What Is Retrograde Motion?

~3 min
Mars in retrograde

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.

You can also check out our infographic, where we explain how retrograde motion works.

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 June 20, the superior planet Jupiter will begin its retrograde motion in the constellation Aquarius. The planet will resume its direct motion only on October 18; at this moment, it will be positioned in the constellation Capricornus. Jupiter goes retrograde every nine months for about four months at a time.

If you’d like to follow the path of any planet in retrograde across the sky, use the stargazing app Star Walk 2. Tap the magnifier icon in the lower-left corner of the screen, type the planet’s name in the search field, and tap the corresponding search result. You will see the current position of the celestial body in the sky. You can also view the planet’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 September 27 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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Image Credit:Vito Technology
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