Retrograde Motion

by Sam Atkins

If you ignore the westward movement of objects across the sky due to Earth’s rotation, the planets’ positions in the sky against the background stars generally move eastward throughout the year (right-to-left when facing south). This is due to their counterclockwise orbits around the Sun. However, periodically, planets will appear to stop, reverse direction, stop again then continue on in their usual direction. This usual direction is known as prograde motion while the backwards motion is known as retrograde motion. Depending on the planet, this retrograde motion can last several weeks to several months. But are the planet’s really doing this? What causes this strange phenomenon?

While prograde and retrograde motion are much more simple than you might be imagining, I want to walk you through them step by step. The main reason being that when it comes to tracking objects in space, especially for people that aren’t used to it, things can get very confusing because everything is moving all the time. In fact, things often appear to move as a result of several things happening simultaneously. It can be difficult to distinguish why something appears to be moving one direction when in reality it’s actually moving in the opposite direction.

REMINDER: Take note when a planet is said to appear to be doing something versus when it is actually doing something. All of this is about perspective and relative motions.

As said before, all the planets revolve around the Sun in the same counterclockwise direction but none at the same speed. The closer a planet is to the Sun, the faster it orbits while more distant planets are slower. Earth is the third planet from the Sun, meaning that Mercury and Venus have faster, inferior orbits around the Sun while the planets of Mars, Jupiter, Saturn, Uranus and Neptune have progressively slower, superior orbits.

Let’s look at the orbit of Jupiter as seen from the orbit of Earth:

Let’s imagine Earth and Jupiter around the Sun, using the hours on a clock to denote their respective positions. Earth is at the Sun’s 9 o’clock and Jupiter is at the Sun’s 12 o’clock. Both are traveling counterclockwise but Earth is traveling much faster. In the time Earth travels all the way around and back to 9 o’clock, Jupiter will only travel one hour on the clock, from 12 to 11. For simplicity’s sake, let’s lock Jupiter to the 12 o’clock position.

As Earth begins to travel around the Sun, Jupiter appears to move from right to left against the background stars. Pay attention to how the red arrow changes direction as Earth’s orientation to Jupiter shifts.

Earth continues on to 6 o’clock where the Sun at the center lies between the two planets. We perceive this on Earth as Jupiter getting closer and closer to the Sun each evening until it disappears into the glare of daylight. Jupiter’s closest pass to the Sun is known as a solar conjunction.

Earth continues past 6 o’clock as Jupiter emerges from the other side of the Sun, still moving from right to left against the background stars but now visible in the morning sky.

Once Earth reaches 3 o’clock, Jupiter’s right to left motion will appear to stop. By this point, Earth’s angle of motion in its orbit is now sending it towards Jupiter.

Remember that both planets are still moving counterclockwise, but as said before, Earth is moving faster. Earth is now coming up behind Jupiter and will gradually close the distance. As Earth catches up to Jupiter, the gas giant’s slower counterclockwise motion not only appears to stop but will actually begin moving backwards (this time from left to right against the background stars). This is the retrograde motion we’ve been looking for.

A good analogy to explain this effect would be you and another car driving parallel on a highway. If you are driving faster, the other car will appear to move backwards even though you both are moving the same direction.

Earth eventually has caught up to Jupiter in its orbit, putting it on the opposite side of Earth as the Sun. This is orientation is known as opposition. It is here that the speed of Jupiter’s retrograde motion is most pronounced because they are closest to each other and Earth’s angle of motion relative to Jupiter is most parallel.

Upon passing opposition, Earth will continue to pull ahead of Jupiter, but the gas giant’s retrograde motion will begin to slow as Earth's orbit changes the angle with which it moves relative to Jupiter. This time, Earth is moving away from the gas giant.

When Earth finally comes back around to its original 9 o’clock position, Jupiter’s retrograde motion comes to a stop. The gas giant will again resume its normal prograde motion as Earth continues back around the far side of the Sun once again to repeat the process over.

Now, if we hadn't locked Jupiter to the 12 o’clock position it would have made it to the 11 o’clock position upon Earth completing a full orbit back to the 9 o’clock position. Honestly, the only difference here would be that Earth would have to reach further around its orbit to hit all of the previously mentioned steps.

We see this same pattern of prograde and retrograde motions among all of the superior planets. However, the further away the planet is, the less pronounced the retrograde motion appears in terms of both speed and angular distance.

Notice the more extreme difference in the two angles for the closer planet, Jupiter, than for Saturn.

In essence, the retrograde motion of planets is simply an illusion as Earth speeds past the more distant and sluggish planets, the same way a car speeds past another on the highway. You should also take note and appreciate how the size and brightness of the planets change as Earth moves closer and further away. It is when a planet’s retrograde motion is most pronounced that it is best positioned for observation through a telescope as that coincides with Earth’s closest approach. The solar system provides us an elegant microcosm of the cosmic ballet that endlessly twirls and glides around us in every direction. When we are able to breakdown each and every flourish, we come closer to a better understanding of how the forces of gravity choreograph this beautifully complex yet intuitively simple dance and the dancers within it.