# Is the retrograde (clockwise) rotation rate of Venus speeding up or slowing down?

In the update of the question "Would it be possible to "ride the wave" on Venus ?" there are links to articles in Science and in Science News that report that the atmosphere of Venus, hitting the mountains on its surface, would accelerate the planets rotation by 2 minutes each Venusian day.

They both refer to the article in Nature Geoscience Atmospheric mountain wave generation on Venus and its influence on the solid planet's rotation rate, but this paper only mentions a change in rotation rate of 2 minutes a Venusian day, also telling it could explain some of the differences in rotation rate measured by spacecraft over the past 40 years.

Wikipedia about the Orbit and rotation of Venus and refering to the ESA article Could Venus be shifting gear ? :

Venus's rotation has slowed down in the 16 years between the Magellan spacecraft and Venus Express visits; each Venusian sidereal day has increased by 6.5 minutes in that time span.

Can this contradiction be explained ?

The solar tide is slowing the rotation of Venus down.

A general rule of tidal effects on rotation goes like this: tidal forces will cause the angular momentum vector from rotation to match the vector from orbital motion. For example, consider the Earth and its Moon. Earth's rotational vector is pointed north with a magnitude of $$2\pi$$ radians per day. The orbital vector is also pointed (roughly) north but it's only about $$2\pi$$ radians per $$27$$ days (using the sidereal orbital period). So to line up the rotation vector with the orbital one Earth's rotation is slowing down.

Now apply this concept to Venus orbiting the Sun. Here the vectors are pointing in opposite directions, so the tidal forces are tending to reverse the rotation vector. The rotation must slow down, and if continued indefinitely we would expect to see it ultimaty stop and then reverse!

• So one (everlasting Venusian) day it will be tidally locked ? And couldn't that dramatically change the weather there ? Jan 3 '20 at 16:58
• @Cornelisinspace That sounds like a good question for SE Astronomy :)
– BMF
Jan 3 '20 at 18:01
• Why would it reverse after stopping? Jan 3 '20 at 21:24
• Because you still have the curvature of Venus's motion relative to the Sun; the rotation has stopped only in the sidereal reference frame. When Venus moves in its curving path the tidal bulge gets out of line with the Sun, and when the Sun pulls it back into line you now get a prograde rotation relative to the distant stars. Jan 3 '20 at 21:33
• You've explained why Venus should slow down with it's rotation, but not the contradiction in the claims of the named articles. Jan 4 '20 at 9:56