Retrograde motion explained: what is retrograde motion in astronomy
Why do planets sometimes seem to move “backward”? Is such a motion even real? Should you fear Mercury retrograde? You’ll find answers to these questions in our article.
Contents
- What does it mean when a planet is retrograde?
- What causes the apparent retrograde motion of the planets?
- What is in retrograde right now?
- How to see the retrograde motion?
- How do planets in retrograde affect us?
- Bottom line
You can also check out our infographic, where we explain how retrograde motion works.
What does it mean when a planet is retrograde?
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?
The apparent retrograde motion is 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 is in retrograde right now?
The elusive planet Mercury will be moving “backwards” in the sky from August 23 to September 15, and from December 13 to January 1, 2024.
The following Solar System planets also go retrograde in 2023:
- Saturn: June 17 - November 4
- Neptune: June 30 - December 6
- Venus: July 22 - September 4
- Uranus: August 29 - January 27, 2024
- Jupiter: September 4 - December 31
To see the retrograde motion of Mars, we’ll have to wait until 2024. It will begin on December 6 and end on February 24, 2025. By the way, in the middle of this period, Mars will be at opposition and shine brighter than usual.
How to see the retrograde motion?
To observe the retrograde motion, you’ll need to mark the position of a planet in the sky every day at the same time. Combine the results of your observations over several weeks, and you’ll see a planet outlining a loop.
If you’d like to see it in fast forward, use the stargazing apps Star Walk 2 or Sky Tonight. Both of them have the Time Panel feature, which allows tracking the motion of any planet on the sky map over the desired period of time. Here’s how to see the retrograde motion of a planet via Sky Tonight:
- The retrograde motion is best seen against the backdrop of the equatorial grid. To enable it, tap the Quick Settings panel at the bottom of the screen and tap the globe icon once;
- Tap the magnifier icon at the bottom of the screen and type the name of a planet (e.g. Venus) into the search field;
- Tap the blue target icon opposite the name of the planet and see its current position on the sky map;
- Open the Time Panel at the upper part of the main screen, set the date (e.g. July 15), then tap the time stamp and drag the slider until Venus is highest in the sky;
- Tap the date and swipe the slider with a quick motion so that the date changes and the time stays the same. You’ll see Venus first moving normally until July 22, then beginning to move “backwards” until September 4, and then returning to prograde motion again.
Also, watch our video tutorial on the Time Panel in Star Walk 2 and Sky Tonight.
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.
Bottom line
Retrograde motion is an apparent reversal of the planetary movement. From the Earth, it appears that the planets in the sky are moving “backwards” at certain times. 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! Also, get Sky Tonight and Star Walk 2 to easily track any planet in the sky.
