What is Planet Retrograde Motion?

If you carefully observe the location of certain planets, you may notice that at times they appear to move backward. After a short period, they return to their normal direction of travel in the night sky. This is caused by planet retrograde motion.

Understanding retrograde planet motion can be a daunting task for beginners in astronomy. However, anyone can attain a conceptual understanding of this feature.

As will soon become apparent, the phenomenon is called apparent retrograde motion. The planets do not actually move backward.

The word retrograde simply means move backward. So what is being discussed is the apparent backwards movements of planets.

In this article, we will investigate why this occurs, identify which planets it is most evident on, and describe when it occurs.

What is Retrograde Planet Motion?

In astronomy, retrograde motion can either be actual or apparent. Retrograde motions occur in a body if it moves in the opposite direction of most other bodies in the solar system. For example Venus and Uranus rotate in the opposite direction of the other planets. This article focuses on the apparent retrograde motion of planets.

Mars retrograde motion

Retrograde planet motion is an illusion caused by the different orbital speeds of planets. When a planet overtakes another, an observer will perceive backward motion. This illusion is known as apparent retrograde planetary motion.

To illustrate the effect, I will use Mars as an example. Because Mars is further from the Sun than Earth, it takes longer to orbit it. As a result, the Earth overtakes Mars periodically. Subsequent Mars retrograde motion events occur about every 26 months. Jupiter and Saturn go into retrograde motion more regularly.

Mars appears to move backwards against the background stars during the overtaking event. The same effect is observable if you are in a car and move past a slower-moving vehicle. The illustration on the right shows Mars appears to slow in Earth’s night sky as it approaches the planet. As it starts to overtake Mars, the planet seems to move backward in relation to the stars behind it.

In the illustration, ‘S’ is the Sun, ‘E’ is the Earth and ‘M’ is for Mars. How the planet appears in Earth’s night sky is denoted by M’. Note that at no time does Mars actually move in the opposite direction.

When Can We Observe this Motion?

There are two situations where apparent planet retrograde motion occurs.

The motion can occur when the inferior planets (i.e. those that orbit between the Earth and Sun – Mercury and Venus) overtake Earth. However, because the planets are located between Earth and the Sun during the event, they are lost in the Sun’s glare. Due to this, amateur astronomers are not able to observe them.

The apparent retrograde motion is easily observable in superior planets (i.e. those that orbit outside of Earth’s orbit around the Sun). The best planets to observe are Mars, Jupiter and Saturn. All these planets can be observed with the naked eye, binoculars or a telescope. The motion occurs when they are in opposition.

Opposition is when a superior planet is on the opposite side of the sky as the Sun. It is the time when they are the closest to Earth.

Mars being closer to Earth makes the effect easier to observe.

If you precisely document your observations over a number of nights, you will notice that the planet also zig-zags a little or forms a loop. This is because the orbits of Mars and Earth are not precisely aligned. The pattern created is different for each event.

Below are the next dates of retrograde motion in these planets. If you wish to observe apparent retrograde motion it is best to begin your observations a little before the retrograde motion begins. Note the location of the planer in relation to the stars near it.

Mars
December 6, 2024 to February 23, 2025 (Currently in retrograde motion)
January 10, 2027 to April 1, 2027

Jupiter
October 9, 2024 to February 4, 2025 (Currently in retrograde motion)
November 11, 2025 to March 11, 2026
December 13, 2026 to April 13, 2027
January 12, 2028 to May 13, 2028

Saturn
June 29, 2024 to November 15, 2024 (Currently in retrograde motion)
July 13, 2025 to November 28, 2025
July 26, 2026 to December 10, 2026
August 9, 2027 to December 24, 2027
August 22, 2028 to January 5, 2029

How Can We Observe Retrograde Motion?

Retrograde planet motion can be observed from Earth by tracking the planet’s position over an extended period. We can use binoculars or telescopes to observe the motion.

You can use planetarium software to help you locate the planet you wish to observe.

To observe the motion, you will need to do so by making observations on different nights. This is where your drawing skills can be practiced. Start your observations before the planet begins its apparent retrograde motion. In each session, draw the planet’s location in relation to the stars in the background. Take the time to precisely draw the relative positions of at least five nearby stars. During the next observing night, compare the planet’s location to the stars. As time progresses, you should see that the speed of the planet’s movement should slow before beginning its backward motion. Before the planet resumes its normal direction of movement, it should again slow.

If your setup allows photographing the planet’s location on each occasion is an excellent option. If you are using a telescope holding a phone to the eyepiece will achieve the required result. This will take a little practice.

Through careful observation, we can better understand retrograde planet motion. This can help us better understand orbital mechanics. Planet retrograde motion can be an exciting and rewarding learning experience for any astronomer!

Motion of the Moon

While this article is about planetary retrograde motion, it would be amiss not to mention the Moon. Because the Earth completes a rotation (i.e. a day) faster than the Moon’s orbit, it appears to orbit from west to east. The Moon orbits from east to west like the planets.

Summary

Retrograde planet motion is an apparent reversal of a planet’s direction of orbit from its usual eastward path across the night sky. It appears that planets are moving backward in their orbit for several months before resuming their regular path. It occurs when the Earth ‘overtakes’ other planets as they move around their orbits.

Anyone can observe the motion and with the aid of the information above you know when it will occur to easily visible planets.

Robert Findlay
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