Anyone with a high school physics education will know that neither the Earth nor the moon orbits the sun. The Earth-moon barycenter, the center of mass which sits on a line between the moon and the earth 1700 km beneath the earth’s crust, orbits the sun. The moon and the Earth rotate around this barycenter as it orbits.

Take a wheel. Paint a bright red spot somewhere on its side. Roll the wheel on a flat surface. Graph the trajectory of the red spot.

The red spot will move in a series of semicircles above the flat surface (moving fastest at maximum distance from the surface and lingering longest at the points nearest the surface).

Would you argue with a straight face that the red spot is not revolving around the centre of the wheel because the graph is not loopy?

“I believe your original statement was that all satellites of slowly rotating planets such as Venus or Mercury must inevitably crash into their planet over the age of the solar system which is still not true. It all depends upon what the offset is of the tidal bulge from the sub-satellite point which depends on many factors such as rotation period of the planet, revolution period of the satellite, viscosity of the planet, etc. Imagine a slowly rotating planet and a distant moon. The tidal bulge lags the sub-moon point due to friction for a stationary planet but the rotation of the planet is just enough to compensate by pushing the bulge forward so that it is always at the sub-moon point. Then the moon’s orbit will neither decrease nor increase over time. So it depends on many, many factors.”

I think we are now in basic agreement – the situation you describe above is the geosynchronous case where the speed of the planet’s rotation matches the orbital speed of the satellite. Satellites further out than the geosynch orbit will tend to grow further away, satellites closer will tend to approach the planet (the viscosity of the planet only affects (I think) the magnitude of the tidal force not the sign). For Venus, “geo”-synch altitude is about a million miles (if I’ve done the back of the envelope calculation correctly) which is outside Venus’s Hill Sphere. So it seems that any satellite that Venus could “own” (ie. hold within its Hill Sphere) would have to orbit in the range that would make its orbit gradually contract – but as you said, the rate of that decrease would depend significantly on the composition of Venus, so my flat statement that the satellite would definitely crash was an overstatement.

Thanks, Andrew