<p>First of all, for the purposes of this question Pluto is a planet. So the eight planets being considered are Mercury, Venus, Mars, Jupter, Saturn, Uranus, Neptune and Pluto. </p>

<p>My question is: can all eight of these planets <em>go retrograde<sup>†</sup></em> at the same time? I know that seven can do so on rare instances; there were 12 days of that in the 20th century all in the 1940's and the 1980's. But to show how rare that is there was a block of 400 years in the middle ages with no days of seven retrogrades. Has eight ever happened? Or is it impossible? </p>

<p><sup>†</sup>Retrograde motion as seen from Earth, not retrograde orbits around the Sun.</p>

CC BY-SA 4.0
2019-06-26T07:20:58.950Z
Since there is no perfect resonance in the planets' orbits, such an event must be possible.
Glorfindel
CC BY-SA 4.0
2019-06-26T07:23:39.847Z
Is there astronomy simulation software that would show me? The problem child planet is Venus because that always remains prograde when the other seven go retrograde.
Snack_Food_Termite
CC BY-SA 4.0
2019-06-26T07:29:55.740Z
There is a perfect mean-motion resonance between pluto and neptune
James K
CC BY-SA 4.0
2019-06-26T21:03:38.640Z
Isn't this basically the same as asking when all planets will be lined up on one side of the Sun?  Inferior planets (Mercury & Venus) need to be near inferior conjunction to be in retrograde, while superior planets (the rest) need to be near opposition.
Michael Seifert
CC BY-SA 4.0
2019-06-26T21:25:05.257Z
Since the periods of all the non-Pluto planets are non-resonant, the probabilities that they're retrograde are effectively independent.  Taking the numbers from the [Wikipedia page on retrograde motion](https://en.wikipedia.org/wiki/Apparent_retrograde_motion), you can find that (for example) Mars is in retrograde 72/780 of the time.  Multiplying all these probabilities together, all eight "real" planets will be in retrograde about 0.0023% of the time.  The chances if you include Pluto are necessarily less than that.
Michael Seifert
CC BY-SA 4.0
2019-06-26T21:34:01.947Z
I am not sure about using probabilities of retrogrades; the main obstacle to 8 retrogrades seems to be having venus and mercury retrograde at the same time. That's why the five inner planets being all retrograde at once is rarer than 7 retrogrades.
Snack_Food_Termite
CC BY-SA 4.0
2019-06-27T20:53:48.280Z
Its a perfect  mean-motion resonance, there is all sorts of complex motions, but on average and in the long term, pluto is locked at 2:3 with Neptune.  It is a fact of no consequience for this question. It is possible for Neptune and pluto to both be at opposion (and hense retrograde) at the same time
James K
<orbital-mechanics><positional-astronomy><ecliptic>
Can Eight Retrograde (seen from Earth) planets occur?
Snack_Food_Termite
# Can Eight Retrograde (seen from Earth) planets occur?

(Asked by: username_0 on 2019-06-26)

First of all, for the purposes of this question Pluto is a planet. So the eight planets being considered are Mercury, Venus, Mars, Jupter, Saturn, Uranus, Neptune and Pluto.

My question is: can all eight of these planets *go retrograde†* at the same time? I know that seven can do so on rare instances; there were 12 days of that in the 20th century all in the 1940's and the 1980's. But to show how rare that is there was a block of 400 years in the middle ages with no days of seven retrogrades. Has eight ever happened? Or is it impossible?

†Retrograde motion as seen from Earth, not retrograde orbits around the Sun.

**Tags:** orbital-mechanics, positional-astronomy, ecliptic
> **Comments:**
> **username_1**: Since there is no perfect resonance in the planets' orbits, such an event must be possible.  
> **username_0**: Is there astronomy simulation software that would show me? The problem child planet is Venus because that always remains prograde when the other seven go retrograde.  
> **username_2**: There is a perfect mean-motion resonance between pluto and neptune  
> **username_3**: Isn't this basically the same as asking when all planets will be lined up on one side of the Sun?  Inferior planets (Mercury & Venus) need to be near inferior conjunction to be in retrograde, while superior planets (the rest) need to be near opposition.  
> **username_3**: Since the periods of all the non-Pluto planets are non-resonant, the probabilities that they're retrograde are effectively independent.  Taking the numbers from the [Wikipedia page on retrograde motion](https://en.wikipedia.org/wiki/Apparent_retrograde_motion), you can find that (for example) Mars is in retrograde 72/780 of the time.  Multiplying all these probabilities together, all eight "real" planets will be in retrograde about 0.0023% of the time.  The chances if you include Pluto are necessarily less than that.  
> **username_0**: I am not sure about using probabilities of retrogrades; the main obstacle to 8 retrogrades seems to be having venus and mercury retrograde at the same time. That's why the five inner planets being all retrograde at once is rarer than 7 retrogrades.  
> **username_2**: Its a perfect  mean-motion resonance, there is all sorts of complex motions, but on average and in the long term, pluto is locked at 2:3 with Neptune.  It is a fact of no consequience for this question. It is possible for Neptune and pluto to both be at opposion (and hense retrograde) at the same time  

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