Mercury is the planet closest to the sun and rotates our star every 88 days compared to Earth’s 365.25 days. Mercury will also be the first planet to be destroyed as the sun expands on its way to becoming a red giant in about 5 billion years.
So it seems a little rough that we Blaming Mercury for all our problems three to four times a year when it is in retrograde. But what does it mean when we say Mercury is “retrograde”?
Image of Mercury from MESSENGER’s wide-angle camera.
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie
A matter of jobs
Retrograde motion means that a planet moves in the opposite direction from normal around the sun. However, the planets never change direction. What we’re talking about is apparently retrograde motion, when it appears to us on Earth that a planet is moving through the sky in the opposite direction of its usual motion.
Because Mercury is closest to the sun and has the fastest orbit, it appears to move backwards in the sky more often than any other planet.
Let’s use my dog Astro to explain what happens when we see a planet in retrograde. Astro is a whippet, or mini greyhound, and he needs speed. When I take Astro for a run at my local cricket oval, he runs super fast laps on the inside while I run much slower on the outside.
As we both go around the cricket pitch in a counter-clockwise direction, it looks like Astro is moving towards me on the other side of the oval, as I jog to the right. But when he gets to the same side of the oval as me, it suddenly looks like he’s running right instead of left (retrograde).
This happens because Astro is moving much faster than I am, and is within my “orbit” of the oval.
Diagram of me (Laura) and Astro running around the oval, from my point of view. At the top we see the top view of the oval. Below we see the side view. From my point of view, it looks like Astro is running from right to left if he’s on the opposite side of the oval to me, but it looks like he’s running from left to right if he’s on the same side of the oval as me.
Laura Driessen (author provided)
Since Mercury’s orbit is inside Earth’s orbit, seeing it from our planet is like watching Astro run.
But Mercury is not the only planet to do this. Venus also rotates in our solar orbit, as it zips around once every 224.7 days. This means that Venus goes retrograde twice every three years.
Scheme of the orbits of the planets (and dwarf planet Pluto) in the solar system. The dwarf planet Ceres orbits between Mars and Jupiter.
NASA space place
The other retrograde
It also works the other way around. The planets beyond our orbit (Mars, Jupiter, Saturn, Uranus, and Neptune) are also going retrograde.
To work this out, we need to switch our perspective. Astro is definitely not a deep thinker, but let’s imagine for a moment that he is and think about what he sees as he runs around the oval.
He runs around the oval and he starts to catch up with me from behind. Right now it looks like we’re both going in the same direction, to the right. But as he begins to pass me, I seem to be going backwards or to the left (retrograde) as he continues to run forward to the right.
This is what happens when we look at the sky and see one of the outer planets retrograde.
Mars is retrograde once every two years. The other planets are so far from the sun and travel so slowly compared to Earth that they almost appear to be standing still. So we see them retrograde about once a year as we fly around the sun that much faster than they do.
Diagram of me (Laura) and Astro running around the oval, this time from Astro’s perspective. At the top we see the top view of the oval. Below we see the side view. From Astro’s point of view, it looks like I’m backing up as he catches up to me.
Laura Driessen (author provided)
A well-known illusion
Retrograde motion has puzzled ancient astronomers since humans began looking up into space, and we didn’t officially find out until Copernicus proposed in 1543 that the planets in orbit around the sun (although he was not the first astronomer to propose this heliocentric model).
Before Copernicus, many astronomers thought that the Earth was the center of the universe and that the planets revolved around us. Astronomers love it Apollonius around 300 BC. saw the planets receding, and explained this by adding more circles called epicycles.
So people discovered that retrograde motion was an optical illusion 500 years ago. However, the pseudoscientific practice of astrology continues to attribute a deeper meaning to this illusion.
Diagram of the dates in 2023 that each planet (and the two dwarf planets Pluto and Ceres) will be retrograde.
Laura Driessen (author provided)
There is usually a retrograde
Looking at the seven planets other than Earth, at least one planet is 244 days retrograde from 2023 — that’s about two-thirds of the year.
Astro the Whippet thinks of all the possums he’s going to bark at later.
Laura Driessen (author provided)
If we include the dwarf planets Pluto and Ceres (and exclude the other seven dwarf planets in the solar system), at least one planet or dwarf planet will be in retrograde for 354 days of 2023, leaving only 11 days without any retrograde motion.
I like to think that the greatest impact the planets have on Earth is bringing wonder and joy every time we turn our eyes (and our telescopes) to the night sky. Astro, on the other hand, is happy as long as he can run around the oval and bark at possums.
Read more: From platypus to parsecs and milliCrab: why do astronomers use such weird units?