TUMBLING VENUS -- WHAT VENUS'S ROTATION MAY BE TELLING US 
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When considering tumbling planets, we usually think of Uranus because it rolls
in its orbit with its rotational axis nearly in its orbital plane.  However,   
this was surely due to a primordial impact because Uranus's moons' orbital plane 
is well-aligned to the planet's equator.  Uranus does not truly tumble.

Venus is likely to be a truly tumbling planet, as per the following evidence:

(1) Venus's sidereal rotational period (243 Earth days) is close to its sidereal 
    orbital period (224.7 Earth days), although retrograde.
(2) Venus's axial tilt is near zero -- almost normal to its orbital plane.
(3) Venus's rotation appears to be slowing (as measured by our sent spacecraft), 
    which would mean it is heading for tidal lock to the Sun.  However, it is
    highly unlikely that this tidal lock would be first reached nowadays rather 
    than gigayears ago.

These conditions can be replicated using a simple model: 

Suppose Venus was a tidally locked planet, always showing the same face to the 
Sun.  As with Mercury which is locked in a rotation-orbital resonance, condition
(2) is met -- both Mercury and Venus have very little axial tilt.  

Now, flip this tidally-locked Venus neatly upside-down.  The rotation is now 
retrograde but otherwise pretty much the same sidereal rotational period.  Now 
let the Sun-Venus tidal forces slow the rotation over some arbitrary time.  
The result: Venus's rotation as it is today.  

Of course, one could question how Venus was flipped over thusly.  The key is  
that we are speaking only of Venus's mantle, not necessarily the full planet.  
As with Earth, Venus is likely to have a core spinning independently from the 
mantle, that action lubricated by pressurized plasma at the core-mantle 
interface.  But Venus has no measurable magnetic field and current thinking is 
that its non-understood internal structure is sealing off its internal processes 
from the upper mantle.  Or more generally, Venus is internally quite different 
to Earth, and we don't yet have a good model for it.  Thus, clues can be gleaned 
from its slow retrograde rotation, as follows:

The Sun's magnetic field flips periodically (happened a few years ago), and so
does the Earth's over longer time scales.  My model is that the rotating core
flips over due to an accumulation of magnetic counter-polarity forces at the 
core-mantle interface, that flip thus re-aligning the core with those forces and 
resulting in the core subsequently accelerating its rotation until equilibrium 
with the interface is reached, after which the counter-polarity forces build up 
again, causing the core to slow and eventually flip again.  The Earth's core 
appears to be wobbling currently (as is seen by the drift of our magnetic poles) 
and may be heading for a flip within some centuries.    

Returning to Venus, the key may be that Venus's core could be very large and 
accounting for upwards of 90% of the planet's mass.  By this model, when the 
time comes for Venus's core to flip, it will instead be its mantle which flips.  
Thus we would see Venus flipping upside-down even though its massive core does 
not flip.  

Thus this model is that of a tidally locked planet which periodically flips, 
resulting in a retrograde rotation of roughly the same sidereal period, 
initially accelerating its retrograde rotation as the built-up magnetic polarity 
force within the core-mantle boundary drives the mantle, reaching its maximum 
siderial rotation of about twice the rate of the sidereal year (although 
retrograde), and then slowing tidally until reaching tidal lockdown to the Sun.  
With the mantle held in tidal lockdown, the core continues its spin which builds 
up the magnetic counter-polarity forces at the core-mantle interface, until the 
mantle finally flips again, starting the cycle anew.  This would explain the 
Venus that we see today.     

Venus's rotational slowdown has been measured by two spacecraft visitors: 
Magellan (NASA) orbited Venus in the 1990's and measured a Venusian sidereal day 
of 243.0185 Earth-days.  The Venus Express satellite (ESA) orbited 2007-2014 and
found that in the 16 years since Magellan, the Venusian rotation was 20km in
arrears of the Magellan rate, well beyond the error margin.  Article:  
https://www.esa.int/Science_Exploration/Space_Science/Venus_Express/Could_Venus_be_shifting_gear 

The cause of the measured rotational slippage between the two Venus orbiters is 
not understood, but if we model that Venus's rotation is slowing down evenly by 
that amount, then the calculation is that Venus will reach tidal lockdown in 
760,000 Earth-years.  This is important because it makes no sense that Venus 
would first achieve tidal lockdown in the human timeline -- it would have 
happened a few billion years ago instead.  Therefore if, say, a third spacecraft 
visit (or Earth radar) confirms that Venus is indeed slowing its rotation rate, 
then it is immediate that this is indeed a continuous cycle and that Venus has 
been tumbling throughout the epochs.

It must be mentioned that Earth-based radar measurements of Venusian rotation 
accord to the Venus Express value, and that the earlier Magellan is an outlier 
in this.  But nobody can find anything wrong with the Magellan measurements -- 
they are taken as correct.  So one idea is that Venus really was rotating faster 
when Magellan visited, but only temporarily -- an idea farcical on its face.  
Others say the impressive Venusian winds affect the rotation -- the winds circle 
Venus in just 4 Earth-days. But no energy equation can support that idea either.   

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This topic originally presented in these newsgroup postings by Eric Flesch:

Subject: Tumbling Venus  
Date: 2011-Sept-02     newsgroup: sci.astro.research 

Subject: Tumbling Venus -- equation needed
Date: 2016-March-09    newsgroup: sci.astro.research