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? Here, we’ll explain the celestial mechanics behind the retrograde motion and show you how to see it with Sky Tonight.
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: retrograde loops
- How do planets in retrograde affect us?
- Retrograde motion of planets: to sum up
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 following Solar System planets will go retrograde in 2025:
- Neptune: July 5, 2025 - December 10, 2025
- Saturn: July 14, 2025 - November 28, 2025
- Mercury: July 17 - August 10, November 9 - December 29
- Uranus: September 6, 2025 - February 4, 2026
- Jupiter: November 11, 2025 - March 11, 2026
How to see the retrograde motion: retrograde loops
To observe a retrograde motion, you need to make a retrograde loop. To do so, mark the position of a planet in the sky at the same time each day. The combined result will show you a planet outlining a loop.
You can do this manually (but it'll take at least several weeks) or you can do it in 3 steps with the stargazing app Sky Tonight. It has a special feature that allows you to instantly build a curve showing the annual movement of any planet. Here’s how to see the retrograde motion of a planet via Sky Tonight:
- Tap the magnifier icon at the bottom of the screen and type the name of a planet (e.g., Mercury) into the search field;
- Tap the blue target icon opposite the name of the planet and see its current position on the sky map;
- Tap the small white camera icon at the bottom part of the screen to enable the curve-building feature. You’ll see the path the planet passes in the sky over several months and will be able to clearly see how the planet “changes direction” at certain moments.
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.
A celestial body that can really affect our lives is an asteroid heading to our home planet. Learn when the next asteroid will hit Earth and what we can do about it.
Retrograde motion of planets: to sum up
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.
For more interesting facts about retrograde motion, have a look at our dedicated infographic!
