixed points of light in a spherical plane surrounding Earth and the planets that orbit it. This is what allowed them to differentiate between stars and planets. On one hand, stars have a fixed position in our night sky relative to us and to each other. On the other hand, planets’ positions seem to deviate slightly from time to time. They used this newfound knowledge to create the geocentric model of the universe, which was believed by one of the great scientific minds of the time, Aristotle.

So, what is Retrograde Motion?

Retrograde Motion is an optical illusion that occurs when Earth passes planets that orbit the Sun further from Earth. If you were to look up into the night sky every night and note the position of Mars relative to the stars, given time, you would notice Mars moving east. In other terms, Mars appears to constantly move from west to east each night.

Every couple of months, however, Mars appears to change course. Its position appears to move from east to west with each passing night, backtracking until again it changes its course to move from west to east.

Why does Retrograde Motion occur?

What happened for this sudden change in the planet’s course to occur? Did the planet change its mind, backtrack, then change it again and continue?

This is why this phenomenon has baffled astronomers for generations, and why many explanations have surfaced over the years, all of which differ in the reason behind this event.

The Ptolemaic Approach

Claudius Ptolemy was an Alexandrian astronomer and mathematician, who created a model of the solar system that persisted for years until Johannes Kepler and Nicolaus Copernicus removed the Earth from the center of the universe and created a system largely similar to our own. Claudius Ptolemy theorized that the Earth was stationary and sat at the center of the universe, wherein planetary bodies such as the Sun, Moon, and other planets orbited the Earth in a series of perfect circles (early astronomers believed that heavenly bodies must move on the most “perfect” path). In the Ptolemaic system planets move along circular paths called epicycles, the centers of which move along a much larger path around the Earth called the deferent. Occasionally, the path a planet is taking will be opposite of the path that the epicycle is moving in, (at the points closest to Earth, planets will move opposite their epicycles, this is because one half of an epicycle runs counter to the general motion of the deferent path). The combined motion will sometimes cause planets to slow down, or even reverse, creating the retrograde motion.

The Truth Behind Retrograde Motion

In the early 16th century, Nicolaus Copernicus arrived at his theory of heliocentrism, wherein the center of the universe lies near the Sun, and that all planets orbit the Sun. Many believe that his model of the solar system may have been the launching point for modern astronomy, as his theory was vastly closer to the true shape of the solar system than Ptolemy’s model. As we have learned more and more about our universe and solar system, we have uncovered and debunked many misconceptions that used to plague ancient astronomers. 

Astronomers now explain retrograde motion in a much simpler and more logical way than before, and the explanation lies at the heart of heliocentrism. The planets orbit the Sun in the order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and so on and so forth. It is logical to expect that the planets with orbits further out from the Sun than our own, such as Mars, would orbit the Sun more slowly, since the path that they have to take is much longer. The time that Mars takes to orbit the Sun is 687 days, whereas Earth orbits the Sun in 365 days. This means that Earth orbits the Sun in about half the time that Mars does. What this means is that every two years or so, the Earth, with its much faster orbit, catches up with, and overtakes, Mars. If you have ever been in a car on the highway, and have overtaken another, had you looked at the car you were passing, it would seem to slow down, stop, and move in reverse. This is merely an optical illusion, as when you finally pass the car it would seem to continue its forward motion. Now if we apply this principle to Mars, everything suddenly makes more sense. As the Earth begins catching up to Mars, its position in our night sky begins to backtrack, moving east to west, until we finally pass its orbit and it continues its motion from west to east.

Nicolaus Copernicus
Apparent retrograde motion of Mars in 2003, Wikipedia

How Galileo Strengthened Heliocentrism

On the night of the 7th of January, Galileo pointed his telescope at Jupiter and noticed three “small fixed stars”, previously gone unnoticed by the naked human eye. He noted their positions relative to each other and to Jupiter for that night and the next few nights. He noticed that their positions were changing. One of them had disappeared, and a fourth one had emerged. He quickly realized that these weren’t dim small stars, but in fact moons. This revelation, the fact that planets other than Earth had moons of their own, kicked up a storm in the scientific community, due to the fact that there had been moons orbiting planets other than our own, and had gone previously unnoticed, and this went a long way in aiding the heliocentric model of the universe.

Advanced Technology Expands Our Perception of the World

We know now that the Earth and all other planets in our solar system orbit the Sun, which is why it is so easy to explain retrograde motion. Yet, with early astronomers and their misconceptions about the way the universe worked, we must acknowledge their genius. The fact that they were able to convincingly explain retrograde motion, is astounding. Early astronomers were mathematical geniuses, able to accurately predict planetary motion, and precisely calculate the radius of the Earth using rudimentary tools and a bit of critical thinking. One such example is when Eratosthenes, an ancient Greek mathematician who lived nearly 2200 years ago, calculated the circumference of the Earth to a frighteningly accurate degree using a stick and some simple math.

We are lucky enough to be living in a time where technology is advancing at an unprecedented rate, and our scientific knowledge along with it, and to have that knowledge be widespread and easily accessible at the touch of a button. Early astronomers and scientists were bound by the technology of their time, yet made great scientific discoveries nonetheless. We are not bound by those same problems, thus we are able to exponentially advance the collective human knowledge of our world, and what lies beyond.

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