What Is Retrograde Motion?

Star Walk
3 min readMay 22, 2021

Why do planets sometimes seem to move “backward”? Is such motion even real? Should you fear Mercury retrograde? You’ll find answers to these questions in our article.

What does retrograde mean?

When a planet seems to reverse its direction in the sky, it’s called retrograde motion (from the Latin word retrogradus — “going backward”).

Day to day and week to week, as the Earth revolves around the Sun, the planets in the sky typically move in the same direction as the Sun — from west to east. Astronomers call it direct or prograde motion. This motion shouldn’t be confused with the daily motion of the planets and the Sun in the sky which goes from east to west and is caused by the Earth’s rotation on its axis.

At specific periods of time, a planet can start moving “backward” — from east to west. This westward movement is called apparent retrograde motion.

What causes the apparent retrograde motion of the planets?

Retrograde motion is an optical illusion caused by differences in the planets’ orbital speed.

Let’s take Mars as an example. This superior planet moves slower in its orbit than the Earth. When we pass Mars, it seems to be moving “backward” because we’re moving faster than it is. The same thing happens when you pass by a slower-moving car on the highway — for a moment, it appears to move in the opposite direction.

This mechanism works for all the superior planets. The inferior planets, Venus and Mercury, that orbit the Sun faster than the Earth, also periodically appear to move “backward”. However, their retrograde motion is hard to observe: when an inferior planet passes us by, it is positioned between the Earth and the Sun, so the Sun’s glare hides it from view.

Ancient astronomers were rather puzzled by the retrograde motion phenomenon — especially those who thought that the Earth was at the center of the Universe. Only in the 16th century, when Nicolaus Copernicus introduced his heliocentric model, scientists understood that retrograde motion was an illusion.

What planets are in retrograde right now?

On May 23, the superior planet Saturn will enter a retrograde loop. It will resume its direct motion only on October 11. During this period, it will be positioned in the constellation Capricornus.

If you’d like to follow Saturn’s path in the sky during this period of time, use the stargazing app Star Walk 2. Tap the magnifier icon in the lower-left corner of the screen, type “Saturn” in the search field, and tap the corresponding search result. You will see the current position of the ringed planet in the sky. You can also view Saturn’s location for any chosen date and time using the Time Machine feature.

In 2021, you’ll also have the chance to witness retrograde motion of other planets. Mercury will go retrograde from May 29 through June 22, and then one more time from September 27 to October 18. Also, Jupiter will be in retrograde from June 20 to October 18 and Venus — from December 19, 2021 to January 29, 2022.

How do planets in retrograde affect us?

Astrologers may warn you against signing contracts during Mercury retrograde or buying metal tools during Mars retrograde. It’s your choice to believe them or not.

However, you now know that retrograde motion is just an illusion that can’t possibly affect your everyday life — at least from the scientific point of view.

We hope that we’ve managed to explain the nature of the retrograde motion phenomenon to you. If you liked the article, please share it with your friends!



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