Counterclockwise, but there are exceptions

Posted by Ethan on October 7, 2010
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“Imperfection clings to a person, and if they wait till they are brushed off entirely, they would spin for ever on their axis, advancing nowhere.” -Thomas Carlyle

When you take a look up at the sky, the two most prominent objects are the Sun and the Moon.

And every day, like clockwork, they rise in the East and set in the West.

Why’s that?

Because the Earth rotates on its North-South axis! Not only does it rotate, but it rotates from West to East, and that’s why we see everything rise in the East and set in the West over the course of a day or night.

Image Credit: Phil Hart.

It gets better, though. If you rose up, above the Earth’s North Pole, and looked down at the Earth as it revolved around the Sun, not only would the Earth spin counterclockwise, it would also orbit the Sun counterclockwise!

So you might ask what other objects in the Solar System do? We can start with the Moon, of course.

Just like you might expect, the Moon both orbits counterclockwise around Earth and rotates counterclockwise on its axis, so that the same side of it always faces us!

Fine, fine, you say. But what about the other planets? Isn’t there some variety there?

Not a chance. Not in the least, in fact!

Not only do all of the rocky planets revolve counterclockwise around the Sun, and not only do all of the gas giant planets revolve counterclockwise around the Sun, but something like 99% of the known asteroids, kuiper belt objects, and comets also revolve counterclockwise around the Sun!

WHY?!

Stars — all of them, as far as we know — form when clouds of gas collapse. Since one direction will be shorter, that will collapse first, and so we’ll get a disc. (Or, in our “complicated scientific jargon,” a pancake. Yes, seriously, you say pancake to an astrophysicist and they’ll know you’re talking about this process.)

But this pancake isn’t just going to sit there. In general, there will be an overall, bulk, rotational motion, due to the law of conservation of angular momentum.

So not only should the planets all revolve in the same direction, but if this idea is correct, the Sun should rotate counterclockwise, too!

And not only does it rotate counterclockwise, we can measure that rotation by following sunspots!

But you might decide to look at the individual planets, and see if they not only revolve counterclockwise about the Sun, but if they also rotate counterclockwise about their own axes, tilted or not. The results may surprise you.

Image credit: Calvin Hamilton, and click for a huge version!

While Mercury, Earth, Mars, Jupiter, Saturn, and Neptune all rotate counterclockwise, at tilts varying from less than a degree up to substantial, significant tilts, two planets stand out as weirdos.

Venus, rotating almost perfectly backwards, and Uranus, rotating on its side, are the unexplained bizarre children of our Solar System.

Unlike some Moons (like Neptune’s Triton) that go backwards, we can’t blame Venus and Uranus on gravitational capture. There are ideas out there, such as big collisions in the young Solar System, but there is currently no compelling evidence that favors any one theory over the other.

So almost everything in the Solar System revolves and rotates counterclockwise (from the North), but Venus? Uranus? We just don’t get it, at least, not yet. Even in something as well-studied as our Solar System, we still have plenty of unexplained mysteries.

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  1. #1 Squiddhartha
    October 7, 2010

    Obviously it’s because their names both end in “nus”, an acronym for “Non-Usual Spin.”

  2. #2 Sarah
    October 7, 2010

    PLUTO! <3

    .. sorry.

  3. #3 CS Shelton
    October 7, 2010

    Good hypothesis, Squiddhy! Let’s find a way to test it…

  4. #4 feedayeen
    October 7, 2010

    What about Pluto? You might want to mention that it has become tidally locked with Charon giving it it’s unusual axis.

  5. #5 christopher
    October 7, 2010

    Before we knew much about Uranus, astrologers were singling that planet out as unusual. They were right but not for the right reasons.

    Venus does not spin backward. It is placed upside down. :D

    Check it out. If you roll it through 180 degrees and keep looking from above (north of the ecliptic) then the same ccw spin becomes a cw spin. So the real question may be what caused Venus to flip upside down. A really good impact might do it.

  6. #6 juanbobsdad
    October 7, 2010

    Ethan,
    You are a born teacher! I look forward to reading you blog like none other – new ones just don’t come fast enough.

    So, is the whole universe spin neutral or not?

  7. #7 Geneva escorts
    October 8, 2010

    Well, in general, an agreement made between a minor and an adult is void. However, if the agreement was for necessities (such as food or an apartment) and the minor benefited from the contract for the necessities, that portion of the contract is enforceable on the minor. Also, if a minor signs an agreement, becomes an adult, and does not revoke the contract within a State-defined period of time after becoming an adult, the contract is considered assumed by the now-adult.

  8. #8 djlactin
    October 8, 2010

    Actually, several asteroids have retrograde orbits:
    http://en.wikipedia.org/wiki/List_of_notable_asteroids#Retrograde_and_highly-inclined

    Also (IIRC) some long-period comets also orbit retrograde.

    p.s. I think “Geneva escorts” posted on the wrong site.

  9. #9 Jonathan
    October 8, 2010

    Uranus and Neptune aren’t really Gas Giants, though. They are Ice Giants; they are of markedly different chemical makeups than the two Gas Giants. Probes have also shown that Neptune and Uranus formed between the orbits of Jupiter and Saturn. It was the gravitational action of these two massive bodies that kicked them out to their current orbits.

    We know the Earth was hit by a dense, Mars-sized planet early in it’s history, working it’s way in from L5, that ended up with the formation of the moon. If Venus had gotten hit by a slightly smaller, less dense planet, working its was in from L4 instead, that would be enough to reverse the spin without leaving enough left over material for a large moon.

  10. #10 buy wow accounts
    October 8, 2010

    Most races (be it humans, machines or animals) on a circular or oval track run in a counterclockwise direction. The exceptions being the Australian version of NASCAR (AUSCAR) and some European horse tracks.
    The reason for the counter clockwise direction for some sport like NASCAR is fairly obvious. Since the driver is on the left side of the car (in this country at least) driving

  11. #11 andy
    October 8, 2010

    Venus does not spin backward. It is placed upside down. :D

    Well the results of the two interpretations, backwards spin vs upside down axis, are the same. The usual system in astronomy is to define the north pole as being the one that lies above the invariable plane of the solar system, rather than defining north by the sense of rotation, which is in some ways a more sensible way to do things. In that interpretation Venus is the right way up (by definition) and spinning backwards.

  12. #12 Nir
    October 8, 2010

    This is northern hemisphere chauvinism. All these heavenly bodies rotate and cycle in the same direction but that direction is anticlockwise only when considering the north as up. If the Aztecs had conquered Spain and not the other way around we all would have said that they rotate clockwise.

  13. #13 lordaxil
    October 8, 2010

    Isn’t there a huge problem with conversation of angular momentum in all of this? Assuming that the angular momentum of universe was originally zero (I’m not a cosmologist, but seems like a reasonable thing to suppose) then where is all the missing clockwise angular momentum? And don’t say dark matter. ;)

  14. #14 lordaxil
    October 8, 2010

    d’oh. CONSERVATION above, obviously.

  15. #15 Lorkas
    October 8, 2010

    @Nir

    Something tells me that Ethan has probably heard of relativity before. We all know that the spin is the other way if you look at it from the south. The point is, either way you look at it, the spin is the same all the time except for a few exceptions.

  16. #16 DTK Greg
    October 8, 2010

    @Nir
    If the Aztecs had conquered Spain, we’d all look at everything from the southern hemisphere POV.
    Everything – including the clocks.
    So we would still say that the solar system rotates counterclockwise, but we would mean the opposite direction.

  17. #17 opony
    October 8, 2010

    People let’s remember about different people in countries like this. Let’s stop the war!

  18. #18 Joe Blow
    October 8, 2010

    It would be interesting to know the solar system’s orientation with respect to the galactic orbital rotation (or maybe the local neighborhood’s motion). Is this sort of rotation more random or do solar systems tend to reflect the galactic rotational direction?

    It seems that the majority of the matter in the solar system follows the sun’s rotation (and probably the proto-solar pancake’s rotation). I would guess that solar systems with respect to the galactic rotation are more random but the tendency would be that on average, more systems would rotate in the same relative direction as the galaxy.

    Joe

  19. #19 rob
    October 8, 2010

    @Geneva escorts and opony:

    the condor flies at noon. bring your watermelon hamster.

  20. #20 djlactin
    October 8, 2010

    Nir; DTH Greg: um… the Aztecs lived in the northern hemisphere. To equator runs through Equador. Perhaps if the Incas had invaded Spain… Or if Australians were in charge of manufacturing globes…

  21. #21 Trevindor
    October 8, 2010

    @5/christopher,

    If I understand you correctly you’re suggesting that Venus may have originally rotated ccw, but a collision or other event effectively flipped it upside down so its north pole now points south and vice versa.

    I think this is exceedingly unlikely. Think about flipping a spinning globe upside down. Due to conservation of angular momentum it would require two perfectly matching forces (collisions) in opposite directions. The first would start the migration of the geographic north pole southward, and the second would strike at just the right place and time to cancel the angular momentum of the first once the poles are flipped 180 degrees. In the interim Venus would have a different axis of rotation.

    I could be completely wrong about this (it’s been ~20 years since classical mechanics) but I don’t see any other way. If I’m wrong I’m sure I’ll be corrected :)

    Another interesting thing about Venus – its day is longer than its year (243 Earth days to 225). It may be rotating backward, but it is barely rotating at all.

  22. #22 OKThen
    October 8, 2010

    OK, so far.

    As well your recent Me At The Bar post shows that the stars of our Milky Way Galaxy rotate; but does not clarify if clockwise, counterclockwise or not aligned to galactic plane.

    Furthermore, some suggest the alignment of spiral galaxies within superclusters. e.g., http://universe-review.ca/F03-supercluster.htm, 2nd figure figure 3-01b shows this. And also wiki, “the orientation of the spin axis of spiral galaxies is not a chance result, but instead they are preferentially aligned along the surface of cosmic voids”….???…But, I can not determine quality of the observation; and strength of this hypothesis. Clarification or comment, anyone please.

  23. #23 Rrr
    October 8, 2010

    IIUC @DTK Greg, “clockwise” is actually DEFINED as a result of looking at the ground for celestial actions (the shadow of a stick planted in the ground (northern hemisphere here, for historic/cultural reasons), later transformed into mechanical watches with moving hands) which will automatically reverse the sense of rotation. Look the other way, you add one minus sign.

    Venus’ “inverted” sense of rotation is still an interesting enigma, I think. But maybe I’m just starstruck.

  24. #24 djlactin
    October 8, 2010

    all of the known asteroids, kuiper belt objects, and comets also revolve counterclockwise around the Sun!

    I’ve already presented counterexamples of retrograde asteroids.

    At the risk of seeming to be (being?) a Prick, I present counterexamples to refute some other assertions.

    Numerous comets orbit retrograde, the most famous being comet Halley:
    http://en.wikipedia.org/wiki/Comet_Halley
    Search for “retrograde” in the text.
    These are considered to be anomalies resulting from interactions with Jupiter (or perhaps other planets).

    One KBO is known to orbit retrograde:
    http://www.cfeps.net/CFEPS/KV42_Press.html
    (On the basis of the orbit given in the image, I hypothesize an interaction with Neptune.)

    If ‘we’ ever detect a minor Trans-neptunian planet that orbits retrograde, I will (as is my right as a Prick) declare it to be an interstellar interloper.

  25. #25 Vicki
    October 8, 2010

    @Trevindor 21:

    Venus is an angular momentum problem no matter what: retrograde or not, a huge amount of angular momentum got lost somewhere. The only other planet with a really long day is Mercury’s, and that’s the result of a 3/2 orbital resonance with the sun.

    Interesting but not directly relevant to this: Venus’s sidereal day (period of rotation) is as described, but the solar day is about 116 Earth days. (That is, if an entity on the surface of Venus could see through the clouds, that’s how long it would take from sunrise to sunrise.) A solar day on Mercury is 176 Earth days, and the sun’s apparent motion is weird.

    Uranus has an even more ridiculously long solar day, because of the axial tilt.

  26. #26 andy
    October 8, 2010

    Well the rotational angular momentum of the planet is several orders of magnitude less than the orbital angular momentum, so at least it is plausible that collisions could be responsible for altering the planet’s spin. This seems to be borne out by simulations of terrestrial planet formation: the rotation axes are essentially randomised during the final stages of accretion by giant collisions (rather like the one that is postulated to have formed the Moon).

  27. #27 Doug
    October 8, 2010

    Perhaps it’s safe to say that due to the sundial->clock face evolution, if the map-makers and clock-inventers come from the same hemisphere, then the solar system will run “ccw”, if they come from different hemispheres, it runs “cw”.

  28. #28 Gollum
    October 8, 2010

    I suspect those speculating on the Aztecs conquering Spain were thinking of the Inca. The Aztecs also lived in the Northern Hemisphere and would have the same view of “clockwise” as Europeans.

  29. #29 Murfomurf
    October 9, 2010

    @Christopher beat me to it- who whacked those planets (Venus & Uranus) so far over?- and since I’m a Southern Hemisphere person, I like to think of everything going clockwise- they use less typing energy and waste less carbon in print!

  30. #30 djlactin
    October 9, 2010

    re: Venus retrograde rotation, Wikipedia (for what it’s worth) reports:

    The present rotation period of Venus represents an equilibrium state between gravitational tidal locking by the Sun that tends to slow the rotation rate, and an atmospheric tide created by the solar heating of the thick Venusian atmosphere.

    and

    A curious aspect of the Venusian orbit and rotation periods is that the 584-day average interval between successive close approaches to the Earth is almost exactly equal to five Venusian solar days. Whether this relationship arose by chance or is the result of tidal locking with the Earth, is unknown.

  31. #31 Lloyd
    October 11, 2010

    Venus is being contrary (just like a woman) while Uranus is being stubborn (just like an ass).

  32. #32 utpal verma
    March 17, 2011

    very strange and not perhaps a coincidence…. in all major religions also counter clock wise motion is performed as in hindu religion in marriage ceremony the bride and the bride groom go around the fire in anti clock wise direction, in mecca muslims go around the the stone anti clock wise ….
    and the dancing dervishes also move that way

  33. #33 John
    April 21, 2011

    earth orbit and gravitational pull since the earths orbit is not completly round is it possable that the gravitational pressure is partly causing some earthquakes
    the pull is strong enough to swing the earth around and if is not a constant that could explane changes is the magnetasphere to ?????

  34. #34 Amrendra kumar Singh
    July 1, 2011

    Hi
    I want to know that ” is Pluto planet or not because according discovery channel from 2006 Pluto is not a planet due to its some spacial character ,and it is known as DOT Planet

  35. #35 Don Van Arnam
    March 21, 2012

    It seems that the majority of the matter in the solar system follows the sun’s rotation (and probably the proto-solar pancake’s rotation).

    Ha-Ha, he said “Proto”

  36. #36 Michael Rowan
    New York
    September 25, 2012

    If political economies were discussed with the same liberation from ideological rigidity and humorous fascination with sharing curious facts as is demonstrated in this string of comments by astrophysicists and cosmophyles, there would be a lot more prosperity and less violence among humans on the planet. Thanks.

  37. #37 Wow
    September 25, 2012

    Or not.

    You humans are weird.

    :-)

  38. #38 Wow
    September 25, 2012

    “@Christopher beat me to it- who whacked those planets (Venus & Uranus) so far over?”

    Its only the presence of the ovesized and nearby moon that’s stopped our earth from flipping over.

    Rotational instability is pretty much inevitable over periods of millions of years without something nearby that would soak up that disturbance.

    So maybe the only weird thing about the two is that we see both being weird at the same time, but a period without one planet playing silly buggers might be fairly unusual.

    NOTE: we’re due to lose a planet in a hundred million years or so. Either Venus or Earth seems most likely, but that darned chaos is playing merry hell with the simulation.

  39. #39 BrianDash
    Oregon
    December 4, 2012

    @Joe Blow
    I agree that :

    “It would be interesting to know the solar system’s orientation with respect to the galactic orbital rotation (or maybe the local neighborhood’s motion). Is this sort of rotation more random or do solar systems tend to reflect the galactic rotational direction?”

    The rotation of the solar system, and of Earth are quite different from the rotation of the galactic plane. It’s always a pleasure to prove this: the milky way does not run along the equator.

    Perhaps someone will fill us both in as to the behavior of the rest of our galactic neighbors…

  40. #40 soomii
    UAE
    March 29, 2013

    can you solve this question ??

    What will happen to these two periods if Earth were to revolve clockwise around the Sun and the Moon clockwise around Earth? Which period will be shorter or longer than the other one

  41. #41 Daraje Garba
    Addis Ababa Ethiopia
    April 25, 2013

    Venus has a thick atomospher solid cloud that is 72-79km the surface is not visible.In that case it slow rotate and backwards.the surface of the temperature does not change day to night.Venus has no magnetic field detection that lack of Iron-core wich develop magnetic field.so it retrograde.

  42. #42 Daraje Garba
    Addis Ababa Ethiopia
    April 25, 2013

    Uranus the only planet lies on side.in the angle of inclination 98 degree verticals.it is nearly parallel to the other planets