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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.

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    $\begingroup$ Since there is no perfect resonance in the planets' orbits, such an event must be possible. $\endgroup$
    – Glorfindel
    Jun 26, 2019 at 7:20
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    $\begingroup$ There is a perfect mean-motion resonance between pluto and neptune $\endgroup$
    – James K
    Jun 26, 2019 at 7:29
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    $\begingroup$ 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. $\endgroup$
    – Michael Seifert
    Jun 26, 2019 at 21:03
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    $\begingroup$ 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, 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. $\endgroup$
    – Michael Seifert
    Jun 26, 2019 at 21:25
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    $\begingroup$ This is interesting and I'm working on a better answer, but if you google "all eight planets retrograde at same time" or similar phrases (no quotes), you'll get a lot of astrology links suggesting up to 7 planets have retrograded at the same time. Regardless of whether you believe in astrology, the calculations they use should be astronomically accurate. A little digging might find an 8 planet retrograde. $\endgroup$
    – user21
    Jun 26, 2019 at 21:52

1 Answer 1

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EDIT: I used the DE431 ephemeris to get the results below. However, if you visit HORIZONS (https://ssd.jpl.nasa.gov/horizons.cgi) and create an ephemeris for Pluto (actually Pluto's barycenter, object 999), the results include this warning:

The Horizons output log may report using DE431 as the source of target or center data, but it is nevertheless using DE433 as an override if Pluto is involved. This is to maintain ephemeris consistency for the New Horizons spacecraft encounter at Pluto.

Therefore, my calculations may be inaccurate, above and beyond the normal inaccuracy of predicting planetary data in the past and future.

There are 2 times in the past 15,000 years and 5 times in the next 15,000 years where all 8 planets are in retrograde as observed from Earth.

The table below lists the times when this happens:

  • The last column is the ephemeris time, the number of seconds since '2000-JAN-01 12:00:00 UTC', which NASA refers to as "the epoch".

  • The first line in each pair shows the start time of when all 8 planets are in retrograde, and the final planet to enter retrograde to make this true.

  • The second line in each pair shows the end time when at least one planet is no longer in retrograde, and the planet whose retrograde period has ended.


B.C. 12435-AUG-10 07:52 JUPITER STARTS RETROGRADE -455482152466.038696
B.C. 12435-AUG-24 17:12 MERCURY ENDS RETROGRADE -455480909251.797852

B.C.  4139-MAR-23 15:02 MARS STARTS RETROGRADE -193692373029.630005
B.C.  4139-MAR-28 21:57 MERCURY ENDS RETROGRADE -193691916114.978455

A.D.  7662-APR-27 19:33 MARS STARTS RETROGRADE 178685595220.195740
A.D.  7662-MAY-12 05:19 NEPTUNE ENDS RETROGRADE 178686840014.478882

A.D.  8807-JAN-07 02:02 MERCURY STARTS RETROGRADE 214808680981.726929
A.D.  8807-JAN-20 01:05 JUPITER ENDS RETROGRADE 214809800771.851685

A.D. 10575-FEB-14 21:34 VENUS STARTS RETROGRADE 270604748086.593933
A.D. 10575-FEB-24 18:53 NEPTUNE ENDS RETROGRADE 270605602468.037781

A.D. 13679-NOV-06 16:03 VENUS STARTS RETROGRADE 368580427477.379639
A.D. 13679-NOV-19 01:05 MERCURY ENDS RETROGRADE 368581496795.683594

A.D. 16364-MAR-11 19:25 MERCURY STARTS RETROGRADE 453290109945.561157
A.D. 16364-MAR-13 08:53 MARS ENDS RETROGRADE 453290244824.895996

Notes and caveats:

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  • $\begingroup$ That is awesome! Unless there's anything else left to add I'm going to tick your response. Thank you very much! Just what I was loking for! $\endgroup$
    – Snack_Food_Termite
    Jun 29, 2019 at 0:45
  • $\begingroup$ What would be even better would be to match up the calculations to a theoretical reason for eight retrograde planets as I listed them being possible. In any case I think that you [barrycarter] should get some sort of medal and/or points bonus for the answer because that was probably one of the hardest questions ever asked on Stack. $\endgroup$
    – Snack_Food_Termite
    Jun 29, 2019 at 1:21
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    $\begingroup$ @getastro Unfortunately, it looks like barrycarter never licensed his github repo. This means that he retains full copyright, and now that he has removed the repository from public access it cannot legally be shared here. $\endgroup$
    – called2voyage ♦
    Sep 9, 2020 at 13:58
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    $\begingroup$ @uhoh See above. $\endgroup$
    – called2voyage ♦
    Sep 9, 2020 at 13:59
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    $\begingroup$ @uhoh I did make a meta post about this, because this made me grumble, lol. $\endgroup$
    – called2voyage ♦
    Sep 9, 2020 at 14:48

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