[DocN (OP)]

Why does Venus rotate in a direction opposite the other planets?  Your guess is as good as mine.
Doc

[Hadrian]

Could it have anything to do with how it was formed?

Is it possible that it was formed the way our moon was but the impact caused it to rotate it the other direction?

What you do speaks so loudly that I cannot hear what you say.

[another_someone]

I don't know that it needs as much as an impact (although it might be a cause).

The question must be, why do the planets rotate at all?  Clearly, their orbits around the sun are caused by the spinning of the gas that formed the solar system itself, but why would the individual planets spin on their own axis within that?  It may be through magnetic interaction with the sun's magnetic field, but I think it more likely that the spin of the planets might be due to eddies in the gas flows that precipitated the solar system (a little like cyclones and anticyclones in the weather system on earth).

But then, what do I know?

George

[daveshorts]

Yeah you don't need any magnetic effects to explain why the planets are rotating. using a very simple model for how they formed from a disk of orbiting matter, they will have coalesed from stuff orbiting below and above their eventual orbit. The stuff below will be moving faster and above slower. This will form something like a vortex as it falls towards the planet , and spin it up.

The final stage of the rocky planet formation was supposed to be collisions of mars sized planetesimals which will make the whole process a lot more random - if the las collision happens to have been on the wrong side then venus may have been caused to spin in the opposite way.

[another_someone]

quote:Originally posted by daveshorts

The final stage of the rocky planet formation was supposed to be collisions of mars sized planetesimals which will make the whole process a lot more random - if the las collision happens to have been on the wrong side then venus may have been caused to spin in the opposite way.

I understood that the collision between our Earth and a Mars sized object was what made our Earth different to its neighbours, and thus such collisions would have been fairly rare.

Ofcourse, for Jupiter sized planets, the collision with Mars sized planetesimals would have had far less significance that such a collision with a Mercury sized planet would have, and might indeed have been common.

I am not saying that collisions between planetesimals would of itself have been uncommon, only that I would have thought that it would have been a very rare event that they would have been of a Mars sized object (or, more precisely, that they would have been of planetesimals of similar size), and would generally have been of objects very much smaller than the planets upon which they were impacting.  I would have thought that the final rotation would have been more a consequence of lots of small events, rather than one cataclysmic event.

George

[daveshorts]

To be honest I am not an expert, but I would have thought that a collision with a fairly large object - much smaller than mars, say between Venus and the moon,  could supply a huge amount of angular momentum, and significantly alter it's spin.

[Hadrian]

I may be my quirky mind playing tricks on me again but it has to be something out of the ordinary to have caused it to be so different and something of a very rare event too. Like being hit by a large object.

Maybe it god having a laugh at us []

What you do speaks so loudly that I cannot hear what you say.

[ROBERT]

Venus' rotation is not only retrograde it is very slow:-

" Venus is strange also in that it spins in the opposite direction to Earth and most of the other planets (the Sun rises in the West instead of the East from Venus' surface) and rotates very slowly, with a Venusian day (243 Earth days) being longer than a Venusian year (225 Earth days) - So every other day on Venus is New Year's eve! "
http://www.solarspace.co.uk/Venus/venus.php#rotation

Could this slow rotation be a clue to Venus' formation ?

[Hadrian]

quote:
Could this slow rotation be a clue to Venus' formation ?

What do you think is might tell us?    

What you do speaks so loudly that I cannot hear what you say.

[DocN (OP)]

These interesting postings reminded me of a book I read, about fifty years ago--Worlds in Collision--whereby Velikovsky relates how Earth may have encountered several near collisions with a "Venus-comet" which later transforms into a planet.  The book is not very scientific being based mainly on past myths, ect.
               "Velikovsky is certainly ingenious. His explanations of parallels among ancient myths are very entertaining, interesting and apparently plausible.  His explanation of universal collective amnesia of these worlds in collision is highly amusing and equally improbable. Imagine we're on earth 3,500 years ago when an object about the same size as our planet is coming at us from outer space! It whacks us a couple of times, spins our planet around so that its rotation stops and starts again, creates great heat and upheavals from within the planet and yet the most anyone can remember about these catastrophes are things like "....and the sun stood still" [Joshua 10: 12-13] and other stories of darkness, storms, upheavals, plagues, floods, snakes and bulls in the sky, etc. No one in ancient times mentions an object the size of earth colliding with us. You'd think someone amongst these ancient peoples, who all loved to tell stories, would have told their grandchildren about it. Someone would have passed it on. But no one on earth seems to remember such an event."                                                                                                                                                                                                  See--
                   http://skepdic.com/velikov.html


Doc

[Hadrian]

How about the hitchhikers guide to the Galaxy

Remember marvin



What you do speaks so loudly that I cannot hear what you say.

[Ophiolite]

Just to confirm and emphasis the point made by daveshorts: collisions between large planetesimals is believed to have been commonplace during the final stages of the formation of the solar system. Each of the terrestrial planets is unique and likely formed by a combination of rare events. Just as it is rare to have 24 turn up twice in succession on a roulette table. Once we gain knowledge of the planetary composition of other solar systems (and not just the giants and ultra giants) we shall be much better placed to explain the precise composition of ours.

Observe; collate; conjecture; analyse; hypothesise; test; validate; theorise. Repeat until complete.

[Hadrian]

So i was not to far off the mark then in thinking it maybe due to a collision.

What you do speaks so loudly that I cannot hear what you say.

[Monox D. I-Fly]

Wait, so in Venus, the sun rises in the west and sets in the east?

[alancalverd]

And why not? The real surprise is that all the other planets orbit and rotate in pretty much the same plane and direction!

[Halc]

Wait, so in Venus, the sun rises in the west and sets in the east?

The convention is usually that the sun always rises in the east, so Venus just has it's north pole pointing the other way.

The weird thing about Venus is it having two days per year despite only rotating once.  If it rotated once in the correct direction, it would have zero days per year, sort of like how Earth never sets as viewed from the moon.

Mercury has a day (sunrise to sunrise) of about 4223 hours, which is thrice the sidereal day (length of the time it takes to make one rotation) of nearly 1408 hours.  With Venus the ratio is more like 0.5x instead of 3x.  With most other planets, the two figures are nearly the same, like Earth with its ratio of 1:1.0027

As to why Venus rotates the other way, the rotation seems quite variable, but a lot of it is due to the thick atmosphere which pushes the planet below it.  The wind on Venus is heavy and very high speed and exerts significant thrust on surface features.  It's kind of amazing that Venus even has surface features anymore with all that continuous punishment.

[Monox D. I-Fly]

Wait, so in Venus, the sun rises in the west and sets in the east?

The convention is usually that the sun always rises in the east, so Venus just has it's north pole pointing the other way.

So, north is the bottom one?

[Halc]

So, north is the bottom one?

That's one way of wording it, yes.  Arbitrary convention is to put north on top, and the north pole of Venus is on the bottom side of the solar system if you consider solar north to be the top side.  Uranus has a tilt of 98° so its north pole is also on the wrong side, but more sideways than on the bottom side.  The tilt of Venus is 177°, almost perfectly inverted.

[Janus]

Wait, so in Venus, the sun rises in the west and sets in the east?

The convention is usually that the sun always rises in the east, so Venus just has it's north pole pointing the other way.

The weird thing about Venus is it having two days per year despite only rotating once.  If it rotated once in the correct direction, it would have zero days per year, sort of like how Earth never sets as viewed from the moon.

.

Not exactly.   In terms of where the Sun rises and sets, the IAU defines the North pole of a planet as the one that lies North of the Solar system plane.  In other words,  Even though we consider Venus as being flipped over, it's North pole is still considered the one that points "up".  By this convention, Venus rotates East to West and the Sun rises in the West and sets in the East.  But there is some dispute about this and not everyone accepts this convention.

If Venus rotated in the same direction as the other planets, then its solar day would be 8.17 (Earth)years long, while its year is only 0.615 yrs long.  There would be a little over 13 1/4 Venerian years to a Venerian day.
This means that from the surface of Venus ( assuming your could see it), the Sun would "bob" North and South several times as it crossed the sky from sunrise to sunset.  With the small tilt Venus has (less than 3 degrees from perpendicular to its orbital plane), this would not be a very noticeable effect.   A larger axial tilt would cause a more noticeable "bob". 

[Halc]

In terms of where the Sun rises and sets, the IAU defines the North pole of a planet as the one that lies North of the Solar system plane.  In other words,  Even though we consider Venus as being flipped over, it's North pole is still considered the one that points "up".  By this convention, Venus rotates East to West and the Sun rises in the West and sets in the East.  But there is some dispute about this and not everyone accepts this convention.

I was unaware of this convention, which seems to only work if there's an obvious upside to an orbital system.  I would have a hard time coming up with a counterexample where a mass has an assortment of objects orbiting in random directions and planes.  Such a system seems inherently unlikely and unstable to boot.

So fine.  Venus has the sun rising in the west by this more common convention.

If Venus rotated in the same direction as the other planets, then its solar day would be 8.17 (Earth)years long, while its year is only 0.615 yrs long.  There would be a little over 13 1/4 Venerian years to a Venerian day.
This means that from the surface of Venus ( assuming your could see it), the Sun would "bob" North and South several times as it crossed the sky from sunrise to sunset.  With the small tilt Venus has (less than 3 degrees from perpendicular to its orbital plane), this would not be a very noticeable effect.   A larger axial tilt would cause a more noticeable "bob".

Mercury is a little like that, with a solar day twice the length of the year.  The eccentricity of Mercury's orbit might be enough that the sun would rise perhaps more than once from some vantage points.
The exact 2x length of the day is due to a strange harmonic of tidal locking which is stable as long as the orbit remains as eccentric as it is.  If the orbit were to sufficiently circularize, the tides would start to lengthen the day again and spin it down to a proper 1-1 tidal lock like the moon has with Earth.